# Programming with >kmm; 4

Murray Cumming
Bernhard Rieder Chapter on "Timeouts".
Jonathon Jongsma Chapter on "Drawing with Cairo". Chapter on "Working with gtkmm's Source Code". Chapter on "Recent Files".
Ole Laursen Parts of chapter on "Internationalization".
Marko Anastasov Chapter on "Printing". Parts of chapter on "Internationalization".
Daniel Elstner Section "Build Structure" of chapter on "Wrapping C Libraries with gmmproc".
Chris Vine Chapter on "Multi-threaded programs".
David King Section on Gtk::Grid.
Pedro Ferreira Chapter on Keyboard Events.
Kjell Ahlstedt Update from gtkmm 3 to gtkmm 4. Chapter on "Building applications".

This book explains key concepts of the >kmm; C++ API for creating user interfaces. It also introduces the main user interface elements ("widgets").

2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Murray Cumming

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. You may obtain a copy of the GNU Free Documentation License from the Free Software Foundation by visiting their Web site or by writing to: Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

## Introduction

### This book

This book explains key concepts of the >kmm; C++ API for creating user interfaces. It also introduces the main user interface elements ("widgets"). Although it mentions classes, constructors, and methods, it does not go into great detail. Therefore, for full API information you should follow the links into the reference documentation.

This book assumes a good understanding of C++, and how to create C++ programs.

We would very much like to hear of any problems you have learning >kmm; with this document, and would appreciate input regarding improvements. Please see the Contributing section for further information.

This book describes >kmm; 4, but some sections have not been fully updated. There are paragraphs that describe >kmm; 3 rather than >kmm; 4. All shown example programs are compatible with >kmm; 4, though.

### gtkmm

>kmm; is a C++ wrapper for GTK, a library used to create graphical user interfaces. It is licensed using the LGPL license, so you can develop open software, free software, or even commercial non-free software using >kmm; without purchasing licenses.

>kmm; was originally named gtk-- because GTK was originally named GTK+ and had a + in the name. However, as -- is not easily indexed by search engines, the package generally went by the name >kmm;, and that's what we stuck with.

#### Why use >kmm; instead of GTK?

>kmm; allows you to write code using normal C++ techniques such as encapsulation, derivation, and polymorphism. As a C++ programmer you probably already realise that this leads to clearer and better organized code.

>kmm; is more type-safe, so the compiler can detect errors that would only be detected at run time when using C. This use of specific types also makes the API clearer because you can see what types should be used just by looking at a method's declaration.

Inheritance can be used to derive new widgets. The derivation of new widgets in GTK C code is so complicated and error prone that almost no C coders do it. As a C++ developer you know that derivation is an essential Object Orientated technique.

Member instances can be used, simplifying memory management. All GTK C widgets are dealt with by use of pointers. As a C++ coder you know that pointers should be avoided where possible.

>kmm; involves less code compared to GTK, which uses prefixed function names and lots of cast macros.

#### >kmm; compared to Qt

Trolltech's Qt is the closest competition to >kmm;, so it deserves discussion.

>kmm; developers tend to prefer >kmm; to Qt because >kmm; does things in a more C++ way. Qt originates from a time when C++ and the standard library were not standardised or well supported by compilers. It therefore duplicates a lot of stuff that is now in the standard library, such as containers and type information. Most significantly, Trolltech modified the C++ language to provide signals, so that Qt classes cannot be used easily with non-Qt classes. >kmm; was able to use standard C++ to provide signals without changing the C++ language. See the FAQ for more detailed differences.

#### >kmm; is a wrapper

>kmm; is not a native C++ toolkit, but a C++ wrapper of a C toolkit. This separation of interface and implementation has advantages. The >kmm; developers spend most of their time talking about how >kmm; can present the clearest API, without awkward compromises due to obscure technical details. We contribute a little to the underlying GTK code base, but so do the C coders, and the Perl coders and the Python coders, etc. Therefore GTK benefits from a broader user base than language-specific toolkits - there are more implementers, more developers, more testers, and more users.

## Installation

### Dependencies

Before attempting to install >kmm;-4.0, you might first need to install these other packages.

- sigc++-3.0

- gtk4

- glibmm-2.68

- cairomm-1.16

- pangomm-2.48

These dependencies have their own dependencies, including the following applications and libraries:

- pkg-config

- glib-2.0

- pango

- cairo

- gdk-pixbuf-2.0

- graphene-1.0

### Unix and Linux

#### Prebuilt Packages

Recent versions of >kmm; are packaged by nearly every major Linux distribution these days. So, if you use Linux, you can probably get started with >kmm; by installing the package from the official repository for your distribution. Distributions that include >kmm; in their repositories include Debian, Ubuntu, Red Hat, Fedora, Mandriva, Suse, and many others.

The names of the >kmm; packages vary from distribution to distribution (e.g. libgtkmm-4.0-dev on Debian and Ubuntu or gtkmm40-devel on Red Hat Fedora), so check with your distribution's package management program for the correct package name and install it like you would any other package.

The package names will not change when new API/ABI-compatible versions of >kmm; are released. Otherwise they would not be API/ABI-compatible. So don't be surprised, for instance, to find >kmm; 4.8 supplied by Debian's libgtkmm-4.0-dev package.

#### Installing From Source

If your distribution does not provide a pre-built >kmm; package, or if you want to install a different version than the one provided by your distribution, you can also install >kmm; from source. The source code for >kmm; can be downloaded from .

After you've installed all of the dependencies, download the >kmm; source code, unpack it, and change to the newly created directory. >kmm; can be built with Meson. See the `README` file in the >kmm; version you've downloaded.

Remember that on a Unix or Linux operating system, you will probably need to be `root` to install software. The `su` or `sudo` command will allow you to enter the `root` password and have `root` status temporarily.

The `configure` script or `meson` will check to make sure all of the required dependencies are already installed. If you are missing any dependencies, it will exit and display an error.

By default, >kmm; if built with Autotools, will be installed under the `/usr/local` directory. On some systems you may need to install to a different location. For instance, on Red Hat Linux systems you might use the `--prefix` option with configure, like so:

    # ./configure --prefix=/usr

You should be very careful when installing to standard system prefixes such as `/usr`. Linux distributions install software packages to `/usr`, so installing a source package to this prefix could corrupt or conflict with software installed using your distribution's package-management system. Ideally, you should use a separate prefix for all software you install from source.

If you want to help develop >kmm; or experiment with new features, you can also install >kmm; from git. Most users will never need to do this, but if you're interested in helping with >kmm; development, see the Working with gtkmm's Source Code appendix.

### Microsoft Windows

GTK and >kmm; were designed to work well with Microsoft Windows, and the developers encourage its use on the win32 platform. However, Windows has no standard installation system for development libraries. Please see the Windows Installation page or the >kmm; and Win32 appendix for Windows-specific installation instructions and notes.

## Basics

This chapter will introduce some of the most important aspects of >kmm; coding. These will be demonstrated with simple working example code. However, this is just a taster, so you need to look at the other chapters for more substantial information.

Your existing knowledge of C++ will help you with >kmm; as it would with any library. Unless we state otherwise, you can expect >kmm; classes to behave like any other C++ class, and you can expect to use your existing C++ techniques with >kmm; classes.

### Simple Example

To begin our introduction to >kmm;, we'll start with the simplest program possible. This program will create an empty 200 x 200 pixel window.

Source Code

We will now explain each line of the example

    #include <gtkmm.h>

All >kmm; programs must include certain >kmm; headers; `gtkmm.h` includes the entire >kmm; kit. This is usually not a good idea, because it includes a megabyte or so of headers, but for simple programs, it suffices.

The next statement:

    auto app = Gtk::Application::create("org.gtkmm.examples.base");

creates a Gtk::Application object, stored in a Glib::RefPtr smartpointer. This is needed in all >kmm; applications. The create() method for this object initializes >kmm;.

The next two lines of code create a window and set its default (initial) size:

    Gtk::Window window;
    window.set_default_size(200, 200);

The last line shows the window and enters the >kmm; main processing loop, which will finish when the window is closed. Your `main()` function will then return with an appropriate success or error code. The argc and argv arguments, passed to your application on the command line, can be checked when run() is called, but this simple application does not use those arguments.

    return app->run(window, argc, argv);

After putting the source code in `simple.cc` you can compile the above program with gcc using:

    g++ simple.cc -o simple `pkg-config gtkmm-4.0 --cflags --libs`

Note that you must surround the `pkg-config` invocation with backquotes. Backquotes cause the shell to execute the command inside them, and to use the command's output as part of the command line. Note also that `simple.cc` must come before the `pkg-config` invocation on the command line.

### Headers and Linking

Although we have shown the compilation command for the simple example, you really should use the automake and autoconf tools, as described in "Autoconf, Automake, Libtool", by G. V. Vaughan et al. The examples used in this book are included in the gtkmm-documentation package, with appropriate build files, so we won't show the build commands in future. You'll just need to find the appropriate directory and type `make`.

To simplify compilation, we use `pkg-config`, which is present in all (properly installed) >kmm; installations. This program 'knows' what compiler switches are needed to compile programs that use >kmm;. The `--cflags` option causes `pkg-config` to output a list of include directories for the compiler to look in; the `--libs` option requests the list of libraries for the compiler to link with and the directories to find them in. Try running it from your shell-prompt to see the results on your system.

However, this is even simpler when using the `PKG_CHECK_MODULES()` macro in a standard configure.ac file with autoconf and automake. For instance:

    PKG_CHECK_MODULES([MYAPP], [gtkmm-4.0 >= 4.8.0])

This checks for the presence of gtkmm and defines MYAPP_LIBS and MYAPP_CFLAGS for use in your Makefile.am files.

gtkmm-4.0 is the name of the current stable API. There are older APIs called gtkmm-2.4 and gtkmm-3.0 which install in parallel when they are available. There are several versions of gtkmm-2.4, such as gtkmm 2.10 and there are several versions of the gtkmm-3.0 API. Note that the API name does not change for every version because that would be an incompatible API and ABI break. There might be a future gtkmm-5.0 API which would install in parallel with gtkmm-4.0 without affecting existing applications.

Note that if you mention extra modules in addition to gtkmm-4.0, they should be separated by spaces, not commas.

The GNU site has more information about autoconf and automake.

If you start by experimenting with a small application that you plan to use just for yourself, it's easier to start with a Makefile similar to the `Makefile.example` files in the Building applications chapter.

### Widgets

>kmm; applications consist of windows containing widgets, such as buttons and text boxes. In some other systems, widgets are called "controls". For each widget in your application's windows, there is a C++ object in your application's code. So you just need to call a method of the widget's class to affect the visible widget.

Widgets are arranged inside container widgets such as frames and notebooks, in a hierarchy of widgets within widgets. Some of these container widgets, such as Gtk::Grid, are not visible - they exist only to arrange other widgets. Here is some example code that adds 2 Gtk::Button widgets to a Gtk::Box container widget:

    m_box.append(m_Button1);
    m_box.append(m_Button2);

and here is how to add the Gtk::Box, containing those buttons, to a Gtk::Frame, which has a visible frame and title:

    m_frame.set_child(m_box);

Most of the chapters in this book deal with specific widgets. See the Container Widgets section for more details about adding widgets to container widgets.

Although you can specify the layout and appearance of windows and widgets with C++ code, you will probably find it more convenient to design your user interfaces with `Glade` and load them at runtime with `Gtk::Builder`. See the Glade and Gtk::Builder chapter.

Although >kmm; widget instances have lifetimes and scopes just like those of other C++ classes, >kmm; has an optional time-saving feature that you will see in some of the examples. The `Gtk::make_managed()` allows you to create a new widget and state that it will become owned by the container into which you place it. This allows you to create the widget, add it to the container and not be concerned about deleting it, since that will occur when the parent container (which may itself be managed) is deleted. You can learn more about >kmm; memory management techniques in the Memory Management chapter.

### Signals

>kmm;, like most GUI toolkits, is event-driven. When an event occurs, such as the press of a mouse button, the appropriate signal will be emitted by the Widget that was pressed. Each Widget has a different set of signals that it can emit. To make a button click result in an action, we set up a signal handler to catch the button's "clicked" signal.

>kmm; uses the libsigc++ library to implement signals. Here is an example line of code that connects a Gtk::Button's "clicked" signal with a signal handler called "on_button_clicked":

    m_button1.signal_clicked().connect( sigc::mem_fun(*this,
      &HelloWorld::on_button_clicked) );

For more detailed information about signals, see the appendix.

For information about implementing your own signals rather than just connecting to the existing >kmm; signals, see the appendix.

### Glib::ustring

You might be surprised to learn that >kmm; doesn't use std::string in its interfaces. Instead it uses Glib::ustring, which is so similar and unobtrusive that you could actually pretend that each Glib::ustring is a std::string and ignore the rest of this section. But read on if you want to use languages other than English in your application.

std::string uses 8 bits per character, but 8 bits aren't enough to encode languages such as Arabic, Chinese, and Japanese. Although the encodings for these languages have been specified by the Unicode Consortium, the C and C++ languages do not yet provide any standardised Unicode support for UTF-8 encoding. GTK and GNOME chose to implement Unicode using UTF-8, and that's what is wrapped by Glib::ustring. It provides almost exactly the same interface as std::string, along with automatic conversions to and from std::string.

One of the benefits of UTF-8 is that you don't need to use it unless you want to, so you don't need to retrofit all of your code at once. std::string will still work for 7-bit ASCII strings. But when you try to localize your application for languages like Chinese, for instance, you will start to see strange errors, and possible crashes. Then all you need to do is start using Glib::ustring instead.

Note that UTF-8 isn't compatible with 8-bit encodings like ISO-8859-1. For instance, German umlauts are not in the ASCII range and need more than 1 byte in the UTF-8 encoding. If your code contains 8-bit string literals, you have to convert them to UTF-8 (e.g. the Bavarian greeting "Grüß Gott" would be "Gr\xC3\xBC\xC3\x9F Gott").

You should avoid C-style pointer arithmetic, and functions such as strlen(). In UTF-8, each character might need anywhere from 1 to 6 bytes, so it's not possible to assume that the next byte is another character. Glib::ustring worries about the details of this for you so you can use methods such as Glib::ustring::substr() while still thinking in terms of characters instead of bytes.

Unlike the Windows UCS-2 Unicode solution, this does not require any special compiler options to process string literals, and it does not result in Unicode executables and libraries which are incompatible with ASCII ones.

Reference

See the Internationalization section for information about providing the UTF-8 string literals.

### Mixing C and C++ APIs

You can use C APIs which do not yet have convenient C++ interfaces. It is generally not a problem to use C APIs from C++, and >kmm; helps by providing access to the underlying C object, and providing an easy way to create a C++ wrapper object from a C object, provided that the C API is also based on the GObject system.

To use a >kmm; instance with a C function that requires a C GObject instance, use the C++ instance’s `gobj()` function to obtain a pointer to the underlying C instance. For example:

    Gtk::Button button("example");
    gtk_button_do_something_that_gtkmm_cannot(button.gobj());

To obtain a >kmm; instance from a C GObject instance, use one of the many overloaded `Glib::wrap()` functions. The C instance’s reference count is not incremented, unless you set the optional take_copy argument to `true`. For example:

    GtkButton* cbutton = get_a_button();
    Gtk::Button* button = Glib::wrap(cbutton);
    button->set_label("Now I speak C++ too!");

The C++ wrapper shall be explicitly deleted if

- it's a widget or other class that inherits from Gtk::Object, and

- the C instance has a floating reference when the wrapper is created, and

- `Gtk::manage()` has not been called on it (which includes if it was created with `Gtk::make_managed()`), or

- `Gtk::manage()` was called on it, but it was never added to, or was later removed from, its parent.

`Glib::wrap()` binds the C and C++ instances to each other. Don't delete the C++ instance before you want the C instance to die.

In all other cases the C++ instance is automatically deleted when the last reference to the C instance is dropped. This includes all `Glib::wrap()` overloads that return a Glib::RefPtr.

### Hello World in >kmm;

We've now learned enough to look at a real example. In accordance with an ancient tradition of computer science, we now introduce Hello World, a la >kmm;:

Source Code

Try to compile and run it before going on. You should see something like this:

Hello World

Pretty thrilling, eh? Let's examine the code. First, the HelloWorld class:

    class HelloWorld : public Gtk::Window
    {
    public:
      HelloWorld();
      ~HelloWorld() override;

    protected:
      //Signal handlers:
      void on_button_clicked();

      //Member widgets:
      Gtk::Button m_button;
    };

This class implements the "Hello World" window. It's derived from Gtk::Window, and has a single Gtk::Button as a member. We've chosen to use the constructor to do all of the initialisation work for the window, including setting up the signals. Here it is, with the comments omitted:

    HelloWorld::HelloWorld()
    : m_button("Hello World")
    {
      m_button.set_margin(10);
      m_button.signal_clicked().connect(sigc::mem_fun(*this,
        &HelloWorld::on_button_clicked));
      set_child(m_button);
    }

Notice that we've used an initialiser statement to give the `m_button` object the label "Hello World".

Next we call the Button's set_margin() method. This sets the amount of space around the button.

We then hook up a signal handler to `m_button`'s `clicked` signal. This prints our friendly greeting to `stdout`.

Next, we use the Window's set_child() method to put `m_button` in the Window. The set_child() method places the Widget in the Window.

Now let's look at our program's `main()` function. Here it is, without comments:

    int main(int argc, char* argv[])
    {
      auto app = Gtk::Application::create("org.gtkmm.example");
      HelloWorld helloworld;
      return app->run(helloworld, argc, argv);
    }

First we instantiate an object stored in a RefPtr smartpointer called `app`. This is of type Gtk::Application. Every >kmm; program must have one of these.

Next we make an object of our HelloWorld class, whose constructor takes no arguments, but it isn't visible yet. When we call Gtk::Application::run(), giving it the helloworld Window and the command-line arguments, it shows the Window and starts the >kmm; event loop. During the event loop >kmm; idles, waiting for actions from the user, and responding appropriately. When the user closes the Window, run() will return, causing the final line of our `main()` function be to executed. The application will then finish.

Like the simple example we showed earlier, this Hello World program does not use the command-line parameters. It's not necessary to pass them to run().

## Changes in >kmm; 3

>kmm;-3.0 is an old version of the >kmm; API that installs in parallel with the still older >kmm;-2.4 API and the new >kmm;-4.0 API. The last version of the >kmm;-2.4 API was >kmm; 2.24. >kmm; 3 has no major fundamental differences to >kmm; 2 but does make several small changes that were not possible while maintaining binary compatibility. If you never used the >kmm;-2.4 API then you can safely ignore this chapter.

>kmm; 3's library is called `libgtkmm-3.0` rather than `libgtkmm-2.4` and installs its headers in a similarly-versioned directory, so your pkg-config check should ask for `gtkmm-3.0` rather than `gtkmm-2.4`.

>kmm; 3 added some new classes:

1. Gtk::AppChooser, Gtk::AppChooserButton, Gtk::AppChooserDialog allow the user to select an installed application to open a particular type of content.

2. Gtk::Grid is a new container widget that will eventually replace Gtk::Box and Gtk::Table. It arranges its children according to properties of those children rather than its own layout details.

3. Gtk::Switch displays On/Off states more explictly than Gtk::CheckButton. It may be useful, for instance, when allowing users to activate hardware.

>kmm; 3 also made several small changes to the API, which you will probably encounter when porting code that used >kmm;-2.4. Here is a short list:

1. Gtk::CellLayout, used by Gtk::IconView, Gtk::TreeView::Column and Gtk::ComboBox, now has a Gtk::CellArea which can be used to specify more details of how the CellRenderers are arranged and aligned.

2. Gtk::ComboBox now derives from CellLayout, allowing easier layout and alignment of its Gtk::CellRenderers.

3. Gtk::Adjustment and IconSet and Gdk::Cursor are now used via Glib::RefPtr.

4. Gtk::Box, Gtk::ButtonBox, Gtk::IconView, Gtk::Paned, Gtk::ProgressBar, Gtk::ScaleButton, Gtk::Scrollbar and Gtk::Separator now derive from Gtk::Orientable, allowing their orientation (vertical or horizontal) to be specified without requiring the use of a derived class such as Gtk::HBox.

5. Gtk::IconView, Gtk::TextView, Gtk::TreeView and other widgets derive from Scrollable instead of having their own methods such as get_vadjustment() and instead of having their own set_scroll_adjustments signal.

6. Gtk::Style and Gtk::Rc were removed, replaced by Gtk::StyleContext, and Gtk::StyleProviders, such as Gtk::CssProvider.

7. Widget::on_expose_event() was replaced by Widget::on_draw(), which assumes that cairomm is used for drawing, via the provided Cairo::Context and does not require you to call Cairo::Context::clip().

8. Gdk::RGBA replaces Color, adding an alpha component for opacity. Colormap was removed, along with its awkward use to allocate colors.

9. Gdk::Pixmap and Gdk::Bitmap were removed in favour of Gdk::Pixbuf.

10. Gdk::Drawable was removed, with its methods moving into Gdk::Window.

11. We now use std::vector in several methods instead of the intermediate *Handle types to make the API clearer.

All deprecated API was removed in >kmm; 3.0, though there have been new deprecations in later >kmm; 3.x versions.

As a first step to porting your source code to >kmm;-3.0 you should probably ensure that your application builds with the deprecated >kmm;-2.4 API disabled, by defining macro such as GTKMM_DISABLE_DEPRECATED. There are some autotools macros that can help with this by defining them optionally at build time. See the gtkmm 3 porting wiki page for more details.

## Changes in >kmm;-4.0 and glibmm-2.68

>kmm;-4.0 is a new version of the >kmm; API that installs in parallel with the older >kmm;-2.4 and >kmm;-3.0 APIs. The last version of the >kmm;-3.0 API is >kmm; 3.24. >kmm; 4 has no major fundamental differences to >kmm; 3 but does make several changes (both small and large ones) that were not possible while maintaining binary compatibility. If you never used the >kmm;-3.0 API then you can safely ignore this chapter.

>kmm; 4's library is called `libgtkmm-4.0` rather than `libgtkmm-3.0` and installs its headers in a similarly-versioned directory, so your pkg-config check should ask for `gtkmm-4.0` rather than `gtkmm-3.0`.

>kmm;-4.0 is used in combination with glibmm-2.68, which sets the global locale for your program. The older glibmm-2.4 does not do that, and >kmm;-3.0 does it only to some extent. What this means is briefly that if your >kmm;-3.0 program contains a call to `std::locale::global(std::locale(""))`, you can probably remove it. If you don't want glibmm or >kmm; to set the global locale for you, you should add a call to `Glib::set_init_to_users_preferred_locale(false)` before any call to `Glib::init()` or Gtk::Application::create().

Reference

There are lots and lots of differences between >kmm;-3.0 and >kmm;-4.0. The following lists are not complete.

Some new classes were added in >kmm; 4 and glibmm 2.68:

1. Glib::ExtraClassInit and Gtk::Snapshot: These classes are needed only for writing custom widgets. See the Custom Widgets section.

2. Gtk::EventControllerKey, Gtk::EventControllerMotion, Gtk::EventControllerScroll and Gtk::GestureStylus

3. Gdk::Paintable, Gdk::Texture, Gtk::Picture and Gtk::WidgetPaintable

4. Gdk::Window has been renamed to Gdk::Surface. (Gtk::Window keeps its name.)

5. Gdk::DrawContext and Gdk::CairoContext are new. Gdk::DrawingContext has been removed.

6. Gtk::Clipboard has been replaced by the new Gdk::Clipboard.

7. Gdk::DragContext has been split into Gdk::Drag and Gdk::Drop.

There have also been several changes to the API, which you will probably encounter when porting code that used >kmm;-3.0 and glibmm-2.4. Here is a short list:

1. A C++17 compiler is required.

2. Gtk::Button, Gtk::ToolButton, Gtk::MenuItem and Gtk::Switch implement the Gtk::Actionable interface instead of the removed Gtk::Activatable interface.

3. Gtk::FontButton implements the Gtk::FontChooser interface.

4. Gtk::Widget: The get_preferred_*_vfunc()s have been replaced by measure_vfunc(). This change only affects custom widgets.

5. sigc::slots use the sigc::slot<R(Args...)> syntax. Example: sigc::slot<void(int, int)> instead of sigc::slot<void, int, int>.

6. Gtk::DrawingArea uses a draw function instead of the draw signal.

7. Glib::ArrayHandle, Glib::StringArrayHandle, Glib::ListHandle and Glib::SListHandle have been removed. They were used in glibmm-2.4, but not used in >kmm;-3.0. If you've ever used these classes, replace them with a standard C++ container, such as std::vector.

8. Gtk::Container has been removed.

9. Gtk::Widget::show_all() has been removed. The default value of Gtk::Widget::property_visible()has been changed from `false` to `true`.

10. All event signals have been removed from Gtk::Widget. In most cases you can use one of the subclasses of Gtk::EventController as a replacement. For instance, use Gtk::GestureMultiPress instead of signal_button_press_event() and signal_button_release_event(), and Gtk::EventControllerKey instead of signal_key_press_event() and signal_key_release_event().

11. Glib::RefPtr is an alias for std::shared_ptr. If you make your own Glib::ObjectBase-derived classes with create() methods that return a Glib::RefPtr, you must use Glib::make_refptr_for_instance() in your create() methods.

12. Gtk::Box::pack_start() and Gtk::Box::pack_end() have been removed. Use the new Gtk::Box methods append(), prepend(), insert_child_after() and insert_child_at_start().

13. Gtk::ButtonBox has been removed.

14. Gtk::RadioButton and Gtk::RadioButtonGroup have been removed. Use Gtk::CheckButton or Gtk::ToggleButton with set_group().

All deprecated API was removed in >kmm; 4.0 and glibmm 2.68, though there will be new deprecations in future versions.

As a first step to porting your source code to >kmm;-4.0 you should probably ensure that your application builds with the deprecated >kmm;-3.0 and glibmm-2.4 API disabled, by defining the macros GTKMM_DISABLE_DEPRECATED, GDKMM_DISABLE_DEPRECATED, GLIBMM_DISABLE_DEPRECATED and GIOMM_DISABLE_DEPRECATED. There are some autotools macros that can help with this by defining them optionally at build time. See the Porting from gtkmm-2.4 to gtkmm-3.0 wiki page for more details.

See also Migrating from GTK 3.x to GTK 4.

## Buttons

>kmm; provides four basic types of buttons:

Push buttons

  Gtk::Button. Standard buttons, usually marked with a label or picture. Pushing one triggers an action. See the Button section.

Toggle buttons

  Gtk::ToggleButton. Unlike a normal Button, which springs back up, a ToggleButton stays down until you press it again. It might be useful as an on/off switch. See the ToggleButton section.

Check buttons

  Gtk::CheckButton. These act like ToggleButtons, but show their state in small squares, with their label at the side. They should be used in most situations which require an on/off setting. See the CheckButton section.

Radio buttons

  Named after the station selectors on old car radios, these buttons are used in groups for options which are mutually exclusive. Pressing one causes all the others in its group to turn off. They are similar to ToggleButtons or CheckButtons (a small widget with a label at the side), but usually look different. There is no separate radio button class. Check buttons and toggle buttons can act as radio buttons. See the Radio Button section.

Note that, due to GTK's theming system, the appearance of these widgets will vary. In the case of check buttons and radio buttons, they may vary considerably.

### Button

There are two ways to create a Button. You can specify a label string in the Gtk::Button constructor, or set it later with set_label().

To define an accelerator key for keyboard navigation, place an underscore before one of the label's characters and specify `true` for the optional `mnemonic` parameter. For instance:

    Gtk::Button* pButton = new Gtk::Button("_Something", true);

Gtk::Button is also a container so you could put any other widget, such as a Gtk::Image into it.

The Gtk::Button widget has the `clicked` signal which is emitted when the button is pressed and released.

Reference

#### Example

This example creates a button with a picture and a label.

buttons example

Source Code

### ToggleButton

ToggleButtons are like normal Buttons, but when clicked they remain activated, or pressed, until clicked again.

To retrieve the state of the ToggleButton, you can use the get_active() method. This returns `true` if the button is "down". You can also set the toggle button's state, with set_active(). Note that, if you do this, and the state actually changes, it causes the "clicked" signal to be emitted. This is usually what you want.

You can use the toggled() method to toggle the button, rather than forcing it to be up or down: This switches the button's state, and causes the `toggled` signal to be emitted.

Reference

### CheckButton

Gtk::CheckButton inherits directly from Gtk::Widget. It is similar to Gtk::ToggleButton. The only real difference between the two is Gtk::CheckButton's appearance. You can check and set a check button using the same member methods as for Gtk::ToggleButton.

Reference

#### Example

CheckButton

Source Code

### Radio Button

There is no separate class for radio buttons. Check buttons and toggle buttons act as radio buttons when they form a group. Only one button in a group can be selected at any one time.

#### Groups

You create the buttons, and set up their group afterwards. In the following example, we put 3 radio buttons in a group:

    auto rb1 = Gtk::make_managed<Gtk::CheckButton>("button1");
    auto rb2 = Gtk::make_managed<Gtk::CheckButton>("button2");
    auto rb3 = Gtk::make_managed<Gtk::CheckButton>("button3");
    rb2->set_group(*rb1);
    rb3->set_group(*rb1);

We told >kmm; to put all three CheckButtons in the same group by using set_group() to tell the other CheckButtons to share group with the first CheckButton.

#### Methods

CheckButtons and ToggleButtons are "off" when created; this means that when you first make a group of them, they will all be off. Don't forget to turn one of them on using set_active().

Reference

#### Example

The following example demonstrates the use of grouped CheckButtons:

RadioButton

Source Code

## Range Widgets

Gtk::Scale and Gtk::Scrollbar both inherit from Gtk::Range and share much functionality. They contain a "trough" and a "slider" (sometimes called a "thumbwheel" in other GUI environments). Dragging the slider with the pointer moves it within the trough, while clicking in the trough advances the slider towards the location of the click, either completely, or by a designated amount, depending on which mouse button is used. This should be familiar scrollbar behaviour.

As will be explained in the Adjustments section, all Range widgets are associated with an Adjustment object. To change the lower, upper, and current values used by the widget you need to use the methods of its Adjustment, which you can get with the get_adjustment() method. The Range widgets' default constructors create an Adjustment automatically, or you can specify an existing Adjustment, maybe to share it with another widget. See the Adjustments section for further details.

Reference

### Scrollbar Widgets

These are standard scrollbars. They should be used only to scroll another widget, such as a Gtk::Entry or a Gtk::Viewport, though it's usually easier to use the Gtk::ScrolledWindow widget in most cases.

The orientation of a Gtk::Scrollbar can be either horizontal or vertical.

Reference

### Scale Widgets

Gtk::Scale widgets (or "sliders") allow the user to visually select and manipulate a value within a specific range. You might use one, for instance, to adjust the magnification level on a zoomed preview of a picture, or to control the brightness of a colour, or to specify the number of minutes of inactivity before a screensaver takes over the screen.

As with Scrollbars, the orientation can be either horizontal or vertical. The default constructor creates an Adjustment with all of its values set to `0.0`. This isn't useful so you will need to set some Adjustment details to get meaningful behaviour.

#### Useful methods

Scale widgets can display their current value as a number next to the trough. By default they show the value, but you can change this with the set_draw_value() method.

The value displayed by a scale widget is rounded to one decimal point by default, as is the `value` field in its Gtk::Adjustment. You can change this with the set_digits() method.

Also, the value can be drawn in different positions relative to the trough, specified by the set_value_pos() method.

Reference

### Example

This example displays a window with three range widgets all connected to the same adjustment, along with a couple of controls for adjusting some of the parameters mentioned above and in the section on adjustments, so you can see how they affect the way these widgets work for the user.

Range Widgets

Source Code

## Miscellaneous Widgets

### Label

Labels are the main method of placing non-editable text in windows, for instance to place a title next to an Entry widget. You can specify the text in the constructor, or later with the set_text() or set_markup() methods.

The width of the label will be adjusted automatically. You can produce multi-line labels by putting line breaks ("\n") in the label string.

The label text can be justified using the set_justify() method. The widget is also capable of word-wrapping, which can be activated with set_line_wrap().

Gtk::Label supports some simple formatting, for instance allowing you to make some text bold, colored, or larger. You can do this by providing a string to set_markup(), using the Pango Markup syntax. For instance, `<b>bold text</b> and <s>strikethrough text</s>` .

Reference

#### Example

Below is a short example to illustrate these functions. This example makes use of the Frame widget to better demonstrate the label styles. (The Frame widget is explained in the Frame section.) It is possible that the first character in `m_Label_Normal` is shown underlined only when you press the Alt key.

Label

Source Code

### Entry

#### Simple Use

Entry widgets allow the user to enter text. You can change the contents with the set_text() method, and read the current contents with the get_text() method.

Occasionally you might want to make an Entry widget read-only. This can be done by passing `false` to the set_editable() method.

For the input of passwords, passphrases and other information you don't want echoed on the screen, calling set_visibility() with `false` will cause the text to be hidden.

You might want to be notified whenever the user types in a text entry widget. Gtk::Entry provides two signals, `activate` and `changed`, for this purpose. `activate` is emitted when the user presses the Enter key in a text-entry widget; `changed` is emitted when the text in the widget changes. You can use these, for instance, to validate or filter the text the user types. Moving the keyboard focus to another widget may also signal that the user has finished entering text. The `focus_out_event` signal that Gtk::Entry inherits from Gtk::Widget can notify you when that happens. The ComboBox with an Entry section contains example programs that use these signals.

If you pass `true` to the set_activates_default() method, pressing Enter in the Gtk::Entry will activate the default widget for the window containing the Gtk::Entry. This is especially useful in dialog boxes. The default widget is usually one of the dialog buttons, which e.g. will close the dialog box. To set a widget as the default widget, use Gtk::Widget::set_can_default() and Gtk::Widget::grab_default().

Reference

##### Simple Entry Example

This example uses Gtk::Entry. It also has two CheckButtons, with which you can toggle the editable and visible flags.

Entry

Source Code

#### Entry Completion

An Entry widget can offer a drop-down list of pre-existing choices based on the first few characters typed by the user. For instance, a search dialog could suggest text from previous searches.

To enable this functionality, you must create an EntryCompletion object, and provide it to the Entry widget via the set_completion() method.

The EntryCompletion may use a TreeModel containing possible entries, specified with set_model(). You should then call set_text_column() to specify which of your model columns should be used to match possible text entries.

Alternatively, if a complete list of possible entries would be too large or too inconvenient to generate, a callback slot may instead be specified with set_match_func(). This is also useful if you wish to match on a part of the string other than the start.

Reference

##### Entry Completion Example

This example creates a Gtk::EntryCompletion and associates it with a Gtk::Entry widget. The completion uses a Gtk::TreeModel of possible entries, and some additional actions.

Entry Completion

Source Code

#### Entry Icons

An Entry widget can show an icon at the start or end of the text area. The icon can be specifed by methods such as set_icon_from_pixbuf() or set_icon_from_icon_name(). An application can respond to the user pressing the icon by handling the signal_icon_press signal.

##### Entry Icon Example

This example shows a Gtk::Entry widget with a named search icon, and prints text to the terminal when the icon is pressed.

Entry with Icon

Source Code

#### Entry Progress

An Entry widget can show a progress bar inside the text area, under the entered text. The progress bar will be shown if the set_progress_fraction() or set_progress_pulse_step() methods are called.

##### Entry Progress Example

This example shows a Gtk::Entry widget with a progress bar.

Entry with Progress Bar

Source Code

### SpinButton

A SpinButton allows the user to select a value from a range of numeric values. It has an Entry widget with increment and decrement buttons at the side. Clicking the buttons causes the value to 'spin' up and down across the range of possible values. The Entry widget may also be used to enter a value directly.

The value can have an adjustable number of decimal places, and the step size is configurable. SpinButtons have an 'auto-repeat' feature as well: holding down the increment or decrement button can optionally cause the value to change more quickly the longer the button is held down.

SpinButtons use an Adjustment object to hold information about the range of values. These Adjustment attributes are used by the Spin Button like so:

- `value`: value for the Spin Button

- `lower`: lower range value

- `upper`: upper range value

- `step_increment`: value to increment/decrement when pressing mouse button 1

- `page_increment`: value to increment/decrement when pressing mouse button 2

- `page_size`: unused

Additionally, mouse button 3 can be used to jump directly to the `upper` or `lower` values.

The SpinButton can create a default Adjustment, which you can access via the get_adjustment() method, or you can specify an existing Adjustment in the constructor.

#### Methods

The number of decimal places can be altered using the set_digits() method.

You can set the spinbutton's value using the set_value() method, and retrieve it with get_value().

The spin() method 'spins' the SpinButton, as if its increment or decrement button had been clicked. You need to specify a Gtk::SpinType to specify the direction or new position.

To prevent the user from typing non-numeric characters into the entry box, pass `true` to the set_numeric() method.

To make the SpinButton 'wrap' between its upper and lower bounds, use the set_wrap() method.

To force it to snap to the nearest `step_increment`, use set_snap_to_ticks().

Reference

#### Example

Here's an example of a SpinButton in action:

SpinButton

Source Code

### ProgressBar

Progress bars are used to show the status of an ongoing operation. For instance, a ProgressBar can show how much of a task has been completed.

To change the value shown, use the set_fraction() method, passing a double between 0.0 and 1.0 to provide the new fraction.

A ProgressBar is horizontal and left-to-right by default, but you can change it to a vertical progress bar by using the set_orientation() method.

Reference

#### Activity Mode

Besides indicating the amount of progress that has occured, the progress bar can also be used to indicate that there is some activity; this is done by placing the progress bar in activity mode. In this mode, the progress bar displays a small rectangle which moves back and forth. Activity mode is useful in situations where the progress of an operation cannot be calculated as a value range (e.g., receiving a file of unknown length).

To do this, you need to call the pulse() method at regular intervals. You can also choose the step size, with the set_pulse_step() method.

The progress bar can also display a configurable text string next to the bar, using the set_text() method.

#### Example

ProgressBar

Source Code

### InfoBar

An InfoBar may show small items of information or ask brief questions. Unlike a Dialog, it appears at the top of the current window instead of opening a new window. Its API is very similar to the Gtk::Dialog API.

Reference

#### Example

InfoBar

Source Code

### Tooltips

Tooltips are the little information windows that pop up when you leave your pointer over a widget for a few seconds. Use set_tooltip_text() to set a text string as a tooltip on any Widget. Gtk::Tooltip is used for more advanced tooltip usage, such as showing an image as well as text.

Widget Reference

Tooltip Reference

#### Example

Tooltip

Source Code

## Container Widgets

Container widgets, like other widgets, derive from Gtk::Widget. Some container widgets, such as Gtk::Grid can hold many child widgets, so these typically have more complex interfaces. Others, such as Gtk::Frame contain only one child widget.

### Single-item Containers

Most single-item container widgets have set_child() and unset_child() methods for the child widget. Gtk::Button and Gtk::Window are technically single-item containers, but we have discussed them already elsewhere.

We also discuss the Gtk::Paned widget, which allows you to divide a window into two separate "panes". This widget actually contains two child widgets, but the number is fixed so it seems appropriate.

#### Frame

Frames can enclose one or a group of widgets within a box, optionally with a title. For instance, you might place a group of ToggleButtons or CheckButtons in a Frame.

Reference

##### Example

Frame

Source Code

#### Paned

Panes divide a widget into two halves, separated by a moveable divider. The two halves (panes) can be oriented either horizontally (side by side) or vertically (one above the other).

Unlike the other widgets in this section, pane widgets contain not one but two child widgets, one in each pane. Therefore, you should use set_start_child() and set_end_child() instead of a set_child() method.

You can adjust the position of the divider using the set_position() method, and you will probably need to do so.

Reference

##### Example

Paned

Source Code

#### ScrolledWindow

ScrolledWindow widgets create a scrollable area. You can insert any type of widget into a ScrolledWindow, and it will be accessible regardless of its size by using the scrollbars. Note that ScrolledWindow is not a Gtk::Window despite the slightly misleading name.

Scrolled windows have scrollbar policies which determine whether the Scrollbars will be displayed. The policies can be set with the set_policy() method. The policy may be for instance `Gtk::PolicyType::AUTOMATIC` or `Gtk::PolicyType::ALWAYS`. `Gtk::PolicyType::AUTOMATIC` will cause the scrolled window to display the scrollbar only if the contained widget is larger than the visible area. `Gtk::PolicyType::ALWAYS` will cause the scrollbar to be displayed always.

Reference

##### Example

Here is a simple example that packs 100 toggle buttons into a ScrolledWindow. Try resizing the window to see the scrollbars react.

ScrolledWindow

Source Code

#### AspectFrame

The AspectFrame widget looks like a Frame widget, but it also enforces the aspect ratio (the ratio of the width to the height) of the child widget, adding extra space if necessary. For instance, this would allow you to display a photograph without allowing the user to distort it horizontally or vertically while resizing.

Reference

##### Example

The following program uses a Gtk::AspectFrame to present a drawing area whose aspect ratio will always be 2:1, no matter how the user resizes the top-level window.

AspectFrame

Source Code

#### Other Single-item Containers

There are other single-item containers. See the reference documentation for a complete list. Here are links to some example programs that show containers, which are not mentioned elsewhere in this tutorial.

Source Code, Expander

Source Code, Popover

### Multiple-item Containers

Multiple-item container widgets have other methods than set_child() and unset_child(). Different containers can have different methods for adding and removing child widgets. For instance, Gtk::Box has append() and remove() as well as other methods. The remove() method for multiple-item containers takes an argument, specifying which widget to remove.

#### Packing

You've probably noticed that >kmm; windows seem "elastic" - they can usually be stretched in many different ways. This is due to the widget packing system.

Many GUI toolkits require you to precisely place widgets in a window, using absolute positioning, often using a visual editor. This leads to several problems:

- The widgets don't rearrange themselves when the window is resized. Some widgets are hidden when the window is made smaller, and lots of useless space appears when the window is made larger.

- It's impossible to predict the amount of space necessary for text after it has been translated to other languages, or displayed in a different font. On Unix it is also impossible to anticipate the effects of every theme and window manager.

- Changing the layout of a window "on the fly", to make some extra widgets appear, for instance, is complex. It requires tedious recalculation of every widget's position.

>kmm; uses the packing system to solve these problems. Rather than specifying the position and size of each widget in the window, you can arrange your widgets in rows, columns, and/or grids. >kmm; can size your window automatically, based on the sizes of the widgets it contains. And the sizes of the widgets are, in turn, determined by the amount of text they contain, or the minimum and maximum sizes that you specify, and/or how you have requested that the available space should be shared between sets of widgets. You can perfect your layout by specifying margins and centering values for each of your widgets. >kmm; then uses all this information to resize and reposition everything sensibly and smoothly when the user manipulates the window.

>kmm; arranges widgets hierarchically, using containers. A container widget contains other widgets. Most >kmm; widgets are containers. Windows, Notebook tabs, and Buttons are all container widgets. There are two flavours of containers: single-child containers and multiple-child containers. Most container widgets in >kmm; are single-child containers, including Gtk::Window.

Yes, that's correct: a Window can contain at most one widget. How, then, can we use a window for anything useful? By placing a multiple-child container in the window. The most useful container widgets are Gtk::Grid and Gtk::Box.

- Gtk::Grid arranges its child widgets in rows and columns. Use attach() and attach_next_to() to insert child widgets.

- Gtk::Box arranges its child widgets vertically or horizontally. Use append() to insert child widgets.

There are several other containers, which we will also discuss.

If you've never used a packing toolkit before, it can take some getting used to. You'll probably find, however, that you don't need to rely on visual form editors quite as much as you might with other toolkits.

#### An improved Hello World

Let's take a look at a slightly improved `helloworld`, showing what we've learnt.

Hello World 2

Source Code

After building and running this program, try resizing the window to see the behaviour. Also, try playing with set_expand(), set_hexpand(), set_vexpand(), set_halign() and set_valign() while reading the Boxes section.

#### Boxes

Most packing uses boxes as in the above example. These are invisible containers into which we can pack our widgets. When packing widgets into a horizontal box, the objects are inserted horizontally from left to right. In a vertical box, widgets are packed from top to bottom. You may use any combination of boxes inside or beside other boxes to create the desired effect.

##### Adding widgets

###### Per-child packing options

The append() method places widgets inside these containers. It will start at the top and work its way down in a Box with vertical orientation, or pack left to right in a Box with horizontal orientation. If it's inconvenient to add widgets in this order, use insert_child_after() or insert_child_at_start(). We will use append() in our examples.

There are several options governing how widgets are to be packed, and this can be confusing at first. You can modify the packing by using set_expand(), set_hexpand(), set_vexpand(), set_halign(), set_valign() and/or set_margin() on the child widgets. If you have difficulties, then it is sometimes a good idea to play with the glade GUI designer to see what is possible. You might even decide to use the Gtk::Builder API to load your GUI at runtime.

There are basically five different styles, as shown in this picture:

Box Packing 1

Each line contains one horizontal Box with several buttons. Each of the buttons on a line is packed into the Box with the same arguments to the set_hexpand(), set_halign(), set_margin_start() and set_margin_end() methods.

Reference

###### Per-container packing options

Here's the constructor for the Box widget, and methods that set per-container packing options:

    Gtk::Box(Gtk::Orientation orientation = Gtk::Orientation::HORIZONTAL, int spacing = 0);
    void set_orientation(Gtk::Orientation orientation);
    void set_spacing(int spacing);
    void set_homogeneous(bool homogeneous = true);

Passing `true` to set_homogeneous() will cause all of the contained widgets to be the same size. spacing is a (minimum) number of pixels to leave between each widget.

What's the difference between spacing (set when the box is created) and margins (set separately for each child widget)? Spacing is added between objects, and margins are added on one or more sides of a widget. The following figure should make it clearer. The shown margins are the left and right margins of each button in the row.

Box Packing 2

##### Gtk::Application and command-line options

The following example program requires a command-line option. The source code shows two ways of handling command-line options in combination with Gtk::Application.

- Handle the options in `main()` and hide them from Gtk::Application by setting `argc = 1` in the call to Gtk::Application::run().

- Give all command-line options to Gtk::Application::run() and add the flag `Gio::Application::Flags::HANDLES_COMMAND_LINE` to Gtk::Application::create(). Connect a signal handler to the `command_line` signal, and handle the command-line options in the signal handler.

  You must set the optional parameter `after = false` in the call to `signal_command_line().connect()`, because your signal handler must be called before the default signal handler. You must also call Gio::Application::activate() in the signal handler, unless you want your application to exit without showing its main window. (Gio::Application is a base class of Gtk::Application.)

##### Example

Here is the source code for the example that produced the screenshots above. When you run this example, provide a number between 1 and 3 as a command-line option, to see different packing options in use.

Source Code

#### Grid

A Grid dynamically lays out child widgets in rows and columns. The dimensions of the grid do not need to be specified in the constructor.

Child widgets can span multiple rows or columns, using attach(), or added next to an existing widget inside the grid with attach_next_to(). Individual rows and columns of the grid can be set to have uniform height or width with set_row_homogeneous() and set_column_homogeneous().

You can set the margin and expand properties of the child Widgets to control their spacing and their behaviour when the Grid is resized.

Reference

##### Example

This example creates a window with three buttons in a grid. The first two buttons are in the upper row, from left to right. A third button is attached underneath the first button, in a new lower row, spanning two columns.

Grid

Source Code

#### Notebook

A Notebook has a set of stacked `pages`, each of which contains widgets. Labelled `tabs` allow the user to select the pages. Notebooks allow several sets of widgets to be placed in a small space, by only showing one page at a time. For instance, they are often used in preferences dialogs.

Use the append_page(), prepend_page() and insert_page() methods to add tabbed pages to the `Notebook`, supplying the child widget and the name for the tab.

To discover the currently visible page, use the get_current_page() method. This returns the page number, and then calling get_nth_page() with that number will give you a pointer to the actual child widget.

To programmatically change the selected page, use the set_current_page() method.

Reference

##### Example

Notebook

Source Code

#### Assistant

An Assistant splits a complex operation into steps. Each step is a page, containing a header, a child widget and an action area. The Assistant's action area has navigation buttons which update automatically depending on the type of the page, set with set_page_type().

Use the append_page(), prepend_page and insert_page() methods to add pages to the Assistant, supplying the child widget for each page.

To determine the currently-visible page, use the get_current_page() method, and pass the result to get_nth_page(), which returns a pointer to the actual widget. To programmatically change the current page, use the set_current_page() method.

To set the title of a page, use the set_page_title() method.

To add widgets to the action area, use the add_action_widget() method. They will be packed alongside the default buttons. Use the remove_action_widget() method to remove widgets.

Reference

##### Example

Assistant

Source Code

#### Other Multi-item Containers

There are other multi-item containers. See the reference documentation for a complete list. Here are links to some example programs that show containers, which are not mentioned elsewhere in this tutorial.

Source Code, ActionBar

Source Code, FlowBox

Source Code, IconView

## The TreeView widget

The Gtk::TreeView widget can contain lists or trees of data, in columns.

### The Model

Each Gtk::TreeView has an associated Gtk::TreeModel, which contains the data displayed by the TreeView. Each Gtk::TreeModel can be used by more than one Gtk::TreeView. For instance, this allows the same underlying data to be displayed and edited in 2 different ways at the same time. Or the 2 Views might display different columns from the same Model data, in the same way that 2 SQL queries (or "views") might show different fields from the same database table.

Although you can theoretically implement your own Model, you will normally use either the ListStore or TreeStore model classes.

Reference

#### ListStore, for rows

The ListStore contains simple rows of data, and each row has no children.

TreeView - ListStore

Reference

#### TreeStore, for a hierarchy

The TreeStore contains rows of data, and each row may have child rows.

TreeView - TreeStore

Reference

#### Model Columns

The TreeModelColumnRecord class is used to keep track of the columns and their data types. You add TreeModelColumn instances to the ColumnRecord and then use those TreeModelColumns when getting and setting the data in model rows. You will probably find it convenient to derive a new TreeModelColumnRecord which has your TreeModelColumn instances as member data.

    class ModelColumns : public Gtk::TreeModelColumnRecord
    {
    public:

      ModelColumns()
        { add(m_col_text); add(m_col_number); }

      Gtk::TreeModelColumn<Glib::ustring> m_col_text;
      Gtk::TreeModelColumn<int> m_col_number;
    };

    ModelColumns m_Columns;

You specify the ColumnRecord when creating the Model, like so:

    Glib::RefPtr<Gtk::ListStore> refListStore =
        Gtk::ListStore::create(m_Columns);

As a TreeModelColumnRecord describes structure, not data, it can be shared among multiple models, and this is preferable for efficiency. However, the instance (such as `m_Columns` here) should usually not be static, because it often needs to be instantiated after glibmm has been initialized. The best solution is to make it a lazily instantiated singleton, so that it will be constructed on-demand, whenever the first model accesses it.

#### Adding Rows

Add rows to the model with the append(), prepend(), or insert() methods.

    auto iter = m_refListStore->append();

You can dereference the iterator to get the Row:

    auto row = *iter;

##### Adding child rows

Gtk::TreeStore models can have child items. Add them with the append(), prepend(), or insert() methods, like so:

    auto iter_child =
        m_refTreeStore->append(row.children());

#### Setting values

You can use the operator[] overload to set the data for a particular column in the row, specifying the TreeModelColumn used to create the model.

    row[m_Columns.m_col_text] = "sometext";

#### Getting values

You can use the operator[] overload to get the data in a particular column in a row, specifying the TreeModelColumn used to create the model.

    auto strText = row[m_Columns.m_col_text];
    auto number = row[m_Columns.m_col_number];

The compiler will complain if you use an inappropriate type. For instance, this would generate a compiler error:

    //compiler error - no conversion from ustring to int.
    int number = row[m_Columns.m_col_text];

#### "Hidden" Columns

You might want to associate extra data with each row. If so, just add it as a Model column, but don't add it to the View.

### The View

The View is the actual widget (Gtk::TreeView) that displays the model (Gtk::TreeModel) data and allows the user to interact with it. The View can show all of the model's columns, or just some, and it can show them in various ways.

Reference

#### Using a Model

You can specify a Gtk::TreeModel when constructing the Gtk::TreeView, or you can use the set_model() method, like so:

    m_TreeView.set_model(m_refListStore);

#### Adding View Columns

You can use the append_column() method to tell the View that it should display certain Model columns, in a certain order, with a certain column title.

    m_TreeView.append_column("Messages", m_Columns.m_col_text);

When using this simple append_column() overload, the TreeView will display the model data with an appropriate CellRenderer. For instance, strings and numbers are shown in a simple Gtk::Entry widget, and booleans are shown in a Gtk::CheckButton. This is usually what you need. For other column types you must either connect a callback that converts your type into a string representation, with TreeViewColumn::set_cell_data_func(), or derive a custom CellRenderer. Note that (unsigned) short is not supported by default - You could use (unsigned) int or (unsigned) long as the column type instead.

#### More than one Model Column per View Column

To render more than one model column in a view column, you need to create the TreeView::Column widget manually, and use pack_start() to add the model columns to it.

Then use append_column() to add the view Column to the View. Notice that Gtk::TreeView::append_column() is overloaded to accept either a prebuilt Gtk::TreeView::Column widget, or just the TreeModelColumn from which it generates an appropriate Gtk::TreeView::Column widget.

Here is some example code, which has a pixbuf icon and a text name in the same column:

    auto pColumn = Gtk::make_managed<Gtk::TreeView::Column>("Icon Name");

    // m_columns.icon and m_columns.iconname are columns in the model.
    // pColumn is the column in the TreeView:
    pColumn->pack_start(m_columns.icon, /* expand= */ false);
    pColumn->pack_start(m_columns.iconname);

    m_TreeView.append_column(*pColumn);

#### Specifying CellRenderer details

The default CellRenderers and their default behaviour will normally suffice, but you might occasionally need finer control. For instance, this example code from `gtkmm/demos/gtk-demo/example_treeview_treestore.cc`, appends a Gtk::CellRenderer widget and instructs it to render the data from various model columns through various aspects of its appearance.

    auto cols_count = m_TreeView.append_column_editable("Alex", m_columns.alex);
    auto pColumn = m_TreeView.get_column(cols_count-1);
    if(pColumn)
    {
      auto pRenderer = static_cast<Gtk::CellRendererToggle*>(pColumn->get_first_cell());
      pColumn->add_attribute(pRenderer->property_visible(), m_columns.visible);
      pColumn->add_attribute(pRenderer->property_activatable(), m_columns.world);

You can also connect to CellRenderer signals to detect user actions. For instance:

    auto pRenderer = Gtk::make_managed<Gtk::CellRendererToggle>();
    pRenderer->signal_toggled().connect(
        sigc::bind( sigc::mem_fun(*this,
            &Example_TreeView_TreeStore::on_cell_toggled), m_columns.dave)
    );

#### Editable Cells

##### Automatically-stored editable cells.

Cells in a TreeView can be edited in-place by the user. To allow this, use the Gtk::TreeView insert_column_editable() and append_column_editable() methods instead of insert_column() and append_column(). When these cells are edited the new values will be stored immediately in the Model. Note that these methods are templates which can only be instantiated for simple column types such as Glib::ustring, int, and long.

##### Implementing custom logic for editable cells.

However, you might not want the new values to be stored immediately. For instance, maybe you want to restrict the input to certain characters or ranges of values.

To achieve this, you should use the normal Gtk::TreeView insert_column() and append_column() methods, then use get_column_cell_renderer() to get the Gtk::CellRenderer used by that column.

You should then cast that Gtk::CellRenderer* to the specific CellRenderer that you expect, so you can use specific API.

For instance, for a CellRendererText, you would set the cell's editable property to true, like so:

    cell->property_editable() = true;

For a CellRendererToggle, you would set the activatable property instead.

You can then connect to the appropriate "edited" signal. For instance, connect to Gtk::CellRendererText::signal_edited(), or Gtk::CellRendererToggle::signal_toggled(). If the column contains more than one CellRenderer then you will need to use Gtk::TreeView::get_column() and then call get_cells() on that view Column.

In your signal handler, you should examine the new value and then store it in the Model if that is appropriate for your application.

### Iterating over Model Rows

Gtk::TreeModel provides a C++ Standard Library-style container of its children, via the children() method. You can use the familiar begin() and end() methods iterator incrementing, like so:

    auto children = refModel->children();
    for (auto iter = children.begin(), end = children.end(); iter != end; ++iter)
    {
      auto row = *iter;
      //Do something with the row - see above for set/get.
    }

If you always want to iterate across the entire range, much more succinct syntax is possible using C++'s range-based `for` loop:

    for (auto row: refModel->children())
    {
      //Do something with the row - see above for set/get.
    }

#### Row children

When using a Gtk::TreeStore, the rows can have child rows, which can have their own children in turn. Use Gtk::TreeModel::Row::children() to get the container of child Rows:

    Gtk::TreeModel::Children children = row.children();

### The Selection

To find out what rows the user has selected, get the Gtk::TreeView::Selection object from the TreeView, like so:

    auto refTreeSelection = m_TreeView.get_selection();

#### Single or multiple selection

By default, only single rows can be selected, but you can allow multiple selection by setting the mode, like so:

    refTreeSelection->set_mode(Gtk::SELECTION_MULTIPLE);

#### The selected rows

For single-selection, you can just call get_selected(), like so:

    auto iter = refTreeSelection->get_selected();
    if(iter) //If anything is selected
    {
      auto row = *iter;
      //Do something with the row.
    }

For multiple-selection, you need to call get_selected_rows() or define a callback, and give it to selected_foreach(), selected_foreach_path(), or selected_foreach_iter(), like so:

    refTreeSelection->selected_foreach_iter(
        sigc::mem_fun(*this, &TheClass::selected_row_callback) );

    void TheClass::selected_row_callback(
        const Gtk::TreeModel::const_iterator& iter)
    {
      auto row = *iter;
      //Do something with the row.
    }

#### The "changed" signal

To respond to the user clicking on a row or range of rows, connect to the signal like so:

    refTreeSelection->signal_changed().connect(
        sigc::mem_fun(*this, &Example_IconTheme::on_selection_changed)
    );

#### Preventing row selection

Maybe the user should not be able to select every item in your list or tree. For instance, in the gtk-demo, you can select a demo to see the source code, but it doesn't make any sense to select a demo category.

To control which rows can be selected, use the set_select_function() method, providing a sigc::slot callback. For instance:

    m_refTreeSelection->set_select_function( sigc::mem_fun(*this,
        &DemoWindow::select_function) );

and then

    bool DemoWindow::select_function(
        const Glib::RefPtr<Gtk::TreeModel>& model,
        const Gtk::TreeModel::Path& path, bool)
    {
      const auto iter = model->get_iter(path);
      return iter->children().empty(); // only allow leaf nodes to be selected
    }

#### Changing the selection

To change the selection, specify a Gtk::TreeModel::iterator or Gtk::TreeModel::Row, like so:

    auto row = m_refModel->children()[5]; //The sixth row.
    if(row)
      refTreeSelection->select(row.get_iter());

or

    auto iter = m_refModel->children().begin()
    if(iter)
      refTreeSelection->select(iter);

### Sorting

The standard tree models (TreeStore and ListStore) derive from TreeSortable, so they offer sorting functionality. For instance, call set_sort_column(), to sort the model by the specified column. Or supply a callback function to set_sort_func() to implement a more complicated sorting algorithm.

TreeSortable Reference

#### Sorting by clicking on columns

So that a user can click on a TreeView's column header to sort the TreeView's contents, call Gtk::TreeView::Column::set_sort_column(), supplying the model column on which model should be sorted when the header is clicked. For instance:

    auto pColumn = treeview.get_column(0);
    if(pColumn)
      pColumn->set_sort_column(m_columns.m_col_id);

#### Independently sorted views of the same model

The TreeView already allows you to show the same TreeModel in two TreeView widgets. If you need one of these TreeViews to sort the model differently than the other then you should use a TreeModelSort instead of just, for instance, Gtk::TreeViewColumn::set_sort_column(). TreeModelSort is a model that contains another model, presenting a sorted version of that model. For instance, you might add a sorted version of a model to a TreeView like so:

    auto sorted_model = Gtk::TreeModelSort::create(model);
    sorted_model->set_sort_column(columns.m_col_name, Gtk::SORT_ASCENDING);
    treeview.set_model(sorted_model);

Note, however, that the TreeView will provide iterators to the sorted model. You must convert them to iterators to the underlying child model in order to perform actions on that model. For instance:

    void ExampleWindow::on_button_delete()
    {
      auto refTreeSelection = m_treeview.get_selection();
      if(refTreeSelection)
      {
        auto sorted_iter = m_refTreeSelection->get_selected();
        if(sorted_iter)
        {
          auto iter = m_refModelSort->convert_iter_to_child_iter(sorted_iter);
          m_refModel->erase(iter);
        }
      }
    }

TreeModelSort Reference

### Drag and Drop

Gtk::TreeView already implements simple drag-and-drop when used with the Gtk::ListStore or Gtk::TreeStore models. If necessary, it also allows you to implement more complex behaviour when items are dragged and dropped, using the normal Drag and Drop API.

#### Reorderable rows

If you call Gtk::TreeView::set_reorderable() then your TreeView's items can be moved within the treeview itself. This is demonstrated in the TreeStore example.

However, this does not allow you any control of which items can be dragged, and where they can be dropped. If you need that extra control then you might create a derived `Gtk::TreeModel` from `Gtk::TreeStore` or `Gtk::ListStore` and override the `Gtk::TreeDragSource::row_draggable_vfunc()` and `Gtk::TreeDragDest::row_drop_possible_vfunc()` virtual methods. You can examine the `Gtk::TreeModel::Path`s provided and allow or disallow dragging or dropping by returning `true` or `false`.

This is demonstrated in the drag_and_drop example.

### Popup Context Menu

Lots of people need to implement right-click context menus for TreeView's so we will explain how to do that here to save you some time. Apart from one or two points, it's much the same as a normal context menu, as described in the menus chapter.

#### Handling `button_press_event`

To detect a click of the right mouse button, you need to handle the `button_press_event` signal, and check exactly which button was pressed. Because the TreeView normally handles this signal completely, you need to either override the default signal handler in a derived TreeView class, use connect_notify() or use connect(slot, /* after= */ false). You probably also want to call the default handler before doing anything else, so that the right-click will cause the row to be selected first.

This is demonstrated in the Popup Context Menu example.

### Examples

Some TreeView examples are shown here. There are more examples in the treeview directory in gtkmm-documentation's examples.

If neither ListStore nor TreeStore is suitable for your application, look at the custom TreeModel example. It shows how you can make your own implementation of the TreeModel interface.

#### ListStore

This example has a Gtk::TreeView widget, with a Gtk::ListStore model.

TreeView - ListStore

Source Code

#### TreeStore

This example is very similar to the ListStore example, but uses a Gtk::TreeStore model instead, and adds children to the rows.

TreeView - TreeStore

Source Code

#### Editable Cells

This example is identical to the ListStore example, but it uses TreeView::append_column_editable() instead of TreeView::append_column().

TreeView - Editable Cells

Source Code

#### Drag and Drop

This example is much like the TreeStore example, but has 2 extra columns to indicate whether the row can be dragged, and whether it can receive drag-and-dropped rows. It uses a derived Gtk::TreeStore which overrides the virtual functions as described in the TreeView Drag and Drop section.

TreeView - Drag And Drop

Source Code

#### Popup Context Menu

This example is much like the ListStore example, but derives a custom TreeView in order to override the `button_press_event`, and also to encapsulate the tree model code in our derived class. See the TreeView Popup Context Menu section.

TreeView - Popup Context Menu

Source Code

## Combo Boxes

The ComboBox widget offers a list (or tree) of choices in a dropdown menu. If appropriate, it can show extra information about each item, such as text, a picture, a check button, or a progress bar. The ComboBox widget usually restricts the user to the available choices, but it can optionally have an Entry, allowing the user to enter arbitrary text if none of the available choices are suitable.

The list is provided via a TreeModel, and columns from this model are added to the ComboBox's view with the ComboBox::pack_start() method. This provides flexibility and compile-time type-safety, but the ComboBoxText class provides a simpler text-based specialization in case that flexibility is not required.

Reference

### The model

The model for a ComboBox can be defined and filled exactly as for a TreeView. For instance, you might derive a ComboBox class with one integer and one text column, like so:

    class ModelColumns : public Gtk::TreeModel::ColumnRecord
    {
    public:
      ModelColumns()
      { add(m_col_id); add(m_col_name); }

      Gtk::TreeModelColumn<int> m_col_id;
      Gtk::TreeModelColumn<Glib::ustring> m_col_name;
    };

    ModelColumns m_columns;

After appending rows to this model, you should provide the model to the ComboBox with the set_model() method. Then use the pack_start() or pack_end() methods to specify what columns will be displayed in the ComboBox. As with the TreeView you may either use the default cell renderer by passing the TreeModelColumn to the pack methods, or you may instantiate a specific CellRenderer and specify a particular mapping with either add_attribute() or set_cell_data_func(). Note that these methods are in the CellLayout base class.

### The chosen item

To discover what item, if any, the user has chosen from the ComboBox, call ComboBox::get_active(). This returns a TreeModel::iterator that you can dereference to a Row in order to read the values in your columns. For instance, you might read an integer ID value from the model, even though you have chosen only to show the human-readable description in the ComboBox. For instance:

    Gtk::TreeModel::iterator iter = m_Combo.get_active();
    if(iter)
    {
      auto row = *iter;

      //Get the data for the selected row, using our knowledge
      //of the tree model:
      auto id = row[m_Columns.m_col_id];
      set_something_id_chosen(id); //Your own function.
    }
    else
      set_nothing_chosen(); //Your own function.

### Responding to changes

You might need to react to every change of selection in the ComboBox, for instance to update other widgets. To do so, you should handle the `changed` signal. For instance:

    m_combo.signal_changed().connect( sigc::mem_fun(*this,
          &ExampleWindow::on_combo_changed) );

### Full Example

ComboBox

Source Code

### Simple Text Example

ComboBoxText

Source Code

### ComboBox with an Entry

A ComboBox may contain an Entry widget for entering of arbitrary text, by specifying `true` for the constructor's `has_entry` parameter.

#### The text column

So that the Entry can interact with the drop-down list of choices, you must specify which of your model columns is the text column, with set_entry_text_column(). For instance:

    m_combo.set_entry_text_column(m_columns.m_col_name);

When you select a choice from the drop-down menu, the value from this column will be placed in the Entry.

#### The entry

Because the user may enter arbitrary text, an active model row isn't enough to tell us what text the user has entered. Therefore, you should retrieve the Entry widget with the ComboBox::get_entry() method and call get_text() on that.

#### Responding to changes

When the user enters arbitrary text, it may not be enough to connect to the `changed` signal, which is emitted for every typed character. It is not emitted when the user presses the Enter key. Pressing the Enter key or moving the keyboard focus to another widget may signal that the user has finished entering text. To be notified of these events, connect to the Entry's `activate` and `focus_out_event` signals, like so

    auto entry = m_Combo.get_entry();
    if (entry)
    {
      // Alternatively you can connect to m_Combo.signal_changed().
      entry->signal_changed().connect(sigc::mem_fun(*this,
        &ExampleWindow::on_entry_changed) );

      entry->signal_activate().connect(sigc::mem_fun(*this,
        &ExampleWindow::on_entry_activate) );

      entry->signal_focus_out_event().connect(sigc::mem_fun(*this,
        &ExampleWindow::on_entry_focus_out_event) );
    }

The `changed` signals of ComboBox and Entry are both emitted for every change. It doesn't matter which one you connect to. But only Entry's `focus_out_event` signal is useful here.

X events are described in more detail in the X Event signals section in the appendix.

#### Full Example

ComboBox with Entry

Source Code

#### Simple Text Example

ComboBoxText with Entry

Source Code

## TextView

The TextView widget can be used to display and edit large amounts of formatted text. Like the TreeView, it has a model/view design. In this case the TextBuffer is the model.

### The Buffer

Gtk::TextBuffer is a model containing the data for the Gtk::TextView, like the Gtk::TreeModel used by Gtk::TreeView. This allows two or more Gtk::TextViews to share the same TextBuffer, and allows those TextBuffers to be displayed slightly differently. Or you could maintain several Gtk::TextBuffers and choose to display each one at different times in the same Gtk::TextView widget.

The TextView creates its own default TextBuffer, which you can access via the get_buffer() method.

Reference

#### Iterators

A Gtk::TextBuffer::iterator and a Gtk::TextBuffer::const_iterator represent a position between two characters in the text buffer. Whenever the buffer is modified in a way that affects the number of characters in the buffer, all outstanding iterators become invalid. Because of this, iterators can't be used to preserve positions across buffer modifications. To preserve a position, use Gtk::TextBuffer::Mark.

Reference

#### Tags and Formatting

##### Tags

To specify that some text in the buffer should have specific formatting, you must define a tag to hold that formatting information, and then apply that tag to the region of text. For instance, to define the tag and its properties:

    auto refTagMatch = Gtk::TextBuffer::Tag::create();
    refTagMatch->property_background() = "orange";

You can specify a name for the Tag when using the create() method, but it is not necessary.

The Tag class has many other properties.

Reference

##### TagTable

Each Gtk::TextBuffer uses a Gtk::TextBuffer::TagTable, which contains the Tags for that buffer. 2 or more TextBuffers may share the same TagTable. When you create Tags you should add them to the TagTable. For instance:

    auto refTagTable = Gtk::TextBuffer::TagTable::create();
    refTagTable->add(refTagMatch);
    //Hopefully a future version of >kmm; will have a set_tag_table() method,
    //for use after creation of the buffer.
    auto refBuffer = Gtk::TextBuffer::create(refTagTable);

You can also use get_tag_table() to get, and maybe modify, the TextBuffer's default TagTable instead of creating one explicitly.

Reference

##### Applying Tags

If you have created a Tag and added it to the TagTable, you may apply that tag to part of the TextBuffer so that some of the text is displayed with that formatting. You define the start and end of the range of text by specifying Gtk::TextBuffer::iterators. For instance:

    refBuffer->apply_tag(refTagMatch, iterRangeStart, iterRangeStop);

Or you could specify the tag when first inserting the text:

    refBuffer->insert_with_tag(iter, "Some text", refTagMatch);

You can apply more than one Tag to the same text, by using apply_tag() more than once, or by using insert_with_tags(). The Tags might specify different values for the same properties, but you can resolve these conflicts by using Tag::set_priority().

#### Marks

TextBuffer iterators are generally invalidated when the text changes, but you can use a Gtk::TextBuffer::Mark to remember a position in these situations. For instance,

    auto refMark = refBuffer->create_mark(iter);

You can then use the get_iter() method later to create an iterator for the Mark's new position.

There are two built-in Marks - `insert` and `selection_bound`, which you can access with TextBuffer's get_insert() and get_selection_bound() methods.

Reference

#### The View

As mentioned above, each TextView has a TextBuffer, and one or more TextViews can share the same TextBuffer.

Like the TreeView, you should probably put your TextView inside a ScrolledWindow to allow the user to see and move around the whole text area with scrollbars.

Reference

##### Default formatting

TextView has various methods which allow you to change the presentation of the buffer for this particular view. Some of these may be overridden by the Gtk::TextTags in the buffer, if they specify the same things. For instance, set_left_margin(), set_right_margin(), set_indent(), etc.

##### Scrolling

Gtk::TextView has various scroll_to() methods. These allow you to ensure that a particular part of the text buffer is visible. For instance, your application's Find feature might use Gtk::TextView::scroll_to() to show the found text.

### Widgets and ChildAnchors

You can embed widgets, such as Gtk::Buttons, in the text. Each such child widget needs a ChildAnchor. ChildAnchors are associated with iterators. For instance, to create a child anchor at a particular position, use Gtk::TextBuffer::create_child_anchor():

    auto refAnchor = refBuffer->create_child_anchor(iter);

Then, to add a widget at that position, use Gtk::TextView::add_child_at_anchor():

    m_TextView.add_child_at_anchor(m_Button, refAnchor);

Reference

### Examples

#### Simple Example

TextView

Source Code

## Menus and Toolbars

There are specific APIs for menus and toolbars, but you should usually deal with them together, creating Gio::SimpleActions that you can refer to in both menus and toolbars. In this way you can handle activation of the action instead of responding to the menu and toolbar items separately. And you can enable or disable both the menu and toolbar item via the action. Gtk::Builder can create menus and toolbars.

This involves the use of the Gio::SimpleActionGroup, Gio::SimpleAction and Gtk::Builder classes, all of which should be instantiated via their create() methods, which return RefPtrs.

### Actions

First create the Gio::SimpleActions and add them to a Gio::SimpleActionGroup, with Gio::ActionMap::add_action(). (Gio::ActionMap is a base class of Gio::SimpleActionGroup.) Then add the action group to your window with Gtk::Widget::insert_action_group().

The arguments to add_action() specify the action's name, which is used in the menu items and toolbar buttons. You can also specify a signal handler when calling add_action(). This signal handler will be called when the action is activated via either a menu item or a toolbar button.

For instance:

    m_refActionGroup = Gio::SimpleActionGroup::create();

    m_refActionGroup->add_action("new", sigc::mem_fun(*this, &ExampleWindow::on_action_file_new));
    m_refActionGroup->add_action("open", sigc::mem_fun(*this, &ExampleWindow::on_action_file_open));
    m_refActionGroup->add_action("quit", sigc::mem_fun(*this, &ExampleWindow::on_action_file_quit));

    insert_action_group("example", m_refActionGroup);

If you use an Gtk::ApplicationWindow, you don't have to create your own action group. Gio::ActionGroup and Gio::ActionMap are base classes of Gtk::ApplicationWindow.

### Menubar and Toolbar

Next you should create a Gtk::Builder. At this point is also a good idea to tell the application to respond to keyboard shortcuts, by using Gtk::Application::set_accel_for_action().

For instance,

    m_refBuilder = Gtk::Builder::create();

    app->set_accel_for_action("example.new", "<Primary>n");
    app->set_accel_for_action("example.quit", "<Primary>q");
    app->set_accel_for_action("example.copy", "<Primary>c");
    app->set_accel_for_action("example.paste", "<Primary>v");

If your main window is derived from ApplicationWindow and you instantiate your menubar with Gtk::Application::set_menubar(), then you don't have to call set_accel_for_action(). See Application Menu and Main Menu example for an example.

Then, you can define the actual visible layout of the menus and toolbars, and add the UI layout to the Builder. This "ui string" uses an XML format, in which you should mention the names of the actions that you have already created. For instance:

    Glib::ustring ui_info =
      "<interface>"
      "  <menu id='menubar'>"
      "    <submenu>"
      "      <attribute name='label' translatable='yes'>_File</attribute>"
      "      <section>"
      "        <item>"
      "          <attribute name='label' translatable='yes'>_New</attribute>"
      "          <attribute name='action'>example.new</attribute>"
      "          <attribute name='accel'>&lt;Primary&gt;n</attribute>"
      "        </item>"
      "      </section>"
      "      <section>"
      "        <item>"
      "          <attribute name='label' translatable='yes'>_Quit</attribute>"
      "          <attribute name='action'>example.quit</attribute>"
      "          <attribute name='accel'>&lt;Primary&gt;q</attribute>"
      "        </item>"
      "      </section>"
      "    </submenu>"
      "    <submenu>"
      "      <attribute name='label' translatable='yes'>_Edit</attribute>"
      "      <item>"
      "        <attribute name='label' translatable='yes'>_Copy</attribute>"
      "        <attribute name='action'>example.copy</attribute>"
      "        <attribute name='accel'>&lt;Primary&gt;c</attribute>"
      "      </item>"
      "      <item>"
      "        <attribute name='label' translatable='yes'>_Paste</attribute>"
      "        <attribute name='action'>example.paste</attribute>"
      "        <attribute name='accel'>&lt;Primary&gt;v</attribute>"
      "      </item>"
      "    </submenu>"
      "  </menu>"
      "</interface>";

    m_refBuilder->add_from_string(ui_info);
    m_refBuilder->add_from_resource("/toolbar/toolbar.glade");

This is where we specify the names of the menu items as they will be seen by users in the menu. Therefore, this is where you should make strings translatable, by adding `translatable='yes'`.

To instantiate a Gtk::MenuBar and Gtk::Toolbar which you can actually show, you should use the Builder::get_object() and Builder::get_widget() methods, and then add the widgets to a container. For instance:

    auto gmenu = m_refBuilder->get_object<Gio::Menu>("menubar");
    auto pMenuBar = Gtk::make_managed<Gtk::MenuBar>(gmenu);
    m_Box.append(*pMenuBar);

    auto toolbar = m_refBuilder->get_widget<Gtk::Toolbar>("toolbar");
    m_Box.append(*toolbar);

### Popup Menus

Menus are normally just added to a window, but they can also be displayed temporarily as the result of a mouse button click. For instance, a context menu might be displayed when the user clicks their right mouse button.

For instance:

    Glib::ustring ui_info =
      "<interface>"
      "  <menu id='menu-examplepopup'>"
      "    <section>"
      "      <item>"
      "        <attribute name='label' translatable='yes'>Edit</attribute>"
      "        <attribute name='action'>examplepopup.edit</attribute>"
      "      </item>"
      "      <item>"
      "        <attribute name='label' translatable='yes'>Process</attribute>"
      "        <attribute name='action'>examplepopup.process</attribute>"
      "      </item>"
      "      <item>"
      "        <attribute name='label' translatable='yes'>Remove</attribute>"
      "        <attribute name='action'>examplepopup.remove</attribute>"
      "      </item>"
      "    </section>"
      "  </menu>"
      "</interface>";

    m_refBuilder->add_from_string(ui_info);

    auto gmenu = m_refBuilder->get_object<Gio::Menu>("menu-examplepopup");
    m_pMenuPopup = std::make_unique<Gtk::Menu>(gmenu);

To show the popup menu, use Gtk::Menu's popup() method, providing the button identifier and the time of activation, as provided by the `button_press_event` signal, which you will need to handle anyway. For instance:

    bool ExampleWindow::on_button_press_event(GdkEventButton* event)
    {
      if( (event->type == GDK_BUTTON_PRESS) && (event->button == 3) )
      {
        if(!m_pMenuPopup->get_attach_widget())
          m_pMenuPopup->attach_to_widget(*this);

        m_pMenuPopup->popup(event->button, event->time);
        return true; //It has been handled.
      }
      else
        return false;
    }

### Gio::Resource and glib-compile-resources

Applications and libraries often contain binary or textual data that is really part of the application, rather than user data. For instance Gtk::Builder `.glade` files, splashscreen images, Gio::Menu markup xml, CSS files, icons, etc. These are often shipped as files in `$datadir/appname`, or manually included as literal strings in the code.

The Gio::Resource API and the glib-compile-resources program provide a convenient and efficient alternative to this, which has some nice properties. You maintain the files as normal files, so it's easy to edit them, but during the build the files are combined into a binary bundle that is linked into the executable. This means that loading the resource files is efficient (as they are already in memory, shared with other instances) and simple (no need to check for things like I/O errors or locate the files in the filesystem). It also makes it easier to create relocatable applications.

Resource bundles are created by the glib-compile-resources program which takes an xml file that describes the bundle, and a set of files that the xml references. These are combined into a binary resource bundle.

Gio::Resource Reference

An example:

    <?xml version="1.0" encoding="UTF-8"?>
    <gresources>
      <gresource prefix="/toolbar">
        <file preprocess="xml-stripblanks">toolbar.glade</file>
        <file>rain.png</file>
      </gresource>
    </gresources>

This will create a resource bundle with the files

- `/toolbar/toolbar.glade`

- `/toolbar/rain.png`

You can then use glib-compile-resources to compile the xml to a binary bundle that you can load with Gio::Resource::create_from_file(). However, it's more common to use the --generate-source argument to create a C source file to link directly into your application. E.g.

    $ glib-compile-resources --target=resources.c --generate-source toolbar.gresource.xml

Once a Gio::Resource has been created and registered all the data in it can be accessed globally in the process by using API calls like Gio::Resource::open_stream_from_global_resources() to stream the data or Gio::Resource::lookup_data_in_global_resources() to get a direct pointer to the data. You can also use URIs like resource:///toolbar/rain.png with Gio::File to access the resource data.

Often you don't need a Gio::Resource instance, because resource data can be loaded with methods such as Gdk::Pixbuf::create_from_resource(), Gtk::Builder::add_from_resource() and Gtk::Image::set_from_resource().

### Examples

#### Application Menu and Main Menu example

This program contains an application menu, a menubar and a toolbar. Classes are derived from Gtk::Application and Gtk::ApplicationWindow.

App and Main Menu

Source Code

#### Main Menu example

This program contains a menubar and a toolbar. A class is derived from Gtk::Window.

Main Menu

Source Code

#### Popup Menu example

Popup Menu

Source Code

## Adjustments

>kmm; has various widgets that can be visually adjusted using the mouse or the keyboard, such as the Range widgets (described in the Range Widgets section). There are also a few widgets that display some adjustable part of a larger area, such as the Viewport widget. These widgets have Gtk::Adjustment objects that express this common part of their API.

So that applications can react to changes, for instance when a user moves a scrollbar, Gtk::Adjustment has a `value_changed` signal. You can then use the get_value() method to discover the new value.

### Creating an Adjustment

The Gtk::Adjustment is created by its create() method which is as follows:

    Glib::RefPtr<Gtk::Adjustment> Gtk::Adjustment::create(
      double value,
      double lower,
      double upper,
      double step_increment = 1,
      double page_increment = 10,
      double page_size = 0);

The value argument is the initial value of the adjustment, usually corresponding to the topmost or leftmost position of an adjustable widget. The lower and upper arguments specify the possible range of values which the adjustment can hold. The step_increment argument specifies the smaller of the two increments by which the user can change the value, while the page_increment is the larger one. The page_size argument usually corresponds somehow to the visible area of a panning widget. The upper argument is used to represent the bottommost or rightmost coordinate in a panning widget's child.

### Using Adjustments the Easy Way

The adjustable widgets can be roughly divided into those which use and require specific units for these values, and those which treat them as arbitrary numbers.

The group which treats the values as arbitrary numbers includes the Range widgets (Scrollbar and Scale), the ScaleButton widget, and the SpinButton widget. These widgets are typically "adjusted" directly by the user with the mouse or keyboard. They will treat the lower and upper values of an adjustment as a range within which the user can manipulate the adjustment's value. By default, they will only modify the value of an adjustment.

The other group includes the Viewport widget and the ScrolledWindow widget. All of these widgets use pixel values for their adjustments. These are also typically adjusted indirectly using scrollbars. While all widgets which use adjustments can either create their own adjustments or use ones you supply, you'll generally want to let this particular category of widgets create its own adjustments.

If you share an adjustment object between a Scrollbar and a TextView widget, manipulating the scrollbar will automagically adjust the TextView widget. You can set it up like this:

    // creates its own adjustments
    Gtk::TextView textview;
    // uses the newly-created adjustment for the scrollbar as well
    Gtk::Scrollbar vscrollbar (textview.get_vadjustment(), Gtk::ORIENTATION_VERTICAL);

### Adjustment Internals

OK, you say, that's nice, but what if I want to create my own handlers to respond when the user adjusts a Range widget or a SpinButton. To access the value of a Gtk::Adjustment, you can use the get_value() and set_value() methods:

As mentioned earlier, Gtk::Adjustment can emit signals. This is, of course, how updates happen automatically when you share an Adjustment object between a Scrollbar and another adjustable widget; all adjustable widgets connect signal handlers to their adjustment's `value_changed` signal, as can your program.

So, for example, if you have a Scale widget, and you want to change the rotation of a picture whenever its value changes, you would create a signal handler like this:

    void cb_rotate_picture (MyPicture* picture)
    {
      picture->set_rotation(adj->get_value());
    ...

and connect it to the scale widget's adjustment like this:

    adj->signal_value_changed().connect(sigc::bind<MyPicture*>(sigc::mem_fun(*this,
        &cb_rotate_picture), picture));

What if a widget reconfigures the upper or lower fields of its Adjustment, such as when a user adds more text to a text widget? In this case, it emits the `changed` signal.

Range widgets typically connect a handler to this signal, which changes their appearance to reflect the change - for example, the size of the slider in a scrollbar will grow or shrink in inverse proportion to the difference between the lower and upper values of its Adjustment.

You probably won't ever need to attach a handler to this signal, unless you're writing a new type of range widget.

    adjustment->signal_changed();

## Dialogs

Dialogs are used as secondary windows, to provide specific information or to ask questions. Gtk::Dialog windows contain a few pre-packed widgets to ensure consistency, and a run() method which blocks until the user dismisses the dialog.

There are several derived Dialog classes which you might find useful. Gtk::MessageDialog is used for most simple notifications. But at other times you might need to derive your own dialog class to provide more complex functionality.

To pack widgets into a custom dialog, you should pack them into the Gtk::Box, available via get_content_area(). To just add a Button to the bottom of the Dialog, you could use the add_button() method.

The run() method returns an `int`. This may be a value from the `Gtk::ResponseType` if the user closed the dialog by clicking a standard button, or it could be the custom response value that you specified when using add_button().

Reference

### MessageDialog

MessageDialog is a convenience class, used to create simple, standard message dialogs, with a message, an icon, and buttons for user response. You can specify the type of message and the text in the constructor, as well as specifying standard buttons via the `Gtk::ButtonsType` enum.

Reference

#### Example

MessageDialog

Source Code

### FileChooserDialog

The FileChooserDialog is suitable for use with "Open" or "Save" menu items.

Most of the useful member methods for this class are actually in the Gtk::FileChooser base class.

Reference

#### Example

FileChooser

Source Code

### ColorChooserDialog

The ColorChooserDialog allows the user to choose a color. The ColorButton opens a color selection dialog when it is clicked.

Reference

#### Example

ColorChooserDialog

Source Code

### FontChooserDialog

The FontChooserDialog allows the user to choose a font. The FontButton opens a font chooser dialog when it is clicked.

Reference

#### Example

FontChooserDialog

Source Code

### Non-modal AboutDialog

The AboutDialog offers a simple way to display information about a program, like its logo, name, copyright, website and license.

Most dialogs in this chapter are modal, that is, they freeze the rest of the application while they are shown. It's also possible to create a non-modal dialog, which does not freeze other windows in the application. The following example shows a non-modal AboutDialog. This is perhaps not the kind of dialog you would normally make non-modal, but non-modal dialogs can be useful in other cases. E.g. gedit's search-and-replace dialog is non-modal.

Reference

#### Example

AboutDialog

Source Code

## The DrawingArea Widget

The DrawingArea widget is a blank window that gives you the freedom to create any graphic you desire. Along with that freedom comes the responsibility to draw on the widget. When a widget is first shown, or when it is covered and then uncovered again it needs to redraw itself. Most widgets have code to do this, but the DrawingArea does not, allowing you to write your own draw function to determine how the contents of the widget will be drawn. This is done by setting a draw function with a call to the set_draw_func() member function.

GTK uses the Cairo drawing API. With >kmm;, you may use the cairomm C++ API for cairo.

You can draw very sophisticated shapes using Cairo, but the methods to do so are quite basic. Cairo provides methods for drawing straight lines, curved lines, and arcs (including circles). These basic shapes can be combined to create more complex shapes and paths which can be filled with solid colors, gradients, patterns, and other things. In addition, Cairo can perform complex transformations, do compositing of images, and render antialiased text.

Cairo and Pango

Although Cairo can render text, it's not meant to be a replacement for Pango. Pango is a better choice if you need to perform more advanced text rendering such as wrapping or ellipsizing text. Drawing text with Cairo should only be done if the text is part of a graphic.

In this section of the tutorial, we'll cover the basic Cairo drawing model, describe each of the basic drawing elements in some detail (with examples), and then present a simple application that uses Cairo to draw a custom clock widget.

### The Cairo Drawing Model

The basic concept of drawing in Cairo involves defining 'invisible' paths and then stroking or filling them to make them visible.

To do any drawing in >kmm; with Cairo, you must first get a Cairo::Context object. This class holds all of the graphics state parameters that describe how drawing is to be done. This includes information such as line width, color, the surface to draw to, and many other things. This allows the actual drawing functions to take fewer arguments to simplify the interface. Usually, you use the Cairo::Context that you get as input data to the draw function that you set with the call to set_draw_func(). It's also possible to create a Cairo::Context by calling the Gdk::Surface::create_cairo_context() and Gdk::CairoContext::cairo_create() functions. Since Cairo contexts are reference-counted objects, cairo_create() returns a Cairo::RefPtr<Cairo::Context> object. (Note the difference between Gdk::CairoContext and Cairo::Context.)

The following example shows how to set up a Cairo context with a foreground color of red and a width of 2. Any drawing functions that use this context will use these settings.

    Gtk::DrawingArea myArea;
    auto gdkCairoContext = myArea.get_surface()->create_cairo_context();
    auto myContext = gdkCairoContext->cairo_create();
    myContext->set_source_rgb(1.0, 0.0, 0.0);
    myContext->set_line_width(2.0);

Each Cairo::Context is associated with a particular Gdk::Surface, so the first line of the above example creates a Gtk::DrawingArea widget and the next two lines use its associated Gdk::Surface to create a Cairo::Context object. The final two lines change the graphics state of the context.

There are a number of graphics state variables that can be set for a Cairo context. The most common context attributes are color (using set_source_rgb() or set_source_rgba() for translucent colors), line width (using set_line_width()), line dash pattern (using set_dash()), line cap style (using set_line_cap()), and line join style (using set_line_join()), and font styles (using set_font_size(), set_font_face() and others). There are many other settings as well, such as transformation matrices, fill rules, whether to perform antialiasing, and others. For further information, see the cairomm API documentation.

The current state of a Cairo::Context can be saved to an internal stack of saved states and later be restored to the state it was in when you saved it. To do this, use the save() method and the restore() method. This can be useful if you need to temporarily change the line width and color (or any other graphics setting) in order to draw something and then return to the previous settings. In this situation, you could call Cairo::Context::save(), change the graphics settings, draw the lines, and then call Cairo::Context::restore() to restore the original graphics state. Multiple calls to save() and restore() can be nested; each call to restore() restores the state from the matching paired save().

It is good practice to put all modifications to the graphics state between save()/restore() function calls. For example, if you have a function that takes a Cairo::Context reference as an argument, you might implement it as follows:

    void doSomething(const Cairo::RefPtr<Cairo::Context>& context, int x)
    {
        context->save();
        // change graphics state
        // perform drawing operations
        context->restore();
    }

The draw function that you set with a call to set_draw_func() is called with a Cairo context that you shall use for drawing in the Gtk::DrawingArea widget. It is not necessary to save and restore this Cairo context in the draw function.

### Drawing Straight Lines

Now that we understand the basics of the Cairo graphics library, we're almost ready to start drawing. We'll start with the simplest of drawing elements: the straight line. But first you need to know a little bit about Cairo's coordinate system. The origin of the Cairo coordinate system is located in the upper-left corner of the window with positive x values to the right and positive y values going down.

Since the Cairo graphics library was written with support for multiple output targets (the X window system, PNG images, OpenGL, etc), there is a distinction between user-space and device-space coordinates. The mapping between these two coordinate systems defaults to one-to-one so that integer values map roughly to pixels on the screen, but this setting can be adjusted if desired. Sometimes it may be useful to scale the coordinates so that the full width and height of a window both range from 0 to 1 (the 'unit square') or some other mapping that works for your application. This can be done with the Cairo::Context::scale() function.

#### Example

In this example, we'll construct a small but fully functional >kmm; program and draw some lines into the window. The lines are drawn by creating a path and then stroking it. A path is created using the functions Cairo::Context::move_to() and Cairo::Context::line_to(). The function move_to() is similar to the act of lifting your pen off of the paper and placing it somewhere else -- no line is drawn between the point you were at and the point you moved to. To draw a line between two points, use the line_to() function.

After you've finished creating your path, you still haven't drawn anything visible yet. To make the path visible, you must use the function stroke() which will stroke the current path with the line width and style specified in your Cairo::Context object. After stroking, the current path will be cleared so that you can start on your next path.

Many Cairo drawing functions have a _preserve() variant. Normally drawing functions such as clip(), fill(), or stroke() will clear the current path. If you use the _preserve() variant, the current path will be retained so that you can use the same path with the next drawing function.

Drawing Area - Lines

Source Code

This program contains a single class, MyArea, which is a subclass of Gtk::DrawingArea and contains an on_draw() member function. This function becomes the draw function by a call to set_draw_func() in MyArea's constructor. on_draw() is then called whenever the image in the drawing area needs to be redrawn. It is passed a Cairo::RefPtr pointer to a Cairo::Context that we use for the drawing. The actual drawing code sets the color we want to use for drawing by using set_source_rgb() which takes arguments defining the Red, Green, and Blue components of the desired color (valid values are between 0 and 1). After setting the color, we created a new path using the functions move_to() and line_to(), and then stroked this path with stroke().

Drawing with relative coordinates

In the example above we drew everything using absolute coordinates. You can also draw using relative coordinates. For a straight line, this is done with the function Cairo::Context::rel_line_to().

#### Line styles

In addition to drawing basic straight lines, there are a number of things that you can customize about a line. You've already seen examples of setting a line's color and width, but there are others as well.

If you've drawn a series of lines that form a path, you may want them to join together in a certain way. Cairo offers three different ways to join lines together: Miter, Bevel, and Round. These are show below:

Different join types in Cairo

The line join style is set using the function Cairo::Context::set_line_join().

Line ends can have different styles as well. The default style is for the line to start and stop exactly at the destination points of the line. This is called a Butt cap. The other options are Round (uses a round ending, with the center of the circle at the end point) or Square (uses a squared ending, with the center of the square at the end point). This setting is set using the function Cairo::Context::set_line_cap().

There are other things you can customize as well, including creating dashed lines and other things. For more information, see the Cairo API documentation.

#### Drawing thin lines

If you try to draw one pixel wide lines, you may notice that the line sometimes comes up blurred and wider than it ought to be. This happens because Cairo will try to draw from the selected position, to both sides (half to each), so if you're positioned right on the intersection of the pixels, and want a one pixel wide line, Cairo will try to use half of each adjacent pixel, which isn't possible (a pixel is the smallest unit possible). This happens when the width of the line is an odd number of pixels (not just one pixel).

The trick is to position in the middle of the pixel where you want the line to be drawn, and thus guaranteeing you get the desired results. See Cairo FAQ.

Drawing Area - Thin Lines

Source Code

### Drawing Curved Lines

In addition to drawing straight lines Cairo allows you to easily draw curved lines (technically a cubic Bézier spline) using the Cairo::Context::curve_to() and Cairo::Context::rel_curve_to() functions. These functions take coordinates for a destination point as well as coordinates for two 'control' points. This is best explained using an example, so let's dive in.

#### Example

This simple application draws a curve with Cairo and displays the control points for each end of the curve.

Drawing Area - Lines

Source Code

The only difference between this example and the straight line example is in the on_draw() function, but there are a few new concepts and functions introduced here, so let's examine them briefly.

We make a call to Cairo::Context::scale(), passing in the width and height of the drawing area. This scales the user-space coordinate system such that the width and height of the widget are both equal to 1.0 'units'. There's no particular reason to scale the coordinate system in this case, but sometimes it can make drawing operations easier.

The call to Cairo::Context::curve_to() should be fairly self-explanatory. The first pair of coordinates define the control point for the beginning of the curve. The second set of coordinates define the control point for the end of the curve, and the last set of coordinates define the destination point. To make the concept of control points a bit easier to visualize, a line has been drawn from each control point to the end-point on the curve that it is associated with. Note that these control point lines are both translucent. This is achieved with a variant of set_source_rgb() called set_source_rgba(). This function takes a fourth argument specifying the alpha value of the color (valid values are between 0 and 1).

### Drawing Arcs and Circles

With Cairo, the same function is used to draw arcs, circles, or ellipses: Cairo::Context::arc(). This function takes five arguments. The first two are the coordinates of the center point of the arc, the third argument is the radius of the arc, and the final two arguments define the start and end angle of the arc. All angles are defined in radians, so drawing a circle is the same as drawing an arc from 0 to 2 * M_PI radians. An angle of 0 is in the direction of the positive X axis (in user-space). An angle of M_PI/2 radians (90 degrees) is in the direction of the positive Y axis (in user-space). Angles increase in the direction from the positive X axis toward the positive Y axis. So with the default transformation matrix, angles increase in a clockwise direction. (Remember that the positive Y axis points downwards.)

To draw an ellipse, you can scale the current transformation matrix by different amounts in the X and Y directions. For example, to draw an ellipse with center at `x`, `y` and size `width`, `height`:

    context->save();
    context->translate(x, y);
    context->scale(width / 2.0, height / 2.0);
    context->arc(0.0, 0.0, 1.0, 0.0, 2 * M_PI);
    context->restore();

#### Example

Here's an example of a simple program that draws an arc, a circle and an ellipse into a drawing area.

Drawing Area - Arcs

Source Code

There are a couple of things to note about this example code. Again, the only real difference between this example and the previous ones is the on_draw() function, so we'll limit our focus to that function. In addition, the first part of the function is nearly identical to the previous examples, so we'll skip that portion.

Note that in this case, we've expressed nearly everything in terms of the height and width of the window, including the width of the lines. Because of this, when you resize the window, everything scales with the window. Also note that there are three drawing sections in the function and each is wrapped with a save()/restore() pair so that we're back at a known state after each drawing.

The section for drawing an arc introduces one new function, close_path(). This function will in effect draw a straight line from the current point back to the first point in the path. There is a significant difference between calling close_path() and manually drawing a line back to the starting point, however. If you use close_path(), the lines will be nicely joined together. If you use line_to() instead, the lines will end at the same point, but Cairo won't do any special joining.

Drawing counter-clockwise

The function Cairo::Context::arc_negative() is exactly the same as Cairo::Context::arc() but the angles go the opposite direction.

### Drawing Text

#### Drawing Text with Pango

Text is drawn via Pango Layouts. The easiest way to create a Pango::Layout is to use Gtk::Widget::create_pango_layout(). Once created, the layout can be manipulated in various ways, including changing the text, font, etc. Finally, the layout can be rendered using the Pango::Layout::show_in_cairo_context() method.

#### Example

Here is an example of a program that draws some text, some of it upside-down. The Printing chapter contains another example of drawing text.

Drawing Area - Text

Source Code

### Drawing Images

There is a method for drawing from a Gdk::Pixbuf to a Cairo::Context. A Gdk::Pixbuf buffer is a useful wrapper around a collection of pixels, which can be read from files, and manipulated in various ways.

Probably the most common way of creating Gdk::Pixbufs is to use Gdk::Pixbuf::create_from_file() or Gdk::Pixbuf::create_from_resource(), which can read an image file, such as a png file into a pixbuf ready for rendering.

The Gdk::Pixbuf can be rendered by setting it as the source pattern of the Cairo context with Gdk::Cairo::set_source_pixbuf(). Then draw the image with either Cairo::Context::paint() (to draw the whole image), or Cairo::Context::rectangle() and Cairo::Context::fill() (to fill the specified rectangle). set_source_pixbuf() is not a member of Cairo::Context. It takes a Cairo::Context as its first parameter.

Here is a small bit of code to tie it all together: (Note that usually you wouldn't load the image every time in the draw signal handler! It's just shown here to keep it all together.)

    void MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr, int width, int height)
    {
      auto image = Gdk::Pixbuf::create_from_file("myimage.png");
      // Draw the image at 110, 90, except for the outermost 10 pixels.
      Gdk::Cairo::set_source_pixbuf(cr, image, 100, 80);
      cr->rectangle(110, 90, image->get_width()-20, image->get_height()-20);
      cr->fill();
    }

#### Example

Here is an example of a simple program that draws an image. The program loads the image from a resource file. See the Gio::Resource and glib-compile-resources section. Use glib-compile-resources to compile the resources into a C source file that can be compiled and linked with the C++ code. E.g.

    $ glib-compile-resources --target=resources.c --generate-source image.gresource.xml

Drawing Area - Image

Source Code

### Example Application: Creating a Clock with Cairo

Now that we've covered the basics of drawing with Cairo, let's try to put it all together and create a simple application that actually does something. The following example uses Cairo to create a custom Clock widget. The clock has a second hand, a minute hand, and an hour hand, and updates itself every second.

Source Code

As before, almost all of the interesting stuff is done in the draw function on_draw(). Before we dig into the draw function, notice that the constructor for the Clock widget connects a handler function on_timeout() to a timer with a timeout period of 1000 milliseconds (1 second). This means that on_timeout() will get called once per second. The sole responsibility of this function is to invalidate the window so that >kmm; will be forced to redraw it.

Now let's take a look at the code that performs the actual drawing. The first section of on_draw() should be pretty familiar by now. This example again scales the coordinate system to be a unit square so that it's easier to draw the clock as a percentage of window size so that it will automatically scale when the window size is adjusted. Furthermore, the coordinate system is scaled over and down so that the (0, 0) coordinate is in the very center of the window.

The function Cairo::Context::paint() is used here to set the background color of the window. This function takes no arguments and fills the current surface (or the clipped portion of the surface) with the source color currently active. After setting the background color of the window, we draw a circle for the clock outline, fill it with white, and then stroke the outline in black. Notice that both of these actions use the _preserve variant to preserve the current path, and then this same path is clipped to make sure that our next lines don't go outside the outline of the clock.

After drawing the outline, we go around the clock and draw ticks for every hour, with a larger tick at 12, 3, 6, and 9. Now we're finally ready to implement the time-keeping functionality of the clock, which simply involves getting the current values for hours, minutes and seconds, and drawing the hands at the correct angles.

## Drag and Drop

Gtk::Widget has several methods and signals which are prefixed with "drag_". These are used for Drag and Drop.

### Sources and Destinations

Things are dragged from `sources` to be dropped on `destinations`. Each source and destination has information about the data formats that it can send or receive, provided by Gdk::ContentFormats. A drop destination will only accept a dragged item if they both share a compatible format. Appropriate signals will then be emitted, telling the signal handlers which format was used.

Gdk::ContentFormats objects contain information about available GTypes and mime types (media types).

### Methods

Widgets can be identified as sources or destinations using these Gtk::Widget methods:

    void drag_source_set(const Glib::RefPtr<Gdk::ContentFormats>& targets,
          Gdk::ModifierType start_button_mask, Gdk::DragAction actions);

- `targets` is a Gdk::ContentFormats object.

- `start_button_mask` is an ORed combination of values, which specify which modifier key or mouse button must be pressed to start the drag.

- `actions` is an ORed combination of values, which specify which Drag and Drop operations will be possible from this source - for instance, copy, move, or link. The user can choose between the actions by using modifier keys, such as Shift to change from `copy` to `move`, and this will be shown by a different cursor.

    void drag_dest_set(const Glib::RefPtr<Gdk::ContentFormats>& targets,
        Gtk::DestDefaults flags, Gdk::DragAction actions);

- `flags` is an ORed combination of values which indicates how the widget will respond visually to Drag and Drop items.

- `actions` indicates the Drag and Drop actions which this destination can receive - see the description above.

There are several methods to add source formats and destination formats. Examples:

- drag_source_add_text_targets()

- drag_source_add_image_targets()

- drag_dest_add_text_targets()

- drag_dest_add_image_targets()

### Signals

When a drop destination has accepted a dragged item, certain signals will be emitted, depending on what action has been selected. For instance, the user might have held down the Shift key to specify a `move` rather than a `copy`. Remember that the user can only select the actions which you have specified in your calls to drag_dest_set() and drag_source_set().

#### Copy

The source widget will emit these signals, in this order:

- `drag_begin`: Provides a Gdk::Drag.

- `drag_data_get`: Provides a Gdk::Drag, and a Gtk::SelectionData object, in which you should put the requested data.

- `drag_end`: Provides a Gdk::Drag.

The destination widget will emit these signals, in this order:

- `drag_motion`: Provides a Gdk::Drop and coordinates. You can call the status() method of the Gdk::Drop to indicate which action will be accepted.

- `drag_drop`: Provides a Gdk::Drop and coordinates. You can call drag_get_data(), which triggers the `drag_data_get` signal in the source widget, and then the `drag_data_received` signal in the destination widget.

- `drag_data_received`: Provides a Gdk::Drop, and a Gtk::SelectionData object which contains the dropped data. You should call the finish() or failed() method of the Gdk::Drop to indicate whether the operation was successful.

#### Move

During a `move`, the source widget will also emit this signal:

- `drag_data_delete`: Gives the source the opportunity to delete the original data if that's appropriate.

### Example

Here is a very simple example, demonstrating a drag and drop `Copy` operation:

Drag and Drop

Source Code

There is a more complex example in examples/others/dnd.

## The Clipboard

Simple text copy-paste functionality is provided for free by widgets such as Gtk::Entry and Gtk::TextView, but you might need special code to deal with your own data formats. For instance, a drawing program would need special code to allow copy and paste within a view, or between documents.

You can get a clipboard instance with Gtk::Widget::get_clipboard() or Gdk::Display::get_clipboard().

Your application doesn't need to wait for clipboard operations, particularly between the time when the user chooses Copy and then later chooses Paste. Many Gdk::Clipboard methods take sigc::slots which specify callback methods. When Gdk::Clipboard is ready, it will call these methods, providing the requested data.

Reference

### Formats

Different applications contain different types of data, and they might make that data available in a variety of formats. >kmm; calls these data types `format`s.

For instance, gedit can supply and receive the `text/plain` mime type, so you can paste data into gedit from any application that supplies that format. Or two different image editing applications might supply and receive a variety of image formats. As long as one application can receive one of the formats that the other supplies then you will be able to copy data from one to the other.

Clipboard data can be in a variety of binary formats. This chapter, and the examples, assume that the data is 8-bit text. This would allow us to use an XML format for the clipboard data. However this would probably not be appropriate for binary data such as images.

The Drag and Drop API uses the same mechanism. You should probably use the same data formats for both Clipboard and Drag and Drop operations.

### Copy

When the user asks to copy some data, you should copy the data to the Clipboard. For instance,

    void ExampleWindow::on_button_copy()
    {
      get_clipboard()->set_text("example_custom_target");
    }

### Paste

When the user asks to paste data from the Clipboard, you should request a specific format and provide a callback method which will be called with the actual data. For instance:

    void ExampleWindow::on_button_paste()
    {
      get_clipboard()->read_text_async(sigc::mem_fun(*this,
                  &ExampleWindow::on_clipboard_received));
    }

Here is an example callback method:

    void ExampleWindow::on_clipboard_received(Glib::RefPtr<Gio::AsyncResult>& result)
    {
      auto text = get_clipboard()->read_text_finish(result);
      //Do something with the pasted data.
    }

#### Discovering the available formats

To find out what formats are currently available on the Clipboard for pasting, call the get_formats() method. Then call a Gdk::ContentFormats method to find out if a format that your application supports is available.

### Examples

#### Simple

This example allows copy and pasting of application-specific data, using the standard text format. Although this is simple, it's not ideal because it does not identify the Clipboard data as being of a particular type.

Clipboard - Simple

Source Code

#### Ideal

This is like the simple example, but it

1. Defines a custom clipboard target, though the format is still text.

2. It uses the Gdk::ContentFormats::signal_changed() signal and disables the Paste button if it can't use anything on the clipboard.

Clipboard - Ideal

Source Code

## Printing

At the application development level, >kmm;'s printing API provides dialogs that are consistent across applications and allows use of Cairo's common drawing API, with Pango-driven text rendering. In the implementation of this common API, platform-specific backends and printer-specific drivers are used.

### PrintOperation

The primary object is Gtk::PrintOperation, allocated for each print operation. To handle page drawing connect to its signals, or inherit from it and override the default virtual signal handlers. PrintOperation automatically handles all the settings affecting the print loop.

#### Signals

The PrintOperation::run() method starts the print loop, during which various signals are emitted:

- `begin_print`: You must handle this signal, because this is where you create and set up a Pango::Layout using the provided Gtk::PrintContext, and break up your printing output into pages.

- `paginate`: Pagination is potentially slow so if you need to monitor it you can call the PrintOperation::set_show_progress() method and handle this signal.

- For each page that needs to be rendered, the following signals are emitted:

  - `request_page_setup`: Provides a PrintContext, page number and Gtk::PageSetup. Handle this signal if you need to modify page setup on a per-page basis.

  - `draw_page`: You must handle this signal, which provides a PrintContext and a page number. The PrintContext should be used to create a Cairo::Context into which the provided page should be drawn. To render text, iterate over the Pango::Layout you created in the `begin_print` handler.

- `end_print`: A handler for it is a safe place to free any resources related to a PrintOperation. If you have your custom class that inherits from PrintOperation, it is naturally simpler to do it in the destructor.

- `done`: This signal is emitted when printing is finished, meaning when the print data is spooled. Note that the provided `Gtk::PrintOperationResult` may indicate that an error occurred. In any case you probably want to notify the user about the final status.

- `status_changed`: Emitted whenever a print job's status changes, until it is finished. Call the PrintOperation::set_track_print_status() method to monitor the job status after spooling. To see the status, use get_status() or get_status_string().

Reference

### Page setup

The PrintOperation class has a method called set_default_page_setup() which selects the default paper size, orientation and margins. To show a page setup dialog from your application, use the Gtk::run_page_setup_dialog() method, which returns a Gtk::PageSetup object with the chosen settings. Use this object to update a PrintOperation and to access the selected Gtk::PaperSize, `Gtk::PageOrientation` and printer-specific margins.

You should save the chosen Gtk::PageSetup so you can use it again if the page setup dialog is shown again.

For instance,

    // Within a class that inherits from Gtk::Window and keeps m_refPageSetup
    // and m_refSettings as members...
    auto new_page_setup = Gtk::run_page_setup_dialog(*this, m_refPageSetup, m_refSettings);
    m_refPageSetup = new_page_setup;

Reference

The Cairo coordinate system, in the `draw_page` handler, is automatically rotated to the current page orientation. It is normally within the printer margins, but you can change that via the PrintOperation::set_use_full_page() method. The default measurement unit is device pixels. To select other units, use the PrintOperation::set_unit() method.

### Rendering text

Text rendering is done using Pango. The Pango::Layout object for printing should be created by calling the PrintContext::create_pango_layout() method. The PrintContext object also provides the page metrics, via get_width() and get_height(). The number of pages can be set with PrintOperation::set_n_pages(). To actually render the Pango text in `on_draw_page`, get a Cairo::Context with PrintContext::get_cairo_context() and show the Pango::LayoutLines that appear within the requested page number.

See an example of exactly how this can be done.

### Asynchronous operations

By default, PrintOperation::run() returns when a print operation is completed. If you need to run a non-blocking print operation, call PrintOperation::set_allow_async(). Note that set_allow_async() is not supported on all platforms, however the `done` signal will still be emitted.

run() may return `PrintOperation::Result::IN_PROGRESS`. To track status and handle the result or error you need to implement signal handlers for the `done` and `status_changed` signals:

For instance,

    // in class ExampleWindow's method...
    auto op = PrintOperation::create();
    // ...set up op...
    op->signal_done().connect(sigc::bind(sigc::mem_fun(
      *this, &ExampleWindow::on_printoperation_done), op));
    // run the op

Second, check for an error and connect to the `status_changed` signal. For instance:

    void ExampleWindow::on_printoperation_done(Gtk::PrintOperationResult result,
      const Glib::RefPtr<PrintOperation>& op)
    {
      if (result == Gtk::PrintOperation::Result::ERROR)
        //notify user
      else if (result == Gtk::PrintOperation::Result::APPLY)
        //Update PrintSettings with the ones used in this PrintOperation

      if (! op->is_finished())
        op->signal_status_changed().connect(sigc::bind(sigc::mem_fun(
          *this, &ExampleWindow::on_printoperation_status_changed), op));
    }

Finally, check the status. For instance,

    void ExampleWindow::on_printoperation_status_changed(const Glib::RefPtr<PrintOperation>& op)
    {
      if (op->is_finished())
        //the print job is finished
      else
        //get the status with get_status() or get_status_string()

      //update UI
    }

### Export to PDF

The 'Print to file' option is available in the print dialog, without the need for extra implementation. However, it is sometimes useful to generate a pdf file directly from code. For instance,

    auto op = Gtk::PrintOperation::create();
    // ...set up op...
    op->set_export_filename("test.pdf");
    auto res = op->run(Gtk::PrintOperation::Action::EXPORT);

### Extending the print dialog

You may add a custom tab to the print dialog:

- Set the title of the tab via PrintOperation::set_custom_tab_label(), create a new widget and return it from the `create_custom_widget` signal handler. You'll probably want this to be a container widget, packed with some others.

- Get the data from the widgets in the `custom_widget_apply` signal handler.

Although the `custom_widget_apply` signal provides the widget you previously created, to simplify things you can keep the widgets you expect to contain some user input as class members. For example, let's say you have a Gtk::Entry called `m_Entry` as a member of your CustomPrintOperation class:

    Gtk::Widget* CustomPrintOperation::on_create_custom_widget()
    {
      set_custom_tab_label("My custom tab");

      auto hbox = new Gtk::Box(Gtk::Orientation::HORIZONTAL, 8);
      hbox->set_margin(6);

      auto label = Gtk::make_managed<Gtk::Label>("Enter some text: ");
      hbox->append(*label);

      hbox->append(m_Entry);

      return hbox;
    }

    void CustomPrintOperation::on_custom_widget_apply(Gtk::Widget* /* widget */)
    {
      auto user_input = m_Entry.get_text();
      //...
    }

The example in examples/book/printing/advanced demonstrates this.

### Preview

The native GTK print dialog has a preview button, but you may also start a preview directly from an application:

    // in a class that inherits from Gtk::Window...
    auto op = PrintOperation::create();
    // ...set up op...
    op->run(Gtk::PrintOperation::Action::PREVIEW, *this);

On Unix, the default preview handler uses an external viewer program. On Windows, the native preview dialog will be shown. If necessary you may override this behaviour and provide a custom preview dialog. See the example located in /examples/book/printing/advanced.

### Example

#### Simple

The following example demonstrates how to print some input from a user interface. It shows how to implement `on_begin_print` and `on_draw_page`, as well as how to track print status and update the print settings.

Printing - Simple

Source Code

## Recently Used Documents

>kmm; provides an easy way to manage recently used documents. This functionality is implemented in the Gtk::RecentManager class.

Each item in the list of recently used files is identified by its URI, and can have associated metadata. The metadata can be used to specify how the file should be displayed, a description of the file, its mime type, which application registered it, whether it's private to the registering application, and several other things.

### RecentManager

RecentManager acts as a database of recently used files. You use this class to register new files, remove files from the list, or look up recently used files. There is one list of recently used files per user.

You can create a new RecentManager, but you'll most likely just want to use the default one. You can get a reference to the default RecentManager with get_default().

#### Adding Items to the List of Recent Files

To add a new file to the list of recent documents, in the simplest case, you only need to provide the URI. For example:

    auto recent_manager = Gtk::RecentManager::get_default();
    recent_manager->add_item(uri);

If you want to register a file with metadata, you can pass a RecentManager::Data parameter to add_item(). The metadata that can be set on a particular file item is as follows:

- `app_exec`: The command line to be used to launch this resource. This string may contain the "f" and "u" escape characters which will be expanded to the resource file path and URI respectively

- `app_name`: The name of the application that registered the resource

- `description`: A short description of the resource as a UTF-8 encoded string

- `display_name`: The name of the resource to be used for display as a UTF-8 encoded string

- `groups`: A list of groups associated with this item. Groups are essentially arbitrary strings associated with a particular resource. They can be thought of as 'categories' (such as "email", "graphics", etc) or tags for the resource.

- `is_private`: Whether this resource should be visible only to applications that have registered it or not

- `mime_type`: The MIME type of the resource

In addition to adding items to the list, you can also look up items from the list and modify or remove items.

#### Looking up Items in the List of Recent Files

To look up recently used files, RecentManager provides several functions. To look up a specific item by its URI, you can use the lookup_item() function, which will return a RecentInfo class. If the specified URI did not exist in the list of recent files, lookup_item() throws a RecentManagerError exception. For example:

    Glib::RefPtr<Gtk::RecentInfo> info;
    try
    {
      info = recent_manager->lookup_item(uri);
    }
    catch(const Gtk::RecentManagerError& ex)
    {
      std::cerr << "RecentManagerError: " << ex.what() << std::endl;
    }
    if (info)
    {
      // item was found
    }

A RecentInfo object is essentially an object containing all of the metadata about a single recently-used file. You can use this object to look up any of the properties listed above.

If you don't want to look for a specific URI, but instead want to get a list of all recently used items, RecentManager provides the get_items() function. The return value of this function is a std::vector of all recently used files. The following code demonstrates how you might get a list of recently used files:

    auto info_list = recent_manager->get_items();

The maximum age of items in the recently used files list can be set with Gtk::Settings::property_gtk_recent_files_max_age(). Default value: 30 days.

#### Modifying the List of Recent Files

There may be times when you need to modify the list of recent files. For instance, if a file is moved or renamed, you may need to update the file's location in the recent files list so that it doesn't point to an incorrect location. You can update an item's location by using move_item().

In addition to changing a file's URI, you can also remove items from the list, either one at a time or by clearing them all at once. The former is accomplished with remove_item(), the latter with purge_items().

The functions move_item(), remove_item() and purge_items() have no effect on the actual files that are referred to by the URIs, they only modify the list of recent files.

### FileChooser

FileChooser is an interface that can be implemented by widgets displaying a list of files. >kmm; provides three built-in implementations for choosing recent files or other files: FileChooserWidget, FileChooserDialog, and FileChooserNative.

FileChooserWidget is a simple widget for displaying a list of recently used files or other files. FileChooserWidget is the basic building block for FileChooserDialog, but you can embed it into your user interface if you want to.

#### Simple FileChooserDialog example

Shown below is a simple example of how to use the FileChooserDialog class in a program. This simple program has a menubar with a File Chooser Dialog menu item. When you select this menu item, a dialog pops up showing a list of files. If you select Recent in the sidebar, the list of recently used files is shown.

If this is the first time you're using a program that uses the Recent Files framework, the dialog may be empty at first. Otherwise it should show the list of recently used documents registered by other applications.

After selecting the File Chooser Dialog menu item and the Recent sidebar item, you should see something similar to the following window.

Source Code

The constructor for ExampleWindow creates the menu and the toolbar using Builder (see for more information). It then adds the menu and the toolbar to the window.

#### Filtering Files

For any of the FileChooser classes, if you don't wish to display all of the items in the list of files, you can filter the list to show only those that you want. You can filter the list with the help of the FileFilter class. This class allows you to filter files by their name (add_pattern()), or their mime type (add_mime_type()).

After you've created and set up the filter to match only the items you want, you can apply a filter to a chooser widget with the FileChooser::add_filter() function.

## Keyboard Events

X events differ in some ways from other signals. These differences are described in the X Event signals section in the appendix. Here we will use keyboard events to show how X events can be used in a program.

### Overview

Whenever you press or release a key, an event is emitted. You can connect a signal handler to handle such events.

The event signal handler will receive an argument that depends on the type of event. For keyboard events it's a GdkEventKey*. As discribed in the appendix, the event signal handler returns a bool value, to indicate that the signal is fully handled (`true`) or allow event propagation (`false`).

To determine which key was pressed or released, you read the value of `GdkEventKey::keyval` and compare it with a constant in the `<gdk/gdkkeysyms.h>` header file. The states of modifier keys (shift, ctrl, etc.) are available as bit-flags in `GdkEventKey::state`.

Here's a simple example:

    bool on_key_press_or_release_event(GdkEventKey* event)
    {
      if (event->type == GDK_KEY_PRESS &&
        event->keyval == GDK_KEY_1 &&
        (event->state & (GDK_SHIFT_MASK | GDK_CONTROL_MASK | GDK_MOD1_MASK)) == GDK_MOD1_MASK)
      {
        handle_alt_1_press(); // GDK_MOD1_MASK is normally the Alt key
        return true;
      }
      return false;
    }

    Gtk::Entry m_entry; // in a class definition

    // in the class constructor
    m_entry.signal_key_press_event().connect( sigc::ptr_fun(&on_key_press_or_release_event) );
    m_entry.signal_key_release_event().connect( sigc::ptr_fun(&on_key_press_or_release_event) );

#### Example

In this example there are three keyboard shortcuts: Alt+1 selects the first radio button, Alt+2 selects the second one, and the Esc key hides (closes) the window. The default event signal handler is overridden, as described in the Overriding default signal handlers section in the appendix.

Keyboard Events - Simple

Source Code

### Event Propagation

Event propagation means that, when an event is emitted on a particular widget, it can be passed to its parent widget (and that widget can pass it to its parent, and so on) and, if the parent has an event handler, that handler will be called.

Contrary to other events, keyboard events are first sent to the toplevel window (Gtk::Window), where it will be checked for any keyboard shortcuts that may be set (accelerator keys and mnemonics, used for selecting menu items from the keyboard). After this (and assuming the event wasn't handled), it is sent to the widget which has focus, and the propagation begins from there.

The event will propagate until it reaches the top-level widget, or until you stop the propagation by returning `true` from an event handler.

Notice, that after canceling an event, no other function will be called (even if it is from the same widget).

#### Example

In this example there are three event handlers that are called after Gtk::Window's default event handler, one in the Gtk::Entry, one in the Gtk::Grid and one in the Gtk::Window.

In the Gtk::Window, we have also the default handler overridden (on_key_release_event()), and another handler being called before the default handler (windowKeyReleaseBefore()).

The purpose of this example is to show the steps the event takes when it is emitted.

When you write in the entry, a key release event will be emitted, which will go first to the toplevel window (Gtk::Window), since we have one event handler set to be called before, that's what is called first (windowKeyReleaseBefore()). Then the default handler is called (which we have overridden), and after that the event is sent to the widget that has focus, the Entry in our example and, depending on whether we let it propagate, it can reach the Grid's and the Window's event handlers. If it propagates, the text you're writing will appear in the Label above the Entry.

Keyboard Events - Event Propagation

Source Code

## Timeouts, I/O and Idle Functions

### Timeouts

You may be wondering how to make >kmm; do useful work while it's idling along. Happily, you have several options. Using the following methods you can create a timeout method that will be called every few milliseconds.

    sigc::connection Glib::SignalTimeout::connect(const sigc::slot<bool()>& slot,
                                          unsigned int interval, int priority = Glib::PRIORITY_DEFAULT);

The first argument is a slot you wish to have called when the timeout occurs. The second argument is the number of milliseconds between calls to that method. You receive a sigc::connection object that can be used to deactivate the connection using its disconnect() method:

    my_connection.disconnect();

Another way of destroying the connection is your signal handler. It has to be of the type sigc::slot<bool()>. As you see from the definition your signal handler has to return a value of the type `bool`. A definition of a sample method might look like this:

    bool MyCallback() { std::cout << "Hello World!\n" << std::endl; return true; }

You can stop the timeout method by returning `false` from your signal handler. Therefore, if you want your method to be called repeatedly, it should return `true`.

Here's an example of this technique:

Source Code

### Monitoring I/O

A nifty feature of Glib (one of the libraries underlying >kmm;) is the ability to have it check for data on a file descriptor for you. This is especially useful for networking applications. The following method is used to do this:

    sigc::connection Glib::SignalIO::connect(const sigc::slot<bool(Glib::IOCondition)>& slot,
                                     Glib::PollFD::fd_t fd, Glib::IOCondition condition,
                                     int priority = Glib::PRIORITY_DEFAULT);

The first argument is a slot you wish to have called when the specified event (see argument 3) occurs on the file descriptor you specify using argument two. Argument three may be one or more (using `|`) of:

- Glib::IO_IN - Call your method when there is data ready for reading on your file descriptor.

- Glib::IO_OUT - Call your method when the file descriptor is ready for writing.

- Glib::IO_PRI - Call your method when the file descriptor has urgent data to be read.

- Glib::IO_ERR - Call your method when an error has occurred on the file descriptor.

- Glib::IO_HUP - Call your method when hung up (the connection has been broken usually for pipes and sockets).

The return value is a sigc::connection that may be used to stop monitoring this file descriptor using its disconnect() method. The slot signal handler should be declared as follows:

    bool input_callback(Glib::IOCondition condition);

where condition is as specified above. As usual the slot is created with `sigc::mem_fun()` (for a member method of an object), or `sigc::ptr_fun()` (for a function).

A little example follows. To use the example just execute it from a terminal; it doesn't create a window. It will create a pipe named `testfifo` in the current directory. Then start another shell and execute `echo "Hello" > testfifo`. The example will print each line you enter until you execute `echo "Q" > testfifo`.

Source Code

### Idle Functions

If you want to specify a method that gets called when nothing else is happening, use the following:

    sigc::connection  Glib::SignalIdle::connect(const sigc::slot<bool()>& slot,
                                        int priority = Glib::PRIORITY_DEFAULT_IDLE);

This causes >kmm; to call the specified method whenever nothing else is happening. You can add a priority (lower numbers are higher priorities). There are two ways to remove the signal handler: calling disconnect() on the sigc::connection object, or returning `false` in the signal handler, which should be declared as follows:

    bool idleFunc();

Since this is very similar to the methods above this explanation should be sufficient to understand what's going on. However, here's a little example:

Source Code

This example points out the difference of idle and timeout methods a little. If you need methods that are called periodically, and speed is not very important, then you want timeout methods. If you want methods that are called as often as possible (like calculating a fractal in background), then use idle methods.

Try executing the example and increasing the system load. The upper progress bar will increase steadily; the lower one will slow down.

## Memory management

### Widgets

#### Normal C++ memory management

>kmm; allows the programmer to control the lifetime (that is, the construction and destruction) of any widget in the same manner as any other C++ object. This flexibility allows you to use `new` and `delete` to create and destroy objects dynamically or to use regular class members (that are destroyed automatically when the class is destroyed) or to use local instances (that are destroyed when the instance goes out of scope). This flexibility is not present in some C++ GUI toolkits, which restrict the programmer to only a subset of C++'s memory management features.

Here are some examples of normal C++ memory management:

##### Class Scope widgets

If a programmer does not need dynamic memory allocation, automatic widgets in class scope may be used. One advantage of automatic widgets in class scope is that memory management is grouped in one place. The programmer does not risk memory leaks from failing to `delete` a widget.

The primary disadvantage of using class scope widgets is revealing the class implementation rather than the class interface in the class header.

    #include <gtkmm/button.h>
    #include <gtkmm/window.h>
    class Foo : public Gtk::Window
    {
    private:
      Gtk::Button theButton;
      // will be destroyed when the Foo object is destroyed
    };

##### Function scope widgets

If a programmer does not need a class scope widget, a function scope widget may also be used. The advantages to function scope over class scope are the increased data hiding and reduced dependencies.

    {
      Gtk::Button aButton;
      aButton.show();
      ...
      app->run();
    }

##### Dynamic allocation with new and delete

Usually, the programmer will prefer to allow containers to automatically destroy their children by creating them using `Gtk::make_managed()` (see below). This is not strictly required, as the `new` and `delete` operators may also be used, but modern C++ style discourages those in favour of safer models of memory management, so it is better to create widgets using `Gtk::make_managed()` and let their parent destroy them, than to manually perform dynamic allocation.

    auto pButton = new Gtk::Button("Test");

    // do something useful with pButton

    delete pButton;

Here, the programmer deletes `pButton` to prevent a memory leak.

#### Managed Widgets

Alternatively, you can let a widget's container control when the widget is destroyed. In most cases, you want a widget to last only as long as the container it is in. To delegate the management of a widget's lifetime to its container, create it with `Gtk::make_managed()` and then pack it into its container with Gtk::Box::append() or a similar method. Now the widget will be destroyed whenever its container is destroyed.

##### Dynamic allocation with make_managed() and append()

>kmm; provides ways including the `make_managed()` function and Gtk::Box::append() method to simplify creation and destruction of widgets whose lifetime can be managed by a parent.

Every widget except a top-level window must be added to a parent container in order to be displayed. The `manage()` function marks a widget so that when that widget is added to a parent container, said container becomes responsible for deleting the widget, meaning the user no longer needs to do so. The original way to create widgets whose lifetime is managed by their parent in this way was to call `manage()`, passing in the result of a `new` expression that created a dynamically allocated widget.

However, usually, when you create such a widget, you will already know that its parent container should be responsible for destroying it, In addition, modern C++ style discourages use of the `new` operator, which was required when passing a newly created widget to `manage()`. Therefore, >kmm; has added `make_managed()`, which combines creation and marking with `manage()` into a single step. This avoids you having to write `new`, which is discouraged in modern C++ style, and more clearly expresses intent to create a managed widget.

    MyContainer::MyContainer()
    {
      auto pButton = Gtk::make_managed<Gtk::Button>("Test");
      append(*pButton); //add *pButton to MyContainer
    }

Now, when objects of type MyContainer are destroyed, the button will also be deleted. It is no longer necessary to delete `pButton` to free the button's memory; its deletion has been delegated to the MyContainer object.

Note that if you never added the widget to any parent container, or you did but later Gtk::Container::remove()d it from said parent, >kmm; restores the widget’s lifetime management to whatever state it had before `manage()` was called, which typically means that the responsibility for `delete`ing the widget returns to the user.

Of course, a top-level container will not be added to another container. The programmer is responsible for destroying the top-level container using one of the traditional C++ techniques. For instance, your top-level Window might just be an instance in your `main()` function.

### Shared resources

Some objects, such as Gdk::Pixbufs and Pango::Fonts, are obtained from a shared store. Therefore you cannot instantiate your own instances. These classes typically inherit from Glib::Object. Rather than requiring you to reference and unreference these objects, >kmm; uses the Glib::RefPtr<> smartpointer. Cairomm has its own smartpointer, Cairo::RefPtr<>.

Objects such as Gdk::Pixbuf can only be instantiated with a create() function. For instance,

    auto pixbuf = Gdk::Pixbuf::create_from_file(filename);

You have no way of getting a bare Gdk::Pixbuf. In the example, `pixbuf` is a smart pointer, so you can do this, much like a normal pointer:

    auto width = 0;
    if(pixbuf)
    {
      width = pixbuf->get_width();
    }

When `pixbuf` goes out of scope an unref() will happen in the background and you don't need to worry about it anymore. There's no `new` so there's no `delete`.

If you copy a RefPtr, for instance

    auto pixbuf2 = pixbuf;

, or if you pass it as a method argument or a return type, then RefPtr will do any necessary referencing to ensure that the instance will not be destroyed until the last RefPtr has gone out of scope.

See the appendix for detailed information about RefPtr.

If you wish to learn more about smartpointers, you might look in these books:

- Bjarne Stroustrup, "The C++ Programming Language" Forth Edition - section 34.3

- Nicolai M. Josuttis, "The C++ Standard Library" - section 4.2

## Glade and Gtk::Builder

Although you can use C++ code to instantiate and arrange widgets, this can soon become tedious and repetitive. And it requires a recompilation to show changes. The Glade application allows you to layout widgets on screen and then save an XML description of the arrangement. Your application can then use the Gtk::Builder API to load that XML file at runtime and obtain a pointer to specifically named widget instances.

This has the following advantages:

1. Less C++ code is required.

2. UI changes can be seen more quickly, so UIs are able to improve.

3. Designers without programming skills can create and edit UIs.

You still need C++ code to deal with User Interface changes triggered by user actions, but using Gtk::Builder for the widget layout allows you to focus on implementing that functionality.

### Loading the .glade file

Gtk::Builder must be used via a Glib::RefPtr. Like all such classes, you need to use a create() method to instantiate it. For instance,

    auto builder = Gtk::Builder::create_from_file("basic.glade");

This will instantiate the windows defined in the `.glade` file.

To instantiate just one window, or just one of the child widgets, you can specify the name of a widget as the second parameter. For instance,

    auto builder = Gtk::Builder::create_from_file("basic.glade", "treeview_products");

### Accessing widgets

To access a widget, for instance to show() a dialog, use the get_widget() method, providing the widget's name. This name should be specified in the Glade Properties window. If the widget could not be found, or is of the wrong type, then the pointer will be set to nullptr.

    auto pDialog = builder->get_widget<Gtk::Dialog>("DialogBasic");

Gtk::Builder checks for a null pointer, and checks that the widget is of the expected type, and will show warnings on the command line about these.

Remember that you are not instantiating a widget with get_widget(), you are just obtaining a pointer to one that already exists. You will always receive a pointer to the same instance when you call get_widget() on the same Gtk::Builder, with the same widget name. The widgets are instantiated during Gtk::Builder::create_from_file().

get_widget() returns child widgets that are `manage()`ed (see the Memory Management chapter), so they will be deleted when their parent container is deleted. Windows (such as Dialogs) cannot be managed because they have no parent container, so you must delete them at some point. The documentation of Gtk::Builder has more to say about the memory management of different kinds of objects.

Reference

#### Example

This simple example shows how to load a Glade file at runtime and access the widgets with Gtk::Builder.

Source Code

### Using derived widgets

You can use Gtk::Builder and Glade to layout your own custom widgets derived from >kmm; widget classes. This keeps your code organized and encapsulated, separating declarative presentation from business logic, avoiding having most of your source just be setting properties and packing in containers.

Use Gtk::Builder::get_widget_derived() like so:

    auto pDialog = Gtk::Builder::get_widget_derived<DerivedDialog>(builder, "DialogDerived");

Your derived class must have a constructor that takes a pointer to the underlying C type, and the Gtk::Builder instance. All relevant classes of >kmm; typedef their underlying C type as BaseObjectType (Gtk::Dialog typedefs BaseObjectType as GtkDialog, for instance).

You must call the base class's constructor in the initialization list, providing the C pointer. For instance,

    DerivedDialog::DerivedDialog(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Builder>& builder)
    : Gtk::Dialog(cobject)
    {
    }

You could then encapsulate the manipulation of the child widgets in the constructor of the derived class, maybe using get_widget() or get_widget_derived() again. For instance,

    DerivedDialog::DerivedDialog(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Builder>& builder)
    : Gtk::Dialog(cobject),
      m_builder(builder),
      //Get the Glade-instantiated Button, and connect a signal handler:
      m_pButton(m_builder->get_widget<Gtk::Button>("quit_button"))
    {
      if(m_pButton)
      {
        m_pButton->signal_clicked().connect( sigc::mem_fun(*this, &DerivedDialog::on_button_quit) );
      }
    }

It's possible to pass additional arguments from get_widget_derived() to the constructor of the derived widget. For instance, this call to get_widget_derived()

    auto pDialog = Gtk::Builder::get_widget_derived<DerivedDialog>(builder, "DialogDerived", true);

can invoke this constructor

    DerivedDialog::DerivedDialog(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Builder>& builder, bool warning)
    : Gtk::Dialog(cobject),
      m_builder(builder),
      m_pButton(m_builder->get_widget<Gtk::Button>("quit_button"))
    {
      // ....
    }

#### Gtk::Builder and Glib::Property

If your derived widget uses Glib::Property, it becomes slightly more complicated. A derived widget that contains Glib::Property members must be registered with its own name in the GType system. It must be registered before any of the create_from_*() or add_from_*() methods are called, meaning that you may have to create an instance of your derived widget just to have its class registered. Your derived widget must have a constructor that has the parameters required by get_widget_derived() and calls the Glib::ObjectBase constructor to register the GType.

    DerivedButton::DerivedButton(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Builder>& builder)
    : Glib::ObjectBase("MyButton"), // The GType name will be gtkmm__CustomObject_MyButton.
      Gtk::Button(cobject),
      prop_ustring(*this, "button-ustring"),
      prop_int(*this, "button-int", 10)
    {
      // ....
    }

When using >kmm; with a version of glibmm from 2.62 onwards, it is possible also to specify properties of derived widgets, declared in C++ using >kmm;, within `.glade` files and load/set these using Gtk::Builder. See the documentation of Gtk::Builder for more details on how to achieve this. Glade won’t recognise such properties as-is, but it should be able to through use of property class definitions and a catalog declaring those new properties.

#### Example

This example shows how to load a Glade file at runtime and access the widgets via derived classes.

Source Code

## Internationalization and Localization

>kmm; applications can easily support multiple languages, including non-European languages such as Chinese and right-to-left languages such as Arabic. An appropriately-written and translated >kmm; application will use the appropriate language at runtime based on the user's environment.

You might not anticipate the need to support additional languages, but you can never rule it out. And it's easier to develop the application properly in the first place rather than retrofitting later.

The process of writing source code that allows for translation is called `internationalization`, often abbreviated to `i18n`. The `Localization` process, sometimes abbreviated as `l10n`, provides translated text for other languages, based on that source code.

The main activity in the internationalization process is finding strings seen by users and marking them for translation. You do not need to do it all at once - if you set up the necessary project infrastructure correctly then your application will work normally regardless of how many strings you've covered.

String literals should be typed in the source code in English, but surrounded by a macro. The gettext (or intltool) utility can then extract the marked strings for translation, and substitute the translated text at runtime.

### Preparing your project

In the instructions below we will assume that you will not be using gettext directly, but intltool, which was written specifically for `GNOME`. intltool uses `gettext()`, which extracts strings from source code, but intltool can also combine strings from other files, for example from desktop menu details, and GUI resource files such as Glade files, into standard gettext `.pot/.po` files.

We also assume that you are using autotools (e.g. automake and autoconf) to build your project, and that you are using `./autogen.sh` from gnome-common or a similar `autogen.sh` file, which, among other things, takes care of some intltool initialization.

An alternative to gnome-common's `autogen.sh` may look like this:

    #! /bin/sh -e
    test -n "$srcdir" || srcdir=`dirname "$0"`
    test -n "$srcdir" || srcdir=.

    autoreconf --force --install --verbose --warnings=all "$srcdir"
    echo "Running intltoolize --copy --force --automake"
    intltoolize --copy --force --automake
    test -n "$NOCONFIGURE" || "$srcdir/configure" "$@"

Create a sub-directory named `po` in your project's root directory. This directory will eventually contain all of your translations. Within it, create a file named `LINGUAS` and a file named `POTFILES.in`. It is common practice to also create a `ChangeLog` file in the `po` directory so that translators can keep track of translation changes.

`LINGUAS` contains an alphabetically sorted list of codes identifying the languages for which your program is translated (comment lines starting with a `#` are ignored). Each language code listed in the `LINGUAS` file must have a corresponding `.po` file. So, if your program has German and Japanese translations, your `LINGUAS` file would look like this:

    # keep this file sorted alphabetically, one language code per line
    de
    ja

(In addition, you'd have the files `ja.po` and `de.po` in your `po` directory which contain the German and Japanese translations, respectively.)

`POTFILES.in` is a list of paths to all files which contain strings marked up for translation, starting from the project root directory. So for example, if your project sources were located in a subdirectory named `src`, and you had two files that contained strings that should be translated, your `POTFILES.in` file might look like this:

    src/main.cc
    src/other.cc

If you are using gettext directly, you can only mark strings for translation if they are in source code file. However, if you use intltool, you can mark strings for translation in a variety of other file formats, including Glade UI files, xml, .desktop files and several more. So, if you have designed some of the application UI in Glade then also add your `.glade` files to the list in `POTFILES.in`.

Now that there is a place to put your translations, you need to initialize intltool and gettext. Add the following code to your `configure.ac`, substituting 'programname' with the name of your program:

    IT_PROG_INTLTOOL([0.35.0])

    GETTEXT_PACKAGE=programname
    AC_SUBST(GETTEXT_PACKAGE)
    AC_DEFINE_UNQUOTED([GETTEXT_PACKAGE], ["$GETTEXT_PACKAGE"],
                       [The domain to use with gettext])
    AM_GNU_GETTEXT([external])
    AM_GNU_GETTEXT_VERSION([0.17])

    PROGRAMNAME_LOCALEDIR=[${datadir}/locale]
    AC_SUBST(PROGRAMNAME_LOCALEDIR)

This `PROGRAMNAME_LOCALEDIR` variable will be used later in the `Makefile.am` file, to define a macro that will be used when you initialize gettext in your source code.

`AM_GLIB_GNU_GETTEXT` has been an alternative to `AM_GNU_GETTEXT` and `AM_GNU_GETTEXT_VERSION`, but `AM_GLIB_GNU_GETTEXT` is now deprecated, and shall not be used in new code.

In the top-level Makefile.am:

- Add `po` to the `SUBDIRS` variable. Without this, your translations won't get built and installed when you build the program

- Define `INTLTOOL_FILES` as:

      INTLTOOL_FILES = intltool-extract.in \
                       intltool-merge.in \
                       intltool-update.in

- Add `INTLTOOL_FILES` to the `EXTRA_DIST` list of files. This ensures that when you do a `make dist`, these files will be included in the source tarball.

- Update your `DISTCLEANFILES`:

      DISTCLEANFILES = ... intltool-extract \
                       intltool-merge \
                       intltool-update \
                       po/.intltool-merge-cache

- Depending on the types of files that contain translatable strings, add code such as

      desktopdir = $(datadir)/applications
      desktop_in_files = programname.desktop.in
      desktop_DATA = $(desktop_in_files:.desktop.in=.desktop)
      @INTLTOOL_DESKTOP_RULE@

In your `src/Makefile.am`, update your `AM_CPPFLAGS` to add the following preprocessor macro definition:

    AM_CPPFLAGS = ... -DPROGRAMNAME_LOCALEDIR=\"${PROGRAMNAME_LOCALEDIR}\"

This macro will be used when you initialize `gettext` in your source code.

### Marking strings for translation

String literals should be typed in the source code in English, but they should be surrounded by a call to the `gettext()` function. These strings will be extracted for translation and the translations may be used at runtime instead of the original English strings.

The GNU gettext package allows you to mark strings in source code, extract those strings for translation, and use the translated strings in your application.

However, Glib defines `gettext()` support macros which are shorter wrappers in an easy-to-use form. To use these macros, include `<glibmm/i18n.h>`, and then, for example, substitute:

    display_message("Getting ready for i18n.");

with:

    display_message(_("Getting ready for i18n."));

For reference, it is possible to generate a file which contains all strings which appear in your code, even if they are not marked for translation, together with file name and line number references. To generate such a file named `my-strings`, execute the following command, within the source code directory:

    xgettext -a -o my-strings --omit-header *.cc *.h

Finally, to let your program use the translation for the current locale, add this code to the beginning of your `main.cc` file, to initialize gettext.

    bindtextdomain(GETTEXT_PACKAGE, PROGRAMNAME_LOCALEDIR);
    bind_textdomain_codeset(GETTEXT_PACKAGE, "UTF-8");
    textdomain(GETTEXT_PACKAGE);

#### How gettext works

The intltool-update or xgettext script extracts the strings and puts them in a `mypackage.pot` file. The translators of your application create their translations by first copying this `.pot` file to a `localename.po` file. A locale identifies a language and an encoding for that language, including date and numerical formats. Later, when the text in your source code has changed, the msgmerge or intltool-update script is used to update the `localename.po` files from the regenerated `.pot` file.

At install time, the `.po` files are converted to a binary format (with the extension `.mo`) and placed in a system-wide directory for locale files, for example `/usr/share/locale/`.

When the application runs, the gettext library checks the system-wide directory to see if there is a `.mo` file for the user's locale environment (you can set the locale with, for instance, "export LANG=de_DE.UTF-8" from a bash console). Later, when the program reaches a `gettext` call, it looks for a translation of a particular string. If none is found, the original string is used.

#### Testing and adding translations

To convince yourself that you've done well, you may wish to add a translation for a new locale. In order to do that, go to the `po` subdirectory of your project and execute the following command:

    intltool-update --pot

That will create a file named `programname.pot`. Now copy that file to `languagecode.po`, such as `de.po` or `hu.po`. Also add that language code to `LINGUAS`. The `.po` file contains a header and a list of English strings, with space for the translated strings to be entered. Make sure you set the encoding of the `.po` file (specified in the header, but also as content) to `UTF-8`.

It's possible that certain strings will be marked as `fuzzy` in the `.po` file. These translations will not substitute the original string. To make them appear, simply remove the `fuzzy` tag.

#### Resources

More information about what lies behind the internationalization and localization process is presented and demonstrated in:

- L10N Guidelines for Developers

- Intltool README

- How to use Git for GNOME translators

- gettext manual

- `gtkmm_hello` example package

- `gnomemm_hello` example package

### Expecting UTF8

A properly internationalized application will not make assumptions about the number of bytes in a character. That means that you shouldn't use pointer arithmetic to step through the characters in a string, and it means you shouldn't use std::string or standard C functions such as `strlen()` because they make the same assumption.

However, you probably already avoid bare char* arrays and pointer arithmetic by using std::string, so you just need to start using Glib::ustring instead. See the Basics chapter about Glib::ustring.

#### Glib::ustring and std::iostreams

Unfortunately, the integration with the standard iostreams is not completely foolproof. >kmm; converts Glib::ustrings to a locale-specific encoding (which usually is not UTF-8) if you output them to an ostream with `operator<<`. Likewise, retrieving Glib::ustrings from istream with `operator>>` causes a conversion in the opposite direction. But this scheme breaks down if you go through a std::string, e.g. by inputting text from a stream to a std::string and then implicitly converting it to a Glib::ustring. If the string contained non-ASCII characters and the current locale is not UTF-8 encoded, the result is a corrupted Glib::ustring. You can work around this with a manual conversion. For instance, to retrieve the std::string from a ostringstream:

    std::locale::global(std::locale("")); // Set the global locale to the user's preferred locale.
                                          // Usually unnecessary here, because Glib::init()
                                          // or Gtk::Application::create() does it for you.
    std::ostringstream output;
    output << percentage << " % done";
    label->set_text(Glib::locale_to_utf8(output.str()));

### Pitfalls

There are a few common mistakes that you would discover eventually yourself. But this section might help you to avoid them.

#### Same strings, different semantics

Sometimes two English strings are identical but have different meanings in different contexts, so they would probably not be identical when translated. Since the English strings are used as look-up keys, this causes problems.

In these cases, you should add extra characters to the strings. For instance, use `"jumps[noun]"` and `"jumps[verb]"` instead of just `"jumps"` and strip them again outside the `gettext` call. If you add extra characters you should also add a comment for the translators before the `gettext` call. Such comments will be shown in the `.po` files. For instance:

    // note to translators: don't translate the "[noun]" part - it is
    // just here to distinguish the string from another "jumps" string
    text = strip(gettext("jumps[noun]"), "[noun]");

If you use Glib's support macros, it's easier. Use `C_()` instead of `_()`. For instance:

    GLib::ustring text(C_("noun", "jumps"));

#### Composition of strings

C programmers use `sprintf()` to compose and concatenate strings. C++ favours streams, but unfortunately, this approach makes translation difficult, because each fragment of text is translated separately, without allowing the translators to rearrange them according to the grammar of the language.

For instance, this code would be problematic:

    std::cout << _("Current amount: ") << amount
              << _(" Future: ") << future << std::endl;

    label.set_text(_("Really delete ") + filename + _(" now?"));

So you should either avoid this situation or use `Glib::ustring::compose()` which supports syntax such as:

    std::cout << Glib::ustring::compose(
                 _("Current amount: %1 Future: %2"), amount, future) << std::endl;

    label.set_text(Glib::ustring::compose(_("Really delete %1 now?"), filename));

#### Assuming the displayed size of strings

You never know how much space a string will take on screen when translated. It might very possibly be twice the size of the original English string. Luckily, most >kmm; widgets will expand at runtime to the required size.

#### Unusual words

You should avoid cryptic abbreviations, slang, or jargon. They are usually difficult to translate, and are often difficult for even native speakers to understand. For instance, prefer "application" to "app"

#### Using non-ASCII characters in strings

Currently, gettext does not support non-ASCII characters (i.e. any characters with a code above 127) in source code. For instance, you cannot use the copyright sign (©).

To work around this, you could write a comment in the source code just before the string, telling the translators to use the special character if it is available in their languages. For English, you could then make an American English `en_US.po` translation which used that special character.

### Getting help with translations

If your program is free software, there is a whole `GNOME` subproject devoted to helping you make translations, the `GNOME` Translation Project.

The way it works is that you upload your source code to a git repository where translators can access it, then contact the gnome-i18n mailing list and ask to have your program added to the list of modules to translate.

Then you make sure you update the file `POTFILES.in` in the `po/` subdirectory (`intltool-update -m` can help with this) so that the translators always access updated `myprogram.pot` files, and simply freeze the strings at least a couple of days before you make a new release, announcing it on gnome-i18n. Depending on the number of strings your program contains and how popular it is, the translations will then start to tick in as `languagename.po` files.

Note that most language teams only consist of 1-3 persons, so if your program contains a lot of strings, it might last a while before anyone has the time to look at it. Also, most translators do not want to waste their time (translating is a very time-consuming task) so if they do not assess your project as being really serious (in the sense that it is polished and being maintained) they may decide to spend their time on some other project.

## Custom Widgets

>kmm; makes it very easy to derive new widgets by inheriting from an existing widget class, either by deriving from a container and adding child widgets, or by deriving from a single-item widget, and changing its behaviour. But you might occasionally find that no suitable starting point already exists. In this case, you can implement a widget from scratch.

### Custom Containers

When deriving a custom container widget directly from Gtk::Widget, you should override the following virtual methods:

- get_request_mode_vfunc(): Return what `Gtk::SizeRequestMode` is preferred by the container.

- measure_vfunc(): Calculate the minimum and natural width or height of the container.

- on_size_allocate(): Position the child widgets, given the height and width that the container has actually been given.

The get_request_mode_vfunc(), measure_vfunc(), and on_size_allocate() virtual methods control the layout of the child widgets. For instance, if your container has 2 child widgets, with one below the other, your get_request_mode_vfunc() might request height-for-width layout. Then your measure_vfunc() might report the maximum of the widths of the child widgets when asked to report width, and it might report the sum of their heights when asked to report height. If you want padding between the child widgets then you would add that to the width and height too. Your widget's container will use this result to ensure that your widget gets enough space, and not less. By examining each widget's parent, and its parent, this logic will eventually decide the size of the top-level window.

You are not guaranteed to get the `Gtk::SizeRequestMode` that you request. Therefore measure_vfunc() must return sensible values for all reasonable values of its input parameters. For a description of measure_vfunc()'s parameters see also the description of Gtk::Widget::measure(), which may be better documented than measure_vfunc().

on_size_allocate() receives the actual height and width that the parent container has decided to give to your widget. This might be more than the minimum, or even more than the natural size, for instance if the top-level window has been expanded. You might choose to ignore the extra space and leave a blank area, or you might choose to expand your child widgets to fill the space, or you might choose to expand the padding between your widgets. It's your container, so you decide.

#### Example

This example implements a container with two child widgets, one above the other. Of course, in this case it would be far simpler just to use a vertical Gtk::Box or Gtk::Grid.

Custom Container

Source Code

### Custom Widgets

By deriving directly from Gtk::Widget you can do all the drawing for your widget directly, instead of just arranging child widgets. For instance, a Gtk::Label draws the text of the label, but does not do this by using other widgets.

When deriving from Gtk::Widget, you should override the following virtual methods. The methods marked (optional) need not be overridden in all custom widgets. The base class's methods may be appropriate.

- get_request_mode_vfunc(): (optional) Return what `Gtk::SizeRequestMode` is preferred by the widget.

- measure_vfunc(): Calculate the minimum and natural width or height of the widget.

- on_size_allocate(): Position the widget, given the height and width that it has actually been given.

- on_realize(): Associate a Gdk::Surface with the widget.

- on_unrealize(): (optional) Break the association with the Gdk::Surface.

- on_map(): (optional)

- on_unmap(): (optional)

- snapshot_vfunc(): Create a render node, e.g. a Cairo::Context node, and draw on it.

The first 3 methods in the previous table are also overridden in custom containers. They are briefly described in the Custom Containers section.

#### Class Init and Instance Init Functions

Some GTK functions, if called at all, must be called from the class init function. Some other GTK functions, if called, must be called from the instance init function. If your custom widget must call any of those functions, you can derive a class from Glib::ExtraClassInit and derive your custom class from that class. The following example shows how that's done.

#### Custom Style Information

Your widget class, whether it's derived directly from Gtk::Widget or from another widget class, can read some style information from a CSS (Cascading Style Sheets) file. The users of your widget, or the users of an application program with your widget, can then modify the style of your widget without modifying the source code. Useful classes are Gtk::StyleContext and Gtk::CssProvider. With the methods of Gtk::StyleContext you can read the values of your widget's style information. CSS files are described in the documentation of GTK. The following example shows a simple use of Gtk::StyleContext::get_padding().

#### Example

This example implements a widget which draws Penrose triangles.

Custom Widget

Source Code

## Multi-threaded programs

### The constraints

Care is required when writing programs based on >kmm; using multiple threads of execution, arising from the fact that libsigc++, and in particular sigc::trackable, are not thread-safe. That's because none of the complex interactions that occur behind the scenes when using libsigc++ are protected by a mutex or other means of synchronization.

These interactions arise from the fact that, amongst other things, a class inheriting from sigc::trackable will, via that inheritance, have a std::list object keeping track of slots created by calls to `sigc::mem_fun()` representing any of its non-static methods (more particularly it keeps a list of callbacks which will null the connected slots on its destruction). Each sigc::slot object also keeps, via sigc::slot_rep, its own sigc::trackable object to track any sigc::connection objects which it needs to inform about its demise, and also has a function to deregister itself from any sigc::trackable on disconnection or destruction. sigc::signal objects also keep lists of slots, which will be updated by a call to their connect() method or calls to any sigc::connection object relating to such a connection.

#### The rules

This requires a number of rules to be observed when writing multi-threaded programs using >kmm;. These are set out below, but one point to note is that extra care is required when deriving classes from sigc::trackable, because the effects are unintuitive (see particularly points 4 and 5 below).

1. Use Glib::Dispatcher to invoke >kmm; functions from worker threads (this is dealt with in more detail in the next section).

2. A sigc::signal object should be regarded as owned by the thread which created it. Only that thread should connect a sigc::slot object to the signal object, and only that thread should emit() or call operator()() on the signal, or null any connected sigc::slot object. It follows (amongst other things) that any signal object provided by a >kmm; widget should only be operated on in the main GUI thread and any object deriving from sigc::trackable having its non-static methods referenced by slots connected to the signal object should only be destroyed in that thread.

3. Any sigc::connection object should be regarded as owned by the thread in which the method returning the sigc::connection object was called. Only that thread should call sigc::connection methods on the object.

4. A sigc::slot object created by a call to `sigc::mem_fun()` which references a method of a class deriving from sigc::trackable should never be copied to another thread, nor destroyed by a different thread than the one which created it.

5. If a particular class object derives from sigc::trackable, only one thread should create sigc::slot objects representing any of the class's non-static methods by calling `sigc::mem_fun()`. The first thread to create such a slot should be regarded as owning the relevant object for the purpose of creating further slots referencing any of its non-static methods using that function, or nulling those slots by disconnecting them or destroying the trackable object.

6. Although glib is itself thread-safe, any glibmm wrappers which use libsigc++ will not be. So for example, only the thread in which a main loop runs should call Glib::SignalIdle::connect(), Glib::SignalIO::connect(), Glib::SignalTimeout::connect(), Glib::SignalTimeout::connect_seconds for that main loop, or manipulate any sigc::connection object returned by them.

   The connect*_once() variants, Glib::SignalIdle::connect_once(), Glib::SignalTimeout::connect_once(), Glib::SignalTimeout::connect_seconds_once(), are thread-safe for any case where the slot is not created by a call to `sigc::mem_fun()` which represents a method of a class deriving from sigc::trackable.

### Using Glib::Dispatcher

The slots connected to sigc::signal objects execute in the thread which calls emit() or operator()() on the signal. Glib::Dispatcher does not behave this way: instead its connected slots execute in the thread in which the Glib::Dispatcher object was constructed (which must have a glib main loop). If a Glib::Dispatcher object is constructed in the main GUI thread (which will therefore be the receiver thread), any worker thread can emit on it and have the connected slots safely execute >kmm; functions.

Some thread safety rules on the use of Glib::Dispatcher still apply. As mentioned, a Glib::Dispatcher object must be constructed in the receiver thread (the thread in whose main loop it will execute its connected slots). By default this is the main program thread, although there is a Glib::Dispatcher constructor which can take the Glib::MainContext object of any thread which has a main loop. Only the receiver thread should call connect() on the Glib::Dispatcher object, or manipulate any related sigc::connection object, unless additional synchronization is employed. However, any worker thread can safely emit on the Glib::Dispatcher object without any locking once the receiver thread has connected the slots, provided that it is constructed before the worker thread is started (if it is constructed after the thread has started, additional synchronization will normally be required to ensure visibility).

Aside from the fact that connected slots always execute in the receiver thread, Glib::Dispatcher objects are similar to sigc::signal<void()> objects. They therefore cannot pass unbound arguments nor return a value. The best way to pass unbound arguments is with a thread-safe (asynchronous) queue. At the time of writing glibmm does not have one, although most people writing multi-threaded code will have one available to them (they are relatively easy to write although there are subtleties in combining thread safety with strong exception safety).

A Glib::Dispatcher object can be emitted on by the receiver thread as well as by a worker thread, although this should be done within reasonable bounds. On unix-like systems Glib::Dispatcher objects share a single common pipe, which could in theory at least fill up on a very heavily loaded system running a program with a very large number of Dispatcher objects in use. Were the pipe to fill up before the receiver thread's main loop has had an opportunity to read from it to empty it, and the receiver thread attempt to emit and so write to it when it is in that condition, the receiver thread would block on the write, so deadlocking. Where the receiver thread is to emit, a normal sigc::signal<void()> object could of course be used instead.

### Example

This is an example program with two threads, one GUI thread, like in all >kmm; programs, and one worker thread. The worker thread is created when you press the `Start work` button. It is deleted when the work is finished, when you press the `Stop work` button, or when you press the `Quit` button.

A Glib::Dispatcher is used for sending notifications from the worker thread to the GUI thread. The ExampleWorker class contains data which is accessed by both threads. This data is protected by a std::mutex. Only the GUI thread updates the GUI.

Compiling and linking a multi-threaded program can require special compiler and linker options. If you use the g++ compiler, add the `-pthread` option. Other compilers may require other options. If you build with meson, it handles the multi-threading complications for you, if you add `dependency('threads')`.

Multi-Threaded Program

Source Code

## Recommended Techniques

This section is simply a gathering of wisdom, general style guidelines and hints for creating >kmm; applications.

Use GNU autoconf and automake! They are your friends :) Automake examines C files, determines how they depend on each other, and generates a `Makefile` so the files can be compiled in the correct order. Autoconf permits automatic configuration of software installation, handling a large number of system quirks to increase portability.

Subclass Widgets to better organize your code. You should probably subclass your main Window at least. Then you can make your child Widgets and signal handlers members of that class.

Create your own signals instead of passing pointers around. Objects can communicate with each other via signals and signal handlers. This is much simpler than objects holding pointers to each other and calling each other's methods. >kmm;'s classes uses special versions of sigc::signal, but you should use normal sigc::signals, as described in the libsigc++ documentation.

### Application Lifetime

Most applications will have only one Window, or only one main window. These applications can use the Gtk::Application::run(Gtk::Window& window) or Gtk::Application::run(Gtk::Window& window, int argc, char** argv) overloads. They show the window and return when the window has been hidden. This might happen when the user closes the window, or when your code decides to hide() the window. You can prevent the user from closing the window (for instance, if there are unsaved changes) by overriding Gtk::Window::on_delete_event().

Most of our examples use this technique.

### Using a >kmm; widget

Our examples all tend to have the same structure. They follow these steps for using a Widget:

1. Declare a variable of the type of Widget you wish to use, generally as member variable of a derived container class. You could also declare a pointer to the widget type, and then create it with `new` in your code. Even when using the widget via a pointer, it's still probably best to make that pointer a member variable of a container class so that you can access it later.

2. Set the attributes of the widget. If the widget has no default constructor, then you will need to initialize the widget in the initalizer list of your container class's constructor.

3. Connect any signals you wish to use to the appropriate handlers.

4. Pack the widget into a container using the appropriate call, e.g. Gtk::Box::append().

If you don't want all widgets to be shown, call Gtk::Widget::hide() on the widgets that you don't want to show. If a container widget is hidden, all of its child widgets are also hidden, even if hide() is not called on the child widgets.

## Building applications

This chapter is similar to the "Building applications" chapter in the GTK4 Reference Manual. The same application is built, but >kmm; is used instead of GTK.

An application consists of a number of files:

The binary file

  This gets installed in `/usr/bin`.

A desktop file

  The desktop file provides important information about the application to the desktop shell, such as its name, icon, D-Bus name, commandline to launch it, etc. It is installed in `/usr/share/applications`.

An icon

  The icon gets installed in `/usr/share/icons/hicolor/48x48/apps`, where it will be found regardless of the current theme.

A settings schema

  If the application uses Gio::Settings, it will install its schema in `/usr/share/glib-2.0/schemas`, so that tools like dconf-editor can find it.

Other resources

  Other files, such as Gtk::Builder ui files, are best loaded from resources stored in the application binary itself. This eliminates the need for most of the files that would traditionally be installed in an application-specific location in `/usr/share`.

>kmm; includes application support that is built on top of Gio::Application. In this chapter we'll build a simple application by starting from scratch, adding more and more pieces over time. Along the way, we'll learn about Gtk::Application, Gtk::Builder, resources, menus, settings, Gtk::HeaderBar, Gtk::Stack, Gtk::SearchBar, Gtk::ListBox, and more.

The full, buildable sources for these examples can be found in the `examples/book/buildapp` directory of the gtkmm-documentation source distribution, or online in the gtkmm-documentation git repository. You can build each example separately by using `meson` and `ninja` with the `meson.build` file or by using `make` with the `Makefile.example` file. For more information, see the `README` included in the `buildapp` directory.

### A trivial application

When using Gtk::Application, the `main()` function can be very simple. We just call Gtk::Application::run() on an instance of our application class.

All the application logic is in the application class, which is a subclass of Gtk::Application. Our example does not yet have any interesting functionality. All it does is open a window when it is activated without arguments, and open the files it is given, if it is started with arguments. (Or rather, our application class tries to open the files, but our subclassed application window does not yet do what it's told to do.)

To handle these two cases, we override signal_activate()'s default handler, which gets called when the application is launched without commandline arguments, and signal_open()'s default handler, which gets called when the application is launched with commandline arguments.

Gio::Application Reference

Gtk::Application Reference

Another important class that is part of the application support in >kmm; is Gtk::ApplicationWindow. It is typically subclassed as well. Our subclass does not do anything yet, so we will just get an empty window.

As part of the initial setup of our application, we also create an icon and a desktop file. Note that @bindir@ in the desktop file needs to be replaced with the actual path to the binary before this desktop file can be used.

Here is what we've achieved so far:

A trivial application

This does not look very impressive yet, but our application is already presenting itself on the session bus, it has single-instance semantics, and it accepts files as commandline arguments.

Source Code

### Populating the window

In this step, we use a Gtk::Builder instance to associate a Gtk::Builder ui file with our application window class.

Our simple ui file gives the window a title, and puts a Gtk::Stack widget as the main content.

To make use of this file in our application, we revisit our Gtk::ApplicationWindow subclass, and call Gtk::Builder::create_from_resource() and Gtk::Builder::get_widget_derived() from the ExampleAppWindow::create() method to get an instance of our subclassed Gtk::ApplicationWindow. See the Using derived widgets section for more information about get_widget_derived().

You may have noticed that we use the _from_resource() variant of the method that reads the ui file. Now we need to use GLib's resource functionality to include the ui file in the binary. This is commonly done by listing all resources in a .gresource.xml file. This file has to be converted into a C source file that will be compiled and linked into the application together with the other source files. To do so, we use the glib-compile-resources utility:

    $ glib-compile-resources --target=resources.c --generate-source exampleapp.gresource.xml

The Gio::Resource and glib-compile-resources section contains more information about resource files. If you build with Meson, use the `compile_resources()` function in Meson's GNOME module.

Our application now looks like this:

Populating the window

Source Code

### Opening files

In this step, we make our application show the contents of all the files that it is given on the commandline.

To this end, we add a data member to our application window and keep a pointer to the Gtk::Stack there. We get the pointer with a call to Gtk::Builder::get_widget() in the application window's constructor.

Now we revisit the ExampleAppWindow::open_file_view() method that is called for each commandline argument, and construct a Gtk::TextView that we then add as a page to the stack.

Lastly, we add a Gtk::StackSwitcher to the titlebar area in the ui file, and we tell it to display information about our stack.

The stack switcher gets all its information it needs to display tabs from the stack that it belongs to. Here, we are passing the label to show for each file as the last argument to the Gtk::Stack::add() method.

Our application is beginning to take shape:

Opening files

Source Code

### A menu

The menu is shown at the right side of the headerbar. It is meant to collect infrequently used actions that affect the whole application.

Just like the application window, we specify our menu in a ui file, and add it as a resource to our binary.

To make the menu appear, we have to load the ui file and associate the resulting menu model with the menu button that we've added to the headerbar. Since menus work by activating Gio::Actions, we also have to add a suitable set of actions to our application.

Adding the actions is best done in the on_startup() default signal handler, which is guaranteed to be called once for each primary application instance.

Our preferences menu item does not do anything yet, but the Quit menu item is fully functional. It can also be activated by the usual Ctrl-Q shortcut. The shortcut is added with Gtk::Application::set_accel_for_action().

The menu looks like this:

A menu

Source Code

### A preference dialog

A typical application will have some preferences that should be remembered from one run to the next. Even for our simple example application, we may want to change the font that is used for the content.

We are going to use Gio::Settings to store our preferences. Gio::Settings requires a schema that describes our settings, in our case the `org.gtkmm.exampleapp.gschema.xml` file.

Before we can make use of this schema in our application, we need to compile it into the binary form that Gio::Settings expects. GIO provides macros to do this in autotools-based projects. See the description of GSettings. Meson provides the `compile_schemas()` function in the GNOME module.

Next, we need to connect our settings to the widgets that they are supposed to control. One convenient way to do this is to use Gio::Settings::bind() to bind settings keys to object properties, as we do for the transition setting in ExampleAppWindow's constructor.

    m_settings = Gio::Settings::create("org.gtkmm.exampleapp");
    m_settings->bind("transition", m_stack->property_transition_type());

The code to connect the font setting is a little more involved, since it corresponds to an object property in a Gtk::TextTag that we must first create. The code is in ExampleAppWindow::open_file_view().

    auto tag = buffer->create_tag();
    m_settings->bind("font", tag->property_font());
    buffer->apply_tag(tag, buffer->begin(), buffer->end());

At this point, the application will already react if you change one of the settings, e.g. using the `gsettings` commandline tool. Of course, we expect the application to provide a preference dialog for these. So lets do that now. Our preference dialog will be a subclass of Gtk::Dialog, and we'll use the same techniques that we've already seen in ExampleAppWindow: a Gtk::Builder ui file and settings bindings.

When we've created the `prefs.ui` file and the ExampleAppPrefs class, we revisit the ExampleApplication::on_action_preferences() method in our application class, and make it open a new preference dialog.

    auto prefs_dialog = ExampleAppPrefs::create(*get_active_window());
    prefs_dialog->present();

After all this work, our application can now show a preference dialog like this:

An preference dialog

Source Code

### Adding a search bar

We continue to flesh out the functionality of our application. For now, we add search. >kmm; supports this with Gtk::SearchEntry and Gtk::SearchBar. The search bar is a widget that can slide in from the top to present a search entry.

We add a toggle button to the header bar, which can be used to slide out the search bar below the header bar. The new widgets are added in the `window.ui` file.

Implementing the search needs quite a few code changes that we are not going to completely go over here. The central piece of the search implementation is a signal handler that listens for text changes in the search entry, shown here without error handling.

    void ExampleAppWindow::on_search_text_changed()
    {
      const auto text = m_searchentry->get_text();
      auto tab = dynamic_cast<Gtk::ScrolledWindow*>(m_stack->get_visible_child());
      auto view = dynamic_cast<Gtk::TextView*>(tab->get_child());

      // Very simple-minded search implementation.
      auto buffer = view->get_buffer();
      Gtk::TextIter match_start;
      Gtk::TextIter match_end;
      if (buffer->begin().forward_search(text, Gtk::TextSearchFlags::CASE_INSENSITIVE,
          match_start, match_end))
      {
        buffer->select_range(match_start, match_end);
        view->scroll_to(match_start);
      }
    }

With the search bar, our application now looks like this:

Adding a search bar

Source Code

### Adding a side bar

As another piece of functionality, we are adding a sidebar, which demonstrates Gtk::Revealer and Gtk::ListBox. The new widgets are added in the `window.ui` file.

The code to populate the sidebar with buttons for the words found in each file is a little too involved to go into here. But we'll look at the code to add a checkbutton for the new feature to the menu. A menu item is added to the ui file `gears_menu.ui`.

To connect the menu item to the new `show-words` setting, we use a Gio::Action corresponding to the given Gio::Settings key. In ExampleAppWindow's constructor:

    // Connect the menu to the MenuButton m_gears, and bind the show-words setting
    // to the win.show-words action and the "Words" menu item.
    // (The connection between action and menu item is specified in gears_menu.ui.)
    auto menu_builder = Gtk::Builder::create_from_resource("/org/gtkmm/exampleapp/gears_menu.ui");
    auto menu = menu_builder->get_object<Gio::MenuModel>("menu");
    m_gears->set_menu_model(menu);
    add_action(m_settings->create_action("show-words"));

What our application looks like now:

Adding a side bar

Source Code

### Properties

Widgets and other objects have many useful properties. Here we show some ways to use them in new and flexible ways, by wrapping them in actions with Gio::PropertyAction or by binding them with Glib::Binding.

To set this up, we add two labels to the header bar in our `window.ui` file, named `lines_label` and `lines`, and get pointers to them in the application window's constructor, as we've seen a couple of times by now. We add a new "Lines" menu item to the gears menu, which triggers the `show-lines` action.

To make this menu item do something, we create a property action for the `visible` property of the `lines` label, and add it to the actions of the window. The effect of this is that the visibility of the label gets toggled every time the action is activated. Since we want both labels to appear and disappear together, we bind the `visible` property of the `lines_label` widget to the same property of the `lines` widget. In ExampleAppWindow's constructor:

    add_action(Gio::PropertyAction::create("show-lines", m_lines->property_visible()));
    m_binding_lines_visible = Glib::Binding::bind_property(m_lines->property_visible(),
      m_lines_label->property_visible());

We also need a function that counts the lines of the currently active tab, and updates the `lines` label. See the full source if you are interested in the details.

This brings our example application to this appearance:

Properties

Source Code

### Header bar

Our application already uses a Gtk::HeaderBar instead of a 'normal' window titlebar. The header bar is a direct child of the window, and its type is `titlebar`. This is set in the `window.ui` file.

Here we'll just make two small changes to the header bar. The `decoration-layout` property is set in the `window.ui` file, to show only the close button, and hide the minimize and maximize buttons. We also include an icon in the resource file, and set up this icon as the window icon. In ExampleAppWindow's constructor:

    Gtk::IconTheme::get_for_display(get_display())->add_resource_path("/org/gtkmm/exampleapp");
    set_icon_name("exampleapp");

Here is how the application now looks:

Header bar

The `window.ui` file sets a header bar title, but this title is not shown. That's because the stack switcher is a child of type `title`. The stack switcher becomes a custom title that hides the title label.

Source Code

## Contributing

This document, like so much other great software out there, was created for free by volunteers. If you are at all knowledgeable about any aspect of >kmm; that does not already have documentation, please consider contributing to this document.

Ideally, we would like you to provide a merge request to the `docs/tutorial/C/index-in.docbook` file. This file is in the `gtkmm-documentation` module in GNOME git.

If you do decide to contribute, please post your contribution to the >kmm; mailing list at <gtkmm-list@gnome.org> or as an issue or merge request to GitLab. Also, be aware that the entirety of this document is free, and any addition you provide must also be free. That is, people must be able to use any portion of your examples in their programs, and copies of this document (including your contribution) may be distributed freely.

## The RefPtr smartpointer

Glib::RefPtr is a smartpointer. Specifically, it is a reference-counting smartpointer. You might be familiar with std::unique_ptr<> and std::shared_ptr<>, which are also smartpointers. In >kmm;-4.0 Glib::RefPtr<> is an alias for std::shared_ptr<>, which is reference-counting. Glib::RefPtr<> was introduced long before there was a reference-counting smartpointer in the C++ Standard Library.

Reference

A smartpointer acts much like a normal pointer. Here are a few examples.

### Copying

You can copy RefPtrs, just like normal pointers. But unlike normal pointers, you don't need to worry about deleting the underlying instance.

    auto refPixbuf = Gdk::Pixbuf::create_from_file(filename);
    auto refPixbuf2 = refPixbuf;

Of course this means that you can store RefPtrs in standard containers, such as std::vector or std::list.

    std::list<Glib::RefPtr<Gdk::Pixbuf>> listPixbufs;
    auto refPixbuf = Gdk::Pixbuf::create_from_file(filename);
    listPixbufs.push_back(refPixbuf);

### Dereferencing

You can dereference a smartpointer with the -> operator, to call the methods of the underlying instance, just like a normal pointer.

    auto refPixbuf = Gdk::Pixbuf::create_from_file(filename);
    auto width = refPixbuf->get_width();

You can also use the * operator and the get() method to access the underlying instance, but it's usually a bad idea to do so. Unless you are careful, you can end up with a pointer or a reference which is not included in the reference count.

    auto refPixbuf = Gdk::Pixbuf::create_from_file(filename);
    auto& underlying = *refPixbuf; // Possible, but not recommended

### Casting

You can cast RefPtrs to base types, just like normal pointers.

    auto refStore = Gtk::TreeStore::create(columns);
    Glib::RefPtr<Gtk::TreeModel> refModel = refStore;

This means that any method which takes a const Glib::RefPtr<BaseType>& argument can also take a const Glib::RefPtr<DerivedType>&. The cast is implicit, just as it would be for a normal pointer.

You can also cast to a derived type, but the syntax is a little different than with a normal pointer.

    auto refStore = std::dynamic_pointer_cast<Gtk::TreeStore>(refModel);
    auto refStore2 = std::static_pointer_cast<Gtk::TreeStore>(refModel);

### Checking for nullptr

Just like normal pointers, you can check whether a RefPtr points to anything.

    auto refModel = m_TreeView.get_model();
    if (refModel)
    {
      auto cols_count = refModel->get_n_columns();
      ...
    }

But unlike normal pointers, RefPtrs are automatically initialized to `nullptr` so you don't need to remember to do that yourself.

### Constness

The use of the `const` keyword in C++ is not always clear. You might not realise that const Something* declares a pointer to a const Something. The pointer can be changed, but not the Something that it points to.

Therefore, the RefPtr equivalent of Something* for a method parameter is const Glib::RefPtr<Something>&, and the equivalent of const Something* is const Glib::RefPtr<const Something>&.

The `const ... &` around both is just for efficiency, like using const std::string& instead of std::string for a method parameter to avoid unnecessary copying.

## Signals

### Connecting signal handlers

>kmm; widget classes have signal accessor methods, such as Gtk::Button::signal_clicked(), which allow you to connect your signal handler. Thanks to the flexibility of libsigc++, the callback library used by >kmm;, the signal handler can be almost any kind of function, but you will probably want to use a class method. Among GTK C coders, these signal handlers are often named callbacks.

Here's an example of a signal handler being connected to a signal:

    #include <gtkmm/button.h>

    void on_button_clicked()
    {
        std::cout << "Hello World" << std::endl;
    }

    int main()
    {
        Gtk::Button button("Hello World");
        button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
    }

There's rather a lot to think about in this (non-functional) code. First let's identify the parties involved:

- The signal handler is on_button_clicked().

- We're hooking it up to the Gtk::Button object called `button`.

- When the Button emits its `clicked` signal, on_button_clicked() will be called.

Now let's look at the connection again:

        ...
        button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
        ...

Note that we don't pass a pointer to on_button_clicked() directly to the signal's connect() method. Instead, we call `sigc::ptr_fun()`, and pass the result to connect().

`sigc::ptr_fun()` generates a sigc::slot. A slot is an object which looks and feels like a function, but is actually an object. These are also known as function objects, or functors. `sigc::ptr_fun()` generates a slot for a standalone function or static method. `sigc::mem_fun()` generates a slot for a member method of a particular instance.

Here's a slightly larger example of slots in action:

    void on_button_clicked();

    class some_class
    {
        void on_button_clicked();
    };

    some_class some_object;

    int main()
    {
        Gtk::Button button;
        button.signal_clicked().connect( sigc::ptr_fun(&on_button_clicked) );
        button.signal_clicked().connect( sigc::mem_fun(some_object, &some_class::on_button_clicked) );
    }

The first call to connect() is just like the one we saw last time; nothing new here.

The next is more interesting. `sigc::mem_fun()` is called with two arguments. The first argument is some_object, which is the object that our new slot will be pointing at. The second argument is a pointer to one of its methods. This particular version of `sigc::mem_fun()` creates a slot which will, when "called", call the pointed-to method of the specified object, in this case some_object.on_button_clicked().

Another thing to note about this example is that we made the call to connect() twice for the same signal object. This is perfectly fine - when the button is clicked, both signal handlers will be called.

We just told you that the button's `clicked` signal is expecting to call a method with no arguments. All signals have requirements like this - you can't hook a function with two arguments to a signal expecting none (unless you use an adapter, such as `sigc::bind()`, of course). Therefore, it's important to know what type of signal handler you'll be expected to connect to a given signal.

### Writing signal handlers

To find out what type of signal handler you can connect to a signal, you can look it up in the reference documentation or the header file. Here's an example of a signal declaration you might see in the >kmm; headers:

    Glib::SignalProxy<bool(Gtk::DirectionType)> signal_focus()

Other than the signal's name (`focus`), the template arguments are important to note here. The first argument, bool, is the type that the signal handler should return; and the type within parentheses, Gtk::DirectionType, is the type of this signal's first, and only, argument. By looking at the reference documentation, you can see the names of the arguments too.

The same principles apply for signals which have more arguments. Here's one with three (taken from `<gtkmm/textbuffer.h>`):

    Glib::SignalProxy<void(TextBuffer::iterator&, const Glib::ustrin&, int)> signal_insert();

It follows the same form. The first type is void, so that should be our signal handler's return type. The following three types are the argument types, in order. Our signal handler's prototype could look like this:

    void on_insert(TextBuffer::iterator& pos, const Glib::ustring& text, int bytes)

### Disconnecting signal handlers

Let's take another look at a Signal's `connect` method:

    sigc::connection signal<void(int)>::connect(const sigc::slot<void(int)>&);

The returned sigc::connection can be used to control the connection. By keeping a connection object you can disconnect its associated signal handler using the sigc::connection::disconnect() method.

### Overriding default signal handlers

So far we've told you to perform actions in response to button-presses and the like by handling signals. That's certainly a good way to do things, but it's not the only way.

Instead of laboriously connecting signal handlers to signals, you can simply make a new class which inherits from a widget - say, a Button - and then override the default signal handler, such as Button::on_clicked(). This can be a lot simpler than hooking up signal handlers for everything.

Subclassing isn't always the best way to accomplish things. It is only useful when you want the widget to handle its own signal by itself. If you want some other class to handle the signal then you'll need to connect a separate handler. This is even more true if you want several objects to handle the same signal, or if you want one signal handler to respond to the same signal from different objects.

>kmm; classes are designed with overriding in mind; they contain virtual member methods specifically intended to be overridden.

Let's look at an example of overriding:

    #include <gtkmm/button.h>

    class OverriddenButton : public Gtk::Button
    {
    protected:
      void on_clicked() override;
    }

    void OverriddenButton::on_clicked()
    {
      std::cout << "Hello World" << std::endl;

      // call the base class's version of the method:
      Gtk::Button::on_clicked();
    }

Here we define a new class called OverriddenButton, which inherits from Gtk::Button. The only thing we change is the on_clicked() method, which is called whenever Gtk::Button emits the `clicked` signal. This method prints "Hello World" to `stdout`, and then calls the original, overridden method, to let Gtk::Button do what it would have done had we not overridden.

You don't always need to call the parent's method; there are times when you might not want to. Note that we called the parent method after writing "Hello World", but we could have called it before. In this simple example, it hardly matters much, but there are times when it will. With connected signal handlers, it's not quite so easy to change details like this, and you can do something here which you can't do at all with connected signal handlers: you can call the parent method in the middle of your custom code.

### Binding extra arguments

If you use one signal handler to catch the same signal from several widgets, you might like that signal handler to receive some extra information. For instance, you might want to know which button was clicked. You can do this with `sigc::bind()`. Here's some code from the helloworld2 example.

    m_button1.signal_clicked().connect(sigc::bind(sigc::mem_fun(*this, &HelloWorld::on_button_clicked), "button 1"));

This says that we want the signal to send an extra Glib::ustring argument to the signal handler, and that the value of that argument should be "button 1". Of course we will need to add that extra argument to the declaration of our signal handler:

    void on_button_clicked(const Glib::ustring& data);

Of course, a normal "clicked" signal handler would have no arguments.

`sigc::bind()` is not commonly used, but you might find it helpful sometimes. If you are familiar with GTK programming then you have probably noticed that this is similar to the extra `gpointer data` arguments which all GTK callbacks have. This is generally overused in GTK to pass information that should be stored as member data in a derived widget, but widget derivation is very difficult in C. We have far less need of this hack in >kmm;.

### X Event signals

The Widget class has some special signals which correspond to the underlying X-Windows events. These are suffixed by `_event`; for instance, Widget::signal_button_press_event().

You might occasionally find it useful to handle X events when there's something you can't accomplish with normal signals. Gtk::Button, for example, does not send mouse-pointer coordinates with its `clicked` signal, but you could handle `button_press_event` if you needed this information. X events are also often used to handle key-presses.

These signals behave slightly differently. The value returned from the signal handler indicates whether it has fully "handled" the event. If the value is `false` then >kmm; will pass the event on to the next signal handler. If the value is `true` then no other signal handlers will need to be called.

Handling an X event doesn't affect the Widget's other signals. If you handle `button_press_event` for Gtk::Button, you'll still be able to get the `clicked` signal. They are emitted at (nearly) the same time.

Here's a simple example:

    bool on_button_press(GdkEventButton* event);
    Gtk::Button button("label");
    button.signal_button_press_event().connect( sigc::ptr_fun(&on_button_press) );

When the mouse is over the button and a mouse button is pressed, on_button_press() will be called.

GdkEventButton is a structure containing the event's parameters, such as the coordinates of the mouse pointer at the time the button was pressed. There are several different types of GdkEvent structures for the various events.

#### Signal Handler sequence

By default, your signal handlers are called after any previously-connected signal handlers. However, this can be a problem with the X Event signals. For instance, the existing signal handlers, or the default signal handler, might return `true` to stop other signal handlers from being called. To specify that your signal handler should be called before the other signal handlers, so that it will always be called, you can specify `false` for the optional `after` parameter. For instance,

    button.signal_button_press_event().connect( sigc::ptr_fun(&on_mywindow_button_press), false );

The event is delivered first to the widget the event occurred in. If all signal handlers in that widget return `false` (indicating that the event has not been handled), then the signal will be propagated to the parent widget and emitted there. This continues all the way up to the top-level widget if no one handles the event.

### Exceptions in signal handlers

When a program is aborted because of an unhandled C++ exception, it's sometimes possible to use a debugger to find the location where the exception was thrown. This is more difficult than usual if the exception was thrown from a signal handler.

This section describes primarily what you can expect on a Linux system, when you use the gdb debugger.

First, let's look at a simple example where an exception is thrown from a normal function (no signal handler).

    // without_signal.cc
    #include <gtkmm.h>

    bool throwSomething()
    {
      throw "Something";
      return true;
    }

    int main(int argc, char** argv)
    {
      throwSomething();
      auto app = Gtk::Application::create("org.gtkmm.without_signal");
      return app->run();
    }

Here is an excerpt from a gdb session. Only the most interesting parts of the output are shown.

    > gdb without_signal
    (gdb) run
    terminate called after throwing an instance of 'char const*'

    Program received signal SIGABRT, Aborted.
    (gdb) backtrace
    #7  0x08048864 in throwSomething () at without_signal.cc:6
    #8  0x0804887d in main (argc=1, argv=0xbfffecd4) at without_signal.cc:12

You can see that the exception was thrown from `without_signal.cc`, line 6 (`throw "Something";`).

Now let's see what happens when an exception is thrown from a signal handler. Here's the source code.

    // with_signal.cc
    #include <gtkmm.h>

    bool throwSomething()
    {
      throw "Something";
      return true;
    }

    int main(int argc, char** argv)
    {
      Glib::signal_timeout().connect(sigc::ptr_fun(throwSomething), 500);
      auto app = Gtk::Application::create("org.gtkmm.with_signal");
      app->hold();
      return app->run();
    }

And here's an excerpt from a gdb session.

    > gdb with_signal
    (gdb) run
    (with_signal:2703): glibmm-ERROR **:
    unhandled exception (type unknown) in signal handler

    Program received signal SIGTRAP, Trace/breakpoint trap.
    (gdb) backtrace
    #2  0x0063c6ab in glibmm_unexpected_exception () at exceptionhandler.cc:77
    #3  Glib::exception_handlers_invoke () at exceptionhandler.cc:150
    #4  0x0063d370 in glibmm_source_callback (data=0x804d620) at main.cc:212
    #13 0x002e1b31 in Gtk::Application::run (this=0x804f300) at application.cc:178
    #14 0x08048ccc in main (argc=1, argv=0xbfffecd4) at with_signal.cc:16

The exception is caught in glibmm, and the program ends with a call to `g_error()`. Other exceptions may result in different behaviour, but in any case the exception from a signal handler is caught in glibmm or >kmm;, and gdb can't see where it was thrown.

To see where the exception is thrown, you can use the gdb command `catch throw`.

    > gdb with_signal
    (gdb) catch throw
    Catchpoint 1 (throw)
    (gdb) run
    Catchpoint 1 (exception thrown), 0x00714ff0 in __cxa_throw ()
    (gdb) backtrace
    #0  0x00714ff0 in __cxa_throw () from /usr/lib/i386-linux-gnu/libstdc++.so.6
    #1  0x08048bd4 in throwSomething () at with_signal.cc:6
    (gdb) continue
    Continuing.
    (with_signal:2375): glibmm-ERROR **
    unhandled exception (type unknown) in signal handler

    Program received signal SIGTRAP, Trace/breakpoint trap.

If there are many caught exceptions before the interesting uncaught one, this method can be tedious. It can be automated with the following gdb commands.

    (gdb) catch throw
    (gdb) commands
    (gdb)   backtrace
    (gdb)   continue
    (gdb)   end
    (gdb) set pagination off
    (gdb) run

These commands will print a backtrace from each `throw` and continue. The backtrace from the last (or possibly the last but one) `throw` before the program stops, is the interesting one.

## Creating your own signals

Now that you've seen signals and signal handlers in >kmm;, you might like to use the same technique to allow interaction between your own classes. That's actually very simple by using the libsigc++ library directly.

This isn't purely a >kmm; or GUI issue. >kmm; uses libsigc++ to implement its proxy wrappers for the GTK signal system, but for new, non-GTK signals, you can create pure C++ signals, using the sigc::signal<> template.

For instance, to create a signal that sends 2 parameters, a bool and an int, just declare a sigc::signal, like so:

    sigc::signal<void(bool, int)> signal_something;

You could just declare that signal as a public member variable, but some people find that distasteful and prefer to make it available via an accessor method, like so:

    class Server
    {
    public:
      //signal accessor:
      using type_signal_something = sigc::signal<void(bool, int)>;
      type_signal_something signal_something();

    protected:
      type_signal_something m_signal_something;
    };

    Server::type_signal_something Server::signal_something()
    {
      return m_signal_something;
    }

You can then connect to the signal using the same syntax used when connecting to >kmm; signals. For instance,

    server.signal_something().connect(
      sigc::mem_fun(client, &Client::on_server_something) );

### Example

This is a full working example that defines and uses custom signals.

Source Code

## Comparison with other signalling systems

(An aside: GTK calls this scheme "signalling"; the sharp-eyed reader with GUI toolkit experience will note that this same design is often seen under the name of "broadcaster-listener" (e.g., in Metrowerks' PowerPlant framework for the Macintosh). It works in much the same way: one sets up `broadcasters`, and then connects `listeners` to them; the broadcaster keeps a list of the objects listening to it, and when someone gives the broadcaster a message, it calls all of its objects in its list with the message. In >kmm;, signal objects play the role of broadcasters, and slots play the role of listeners - sort of. More on this later.)

>kmm; signal handlers are strongly-typed, whereas GTK C code allows you to connect a callback with the wrong number and type of arguments, leading to a segfault at runtime. And, unlike Qt, >kmm; achieves this without modifying the C++ language.

Re. Overriding signal handlers: You can do this in the straight-C world of GTK too; that's what GTK's object system is for. But in GTK, you have to go through some complicated procedures to get object-oriented features like inheritance and overloading. In C++, it's simple, since those features are supported in the language itself; you can let the compiler do the dirty work.

This is one of the places where the beauty of C++ really comes out. One wouldn't think of subclassing a GTK widget simply to override its action method; it's just too much trouble. In GTK, you almost always use signals to get things done, unless you're writing a new widget. But because overriding methods is so easy in C++, it's entirely practical - and sensible - to subclass a button for that purpose.

## >kmm; and Win32

One of the major advantages of >kmm; is that it is crossplatform. >kmm; programs written on other platforms such as GNU/Linux can generally be transferred to Windows (and vice versa) with few modifications to the source.

>kmm; currently works with the MinGW/GCC compiler with a compiler version that supports C++17, such as gcc 7 or 8. It also works with Microsoft Visual C++ 2017 15.7.x or later (including the freely available express/community editions) on the Windows platform. There is an installer available for >kmm; on Microsoft Windows, but as of this writing (October 2020) it has not been updated for a long time. Please be aware that although normally it is fine to mix builds done with Visual Studio 2017 and 2019, please do not do so when building >kmm; with its -mm dependencies.

Refer to the README.win32, as well as the README files in the >kmm;, pangomm and glibmm for instructions on how to build >kmm; on Windows.

## Working with gtkmm's Source Code

If you are interested in helping out with the development of >kmm;, or fixing a bug in >kmm;, you'll probably need to build the development version of >kmm;. However, you should not install a development version over your stable version. Instead, you should install it alongside your existing >kmm; installation, in a separate path.

The easiest way to do this is using jhbuild. jhbuild is a program that makes building GNOME software much easier by calculating dependencies and building things in the correct order. This section will give a brief explanation of how to set up jhbuild to build and install >kmm; from the source repository (git). For up-to-date information on jhbuild, please refer to the jhbuild manual.

Note that to build >kmm; from git, you'll often need to build many of its dependencies from git as well. jhbuild makes this easier than it would normally be, but it will take quite a while to build and install them all. You will probably encounter build problems, though these will usually be corrected quickly if you report them.

gnome-build-meta is an alternative to jhbuild. It is described at the Building system components wiki page, but here we concentrate on jhbuild.

### Setting up jhbuild

To set up jhbuild, follow the basic installation instructions from the jhbuild manual. After you have installed jhbuild, you should copy the sample jhbuild configuration file into your home directory by executing the following command from the jhbuild directory:

    $ cp examples/sample.jhbuildrc ~/.config/jhbuildrc

The >kmm; module is defined in the `gnome-suites-core-deps-latest.modules` moduleset. So edit your `jhbuildrc` file and set your moduleset setting like so:

    moduleset = 'gnome-suites-core-deps-latest'

After setting the correct moduleset, you need to tell jhbuild which module or modules to build. To build >kmm; and all of its dependencies, set `modules` like so:

    modules = [ 'gtkmm' ]

You can build several modules by setting the `modules` variable to a meta-package, e.g. `meta-gnome-core`, or listing more than one module name. The `modules` variable specifies which modules will be built when you don't explicitly specify anything on the command line. You can always build a different moduleset later by specifying it on the commandline (e.g. `jhbuild build gtkmm`).

Setting a prefix

By default, jhbuild's configuration is configured to install all software built with jhbuild under the `~/jhbuild/install` prefix. You can choose a different prefix, but it is recommended that you keep this prefix different from other software that you've installed (don't set it to `/usr`!) If you've followed the jhbuild instructions then this prefix belongs to your user, so you don't need to run jhbuild as `root`.

When you downloaded jhbuild from the git repository, you got a number of `.modules` files, specifying dependencies between modules. By default jhbuild does not use the downloaded versions of these files, but reads the latest versions in the git repository. This is usually what you want. If you don't want it, use the `use_local_modulesets` variable in `.jhbuildrc`.

### Installing and Using the git version of >kmm;

Once you've configured jhbuild as described above, building >kmm; should be relatively straightforward. The first time you run jhbuild, you should run the following sequence of commands to ensure that jhbuild has the required tools and verify that it is set up correctly:

    $ jhbuild bootstrap
    $ jhbuild sanitycheck

#### Installing >kmm; with jhbuild

If everything worked correctly, you should be able to build >kmm; and all of its dependencies from git by executing `jhbuild build` (or, if you didn't specify >kmm; in the `modules` variable, with the command `jhbuild build gtkmm`).

This command will build and install a series of modules and will probably take quite a long time the first time through. After the first time, however, it should go quite a bit faster since it only needs to rebuild files that changed since the last build. Alternatively, after you've built and installed >kmm; the first time, you can rebuild >kmm; by itself (without rebuilding all of its dependencies) with the command `jhbuild buildone gtkmm`.

#### Using the git version of >kmm;

After you've installed the git version of >kmm;, you're ready to start using and experimenting with it. In order to use the new version of >kmm; you've just installed, you need to set some environment variables so that your `configure` or `meson.build` script knows where to find the new libraries. Fortunately, jhbuild offers an easy solution to this problem. Executing the command `jhbuild shell` will start a new shell with all of the correct environment variables set. Now if you re-configure and build your project just as you usually do, it should link against the newly installed libraries. To return to your previous environment, simply exit the jhbuild shell.

Once you've built your software, you'll need to run your program within the jhbuild environment as well. To do this, you can again use the `jhbuild shell` command to start a new shell with the jhbuild environment set up. Alternatively, you can execute a one-off command in the jhbuild environment using the following command: `jhbuild run command-name`. In this case, the command will be run with the correct environment variables set, but will return to your previous environment after the program exits.

## Wrapping C Libraries with gmmproc

>kmm; uses the `gmmproc` tool to generate most of its source code, using .defs files that define the APIs of GObject-based libraries. So it's quite easy to create additional gtkmm-style wrappers of other glib/GObject-based libraries.

This involves a variety of tools, some of them crufty, but at least they work, and has been used successfully by several projects.

### The build structure

Generation of the source code for a gtkmm-style wrapper API requires use of tools such as `gmmproc` and `generate_wrap_init.pl`, which are included in glibmm. In theory you could write your own build files to use these appropriately, but a much better option is to make use of the build infrastructure provided by the mm-common module. To get started, it helps a lot to pick an existing binding module as an example to look at.

For instance, let's pretend that we are wrapping a C library called libsomething. It provides a GObject-based API with types named, for instance, SomeWidget and SomeStuff.

#### Copying the skeleton project

Typically our wrapper library would be called libsomethingmm. We can start by copying the skeleton source tree from the mm-common module. Starting with mm-common 1.0.0 this skeleton application is built with the Meson build system.

      $ git clone https://gitlab.gnome.org/GNOME/mm-common.git
      $ cp -a mm-common/skeletonmm libsomethingmm

This provides a directory structure for the source .hg and .ccg files and the hand-written .h and .cc files, with `meson.build` files that can specify the various files in use, in terms of Meson variables. The directory structure usually looks like this, after we have renamed the directories appropriately:

- `libsomethingmm`: The top-level directory.

  - `libsomething`: Contains the main include file and the pkg-config .pc file.

    - `src`: Contains .hg and .ccg source files.

    - `libsomethingmm`: Contains hand-written .h and .cc files.

As well as renaming the directories, we should rename some of the source files. For instance:

    $ for f in $(find libsomethingmm -depth -name '*skeleton*'); do \
        d="${f%/*}"; b="${f##*/}"; mv "$f" "$d/${b//skeleton/libsomething}"; \
      done

A number of the skeleton files must still be filled in with project-specific content later.

Note that files ending in `.in` will be used to generate files with the same name but without the `.in` suffix, by replacing some variables with actual values during the configure stage.

Generated files are saved in the build tree, which is separated from the source tree when `meson` and `ninja` are used.

#### Modifying build files

Now we edit the files to adapt them to our needs. You might prefer to use a multiple-file search-replace utility for this, such as `regexxer`. Note that nearly all of the files provided with the skeleton source tree contain placeholder text. Thus, the substitutions should be performed globally, and not be limited to the Meson files.

All mentions of `skeleton` should be replaced by the correct name of the C library you are wrapping, such as "something" or "libsomething". In the same manner, all instances of `SKELETON` should be replaced by "SOMETHING" or "LIBSOMETHING", and all occurrences of `Skeleton` changed to "Something".

Likewise, replace all instances of `Joe Hacker` by the name of the intended copyright holder, which is probably you. Do the same for the `joe@example.com` email address.

##### meson.build in the top-level directory

- It is common for binding modules to track the version number of the library they are wrapping. So, for instance, if the C library is at version 1.23.4, then the initial version of the binding module would be 1.23.0. However, avoid starting with an even minor version number as that usually indicates a stable release.

- In the `project()` function, change the license and the C++ version, if necessary.

- You probably need to add more required modules than glibmm and skeleton (libsomething).

##### Other meson.build files

Next we must adapt the other `meson.build` files:

- `skeleton/meson.build`: Perhaps not much to change here more than the global name substitutions.

- `skeleton/skeletonmm/meson.build`

  `defs_basefiles`

    If we have more .defs and docs.xml files, we add them here.

  `hg_ccg_basenames`

    We must mention all of our `.hg` and `.ccg` files here.

  `extra_cc_files, extra_h_files`

    Any additional hand-written `.h` and `.cc` source files go here.

##### Creating .hg and .ccg files

We should now create our first `.hg` and `.ccg` files, to wrap one of the objects in the C library. One pair of example source files already exists: `skeleton.ccg` and `skeleton.hg`. Create copies of these files as necessary.

In the .hg and .ccg files section you can learn about the syntax used in these files.

### Generating the .defs files.

The `.defs` files are text files, in a lisp format, that describe the API of a C library, including its

- objects (GObjects, widgets, interfaces, boxed-types and plain structs)

- functions

- enums

- signals

- properties

- vfuncs

At the moment, we have separate tools for generating different parts of these `.defs`, so we split them up into separate files. For instance, in the `gtk/src` directory of the >kmm; sources, you will find these files:

`gtk.defs`

  Includes the other files.

`gtk_methods.defs`

  Objects and functions.

`gtk_enums.defs`

  Enumerations.

`gtk_signals.defs`

  Signals and properties.

`gtk_vfuncs.defs`

  vfuncs (function pointer member fields in structs), written by hand.

The `skeletonmm/tools/generate_defs_and_docs.sh` script generates all `.defs` files and the `*_docs.xml` file, described in the Documentation section.

#### Generating the methods .defs

This `.defs` file describes objects and their functions. It is generated by the `h2def.py` script which you can find in glibmm's `tools/defs_gen` directory. For instance,

    $ ./h2def.py /usr/include/gtk-4.0/gtk/*.h > gtk_methods.defs

#### Generating the enums .defs

This `.defs` file describes enum types and their possible values. It is generated by the `enum.pl` script which you can find in glibmm's `tools` directory. For instance,

    $ ./enum.pl /usr/include/gtk-4.0/gtk/*.h > gtk_enums.defs

#### Generating the signals and properties .defs

This `.defs` file describes signals and properties. It is generated by the special `generate_extra_defs` utility that is in every wrapping project, such as `gtkmm/tools/extra_defs_gen/`. For instance

    $ cd tools/extra_defs_gen
    $ ./generate_extra_defs > gtk_signals.defs

You must edit the source code of your own `generate_extra_defs` tool in order to generate the `.defs` for the GObject C types that you wish to wrap. In the skeleton source tree, the source file is named `tools/extra_defs_gen/generate_defs_skeleton.cc`. If not done so already, the file should be renamed, with the basename of your new binding substituted for the `skeleton` placeholder. The `tools/extra_defs_gen/meson.build` file should also mention the new source filename.

Then edit the `.cc` file to specify the correct types. For instance, your `main()` function might look like this:

    #include <glibmm_generate_extra_defs/generate_extra_defs.h>
    #include <libsomething.h>
    #include <iostream>

    int main(int, char**)
    {
      something_init();

      std::cout << get_defs(SOME_TYPE_WIDGET)
                << get_defs(SOME_TYPE_STUFF);
      return 0;
    }

#### Writing the vfuncs .defs

This `.defs` file describes virtual functions (vfuncs). It must be written by hand. There is the skeleton file `skeleton/src/skeleton_vfunc.defs` to start from. You can also look at >kmm;'s `gtk/src/gtk_vfuncs.defs` file.

### The .hg and .ccg files

The .hg and .ccg source files are very much like .h and .cc C++ source files, but they contain extra macros, such as `_CLASS_GOBJECT()` and `_WRAP_METHOD()`, from which `gmmproc` generates appropriate C++ source code, usually at the same position in the header. Any additional C++ source code will be copied verbatim into the corresponding .h or .cc file.

A .hg file will typically include some headers and then declare a class, using some macros to add API or behaviour to this class. For instance, >kmm;'s `button.hg` looks roughly like this:

    #include <gtkmm/widget.h>
    #include <gtkmm/actionable.h>
    _DEFS(gtkmm,gtk)
    _PINCLUDE(gtkmm/private/widget_p.h)

    namespace Gtk
    {

    class Button
      : public Widget,
        public Actionable
    {
      _CLASS_GTKOBJECT(Button, GtkButton, GTK_BUTTON, Gtk::Widget, GtkWidget)
      _IMPLEMENTS_INTERFACE(Actionable)
    public:

      _CTOR_DEFAULT
      explicit Button(const Glib::ustring& label, bool mnemonic = false);

      _WRAP_METHOD(void set_label(const Glib::ustring& label), gtk_button_set_label)

      ...

      _WRAP_SIGNAL(void clicked(), "clicked")

      ...

      _WRAP_PROPERTY("label", Glib::ustring)
    };

    } // namespace Gtk

The macros in this example do the following:

`_DEFS()`

  Specifies the destination directory for generated sources, and the name of the main .defs file that `gmmproc` should parse.

`_PINCLUDE()`

  Tells `gmmproc` to include a header in the generated `private/button_p.h` file.

`_CLASS_GTKOBJECT()`

  Tells `gmmproc` to add some typedefs, constructors, and standard methods to this class, as appropriate when wrapping a widget.

`_IMPLEMENTS_INTERFACE()`

  Tells `gmmproc` to add initialization code for the interface.

`_CTOR_DEFAULT`

  Adds a default constructor.

`_WRAP_METHOD()`, `_WRAP_SIGNAL()`, and `_WRAP_PROPERTY()`

  Add methods to wrap parts of the C API.

The .h and .cc files will be generated from the .hg and .ccg files by processing them with `gmmproc` like so, though this happens automatically when using the above build structure:

    $ cd gtk/src
    $ /usr/lib/glibmm-2.68/proc/gmmproc -I ../../tools/m4 --defs . button . ./../gtkmm

Notice that we provided `gmmproc` with the path to the .m4 convert files, the path to the .defs file, the name of a .hg file, the source directory, and the destination directory.

You should avoid including the C header from your C++ header, to avoid polluting the global namespace, and to avoid exporting unnecessary public API. But you will need to include the necessary C headers from your .ccg file.

The macros are explained in more detail in the following sections.

#### m4 Conversions

The macros that you use in the .hg and .ccg files often need to know how to convert a C++ type to a C type, or vice-versa. `gmmproc` takes this information from an .m4 file in your `tools/m4/` or `codegen/m4/` directory. This allows it to call a C function in the implementation of your C++ method, passing the appropriate parameters to that C functon. For instance, this tells `gmmproc` how to convert a GtkTreeView pointer to a Gtk::TreeView pointer:

    _CONVERSION(`GtkTreeView*',`TreeView*',`Glib::wrap($3)')

`$3` will be replaced by the parameter name when this conversion is used by `gmmproc`.

Some extra macros make this easier and consistent. Look in >kmm;'s .m4 files for examples. For instance:

    _CONVERSION(`PrintSettings&',`GtkPrintSettings*',__FR2P)
    _CONVERSION(`const PrintSettings&',`GtkPrintSettings*',__FCR2P)
    _CONVERSION(`const Glib::RefPtr<Printer>&',`GtkPrinter*',__CONVERT_REFPTR_TO_P($3))

#### m4 Initializations

Often when wrapping methods, it is desirable to store the return of the C function in what is called an output parameter. In this case, the C++ method returns void but an output parameter in which to store the value of the C function is included in the argument list of the C++ method. `gmmproc` allows such functionality, but appropriate initialization macros must be included to tell `gmmproc` how to initialize the C++ parameter from the return of the C function.

For example, if there was a C function that returned a GtkWidget* and for some reason, instead of having the C++ method also return the widget, it was desirable to have the C++ method place the widget in a specified output parameter, an initialization macro such as the following would be necessary:

    _INITIALIZATION(`Gtk::Widget&',`GtkWidget*',`$3 = Glib::wrap($4)')

`$3` will be replaced by the output parameter name of the C++ method and `$4` will be replaced by the return of the C function when this initialization is used by `gmmproc`. For convenience, `$1` will also be replaced by the C++ type without the ampersand (&) and `$2` will be replaced by the C type.

#### Class macros

The class macro declares the class itself and its relationship with the underlying C type. It generates some internal constructors, the member `gobject_`, typedefs, the `gobj()` accessors, type registration, and the `Glib::wrap()` method, among other things.

Other macros, such as `_WRAP_METHOD()` and `_WRAP_SIGNAL()` may only be used after a call to a `_CLASS_*` macro.

##### _CLASS_GOBJECT

This macro declares a wrapper for a type that is derived from GObject, but whose wrapper is not derived from Gtk::Object.

`_CLASS_GOBJECT( C++ class, C class, C casting macro, C++ base class, C base class )`

For instance, from `adjustment.hg`:

    _CLASS_GOBJECT(Adjustment, GtkAdjustment, GTK_ADJUSTMENT, Glib::Object, GObject)

##### _CLASS_GTKOBJECT

This macro declares a wrapper for a type whose wrapper is derived from Gtk::Object, such as a widget or dialog.

`_CLASS_GTKOBJECT( C++ class, C class, C casting macro, C++ base class, C base class )`

For instance, from `button.hg`:

    _CLASS_GTKOBJECT(Button, GtkButton, GTK_BUTTON, Gtk::Widget, GtkWidget)

You will typically use this macro when the class already derives from Gtk::Object. For instance, you will use it when wrapping a GTK Widget, because Gtk::Widget derives from Gtk::Object.

You might also derive non-widget classes from Gtk::Object so they can be used without Glib::RefPtr. For instance, they could then be instantiated with `Gtk::make_managed()` or on the stack as a member variable. This is convenient, but you should use this only when you are sure that true reference-counting is not needed. We consider it useful for widgets.

##### _CLASS_BOXEDTYPE

This macro declares a wrapper for a non-GObject struct, registered with `g_boxed_type_register_static()`.

`_CLASS_BOXEDTYPE( C++ class, C class, new function, copy function, free function )`

For instance, from Gdk::RGBA:

    _CLASS_BOXEDTYPE(RGBA, GdkRGBA, NONE, gdk_rgba_copy, gdk_rgba_free)

##### _CLASS_BOXEDTYPE_STATIC

This macro declares a wrapper for a simple assignable struct such as GdkRectangle. It is similar to `_CLASS_BOXEDTYPE`, but the C struct is not allocated dynamically.

`_CLASS_BOXEDTYPE_STATIC( C++ class, C class )`

For instance, for Gdk::Rectangle:

    _CLASS_BOXEDTYPE_STATIC(Rectangle, GdkRectangle)

##### _CLASS_OPAQUE_COPYABLE

This macro declares a wrapper for an opaque struct that has copy and free functions. The new, copy and free functions will be used to instantiate the default constructor, copy constructor and destructor.

`_CLASS_OPAQUE_COPYABLE( C++ class, C class, new function, copy function, free function )`

For instance, from Glib::VariantType:

    _CLASS_OPAQUE_COPYABLE(VariantType, GVariantType, NONE, g_variant_type_copy, g_variant_type_free)

##### _CLASS_OPAQUE_REFCOUNTED

This macro declares a wrapper for a reference-counted opaque struct. The C++ wrapper cannot be directly instantiated and can only be used with Glib::RefPtr.

`_CLASS_OPAQUE_REFCOUNTED( C++ class, C class, new function, ref function, unref function )`

For instance, for Gtk::CssSection:

    _CLASS_OPAQUE_REFCOUNTED(CssSection, GtkCssSection, NONE, gtk_css_section_ref, gtk_css_section_unref)

##### _CLASS_GENERIC

This macro can be used to wrap structs which don't fit into any specialized category.

`_CLASS_GENERIC( C++ class, C class )`

For instance, for Gdk::TimeCoord:

    _CLASS_GENERIC(TimeCoord, GdkTimeCoord)

##### _CLASS_INTERFACE

This macro declares a wrapper for a type that is derived from GTypeInterface.

`_CLASS_INTERFACE( C++ class, C class, C casting macro, C interface struct, Base C++ class (optional), Base C class (optional) )`

For instance, from `celleditable.hg`:

    _CLASS_INTERFACE(CellEditable, GtkCellEditable, GTK_CELL_EDITABLE, GtkCellEditableIface)

Two extra optional parameters were once added, for the case that the interface derives from another interface, which was believed to be the case when the GInterface has another GInterface as a prerequisite. This is a misunderstanding, though. When GInterface A has GInterface B as a prerequisite, it means that every class that implements A shall also implement B. For instance, from `loadableicon.hg` in glibmm-2.4:

    _CLASS_INTERFACE(LoadableIcon, GLoadableIcon, G_LOADABLE_ICON, GLoadableIconIface, Icon, GIcon)

#### Constructor macros

The `_CTOR_DEFAULT()` and `_WRAP_CTOR()` macros add constructors, wrapping the specified `*_new()` C functions. These macros assume that the C object has properties with the same names as the function parameters, as is usually the case, so that it can supply the parameters directly to a `g_object_new()` call. These constructors never actually call the `*_new()` C functions, because >kmm; must actually instantiate derived GTypes, and the `*_new()` C functions are meant only as convenience functions for C programmers.

When using `_CLASS_GOBJECT()`, the constructors should be protected (rather than public) and each constructor should have a corresponding `_WRAP_CREATE()` in the public section. This prevents the class from being instantiated without using a RefPtr. For instance:

    class TextMark : public Glib::Object
    {
      _CLASS_GOBJECT(TextMark, GtkTextMark, GTK_TEXT_MARK, Glib::Object, GObject)

    protected:
      _WRAP_CTOR(TextMark(const Glib::ustring& name, bool left_gravity = true), gtk_text_mark_new)

    public:
      _WRAP_CREATE(const Glib::ustring& name, bool left_gravity = true)

##### _CTOR_DEFAULT

This macro creates a default constructor with no arguments.

##### _WRAP_CTOR

This macro creates a constructor with arguments, equivalent to a `*_new()` C function. It won't actually call the `*_new()` function, but will simply create an equivalent constructor with the same argument types. It takes a C++ constructor signature, and a C function name.

It also takes an optional extra argument:

errthrow

  This tells `gmmproc` that the C `*_new()` has a final GError** parameter which should be ignored.

##### Hand-coding constructors

When a constructor must be partly hand written because, for instance, the `*_new()` C function's parameters do not correspond directly to object properties, or because the `*_new()` C function does more than call `g_object_new()`, the `_CONSTRUCT()` macro may be used in the .ccg file to save some work. The `_CONSTRUCT` macro takes a series of property names and values. For instance, from `button.ccg`:

    Button::Button(const Glib::ustring& label, bool mnemonic)
    :
      _CONSTRUCT("label", label.c_str(), "use_underline", gboolean(mnemonic))
    {}

#### Macros that suppress generation of some code

Some macros suppress the generation of some code when they are used after a `_CLASS_*` macro. Some suppress the definition in the generated .cc file, others suppress both the declaration in the .h file and the definition in the .cc file.

##### _CUSTOM_DEFAULT_CTOR

Suppresses declaration and definition of default constructor in `_CLASS_BOXEDTYPE`, `_CLASS_BOXEDTYPE_STATIC` and `_CLASS_OPAQUE_COPYABLE`.

##### _CUSTOM_CTOR_CAST

Suppresses declaration and definition of the constructor that takes a pointer to the wrapped C object in `_CLASS_BOXEDTYPE` and `_CLASS_BOXEDTYPE_STATIC`.

Suppresses definition of the constructor that takes a pointer to the wrapped C object in `_CLASS_INTERFACE` and `_CLASS_OPAQUE_COPYABLE`.

Suppresses definition of the constructor that takes a pointer to the wrapped C object and the constructor that takes construct_params in `_CLASS_GOBJECT` and `_CLASS_GTKOBJECT`.

##### _CUSTOM_DTOR

Suppresses definition of destructor in `_CLASS_GOBJECT` and `_CLASS_GTKOBJECT`.

##### _CUSTOM_MOVE_OPERATIONS

Suppresses declaration and definition of move constructor and move assignment operator in `_CLASS_GOBJECT` and `_CLASS_GTKOBJECT`.

For example:

    class Derived : public Glib::Object
    {
      _CLASS_GOBJECT(Derived, GDerived, G_DERIVED, Glib::Object, GObject)

      _CUSTOM_MOVE_OPERATIONS

    public:
      Derived(Derived&& src) noexcept;
      Derived& operator=(Derived&& src) noexcept;
      // ...
    };

##### _CUSTOM_WRAP_NEW

Suppresses definition of `Glib::wrap_new()` function in `_CLASS_GOBJECT`.

##### _CUSTOM_WRAP_FUNCTION

Suppresses definition of `Glib::wrap()` function in `_CLASS_GOBJECT` and `_CLASS_GTKOBJECT`.

##### _NO_WRAP_FUNCTION

Suppresses declaration and definition of `Glib::wrap()` function in `_CLASS_GOBJECT`, `_CLASS_BOXEDTYPE`, `_CLASS_BOXEDTYPE_STATIC`, `_CLASS_OPAQUE_COPYABLE`, `_CLASS_INTERFACE` and `_CLASS_GTKOBJECT`.

#### Method macros

##### _WRAP_METHOD

This macro generates the C++ method to wrap a C function.

`_WRAP_METHOD( C++ method signature, C function name)`

For instance, from `entry.hg`:

    _WRAP_METHOD(void set_text(const Glib::ustring& text), gtk_entry_set_text)

The C function (e.g. `gtk_entry_set_text`) is described more fully in the .defs file, and the `convert*.m4` files contain the necessary conversion from the C++ parameter type to the C parameter type. This macro also generates doxygen documentation comments based on the `*_docs.xml` and `*_docs_override.xml` files.

There are some optional extra arguments:

refreturn

  Do an extra `reference()` on the return value, in case the C function does not provide a reference.

errthrow ["<exceptions>"]

  Use the last GError** parameter of the C function to throw an exception. The optional "<exceptions>" is a comma-separated list of exceptions that can be thrown. It determines which @throws Doxygen commands are added to the documentation. Default value is Glib::Error. If you want a comma in the description of an exception, precede it by a backslash. Example: `errthrow "Glib::OptionError Hello\, world, Glib::ConvertError"`

deprecated ["<text>"]

  Puts the generated code in #ifdef blocks. Text about the deprecation can be specified as an optional parameter.

constversion

  Just call the non-const version of the same function, instead of generating almost duplicate code.

newin "<version>"

  Adds a @newin Doxygen command to the documentation, or replaces the @newin command generated from the C documentation.

ifdef <identifier>

  Puts the generated code in #ifdef blocks.

slot_name <parameter_name>

  Specifies the name of the slot parameter of the method, if it has one. This enables `gmmproc` to generate code to copy the slot and pass the copy on to the C function in its final `gpointer user_data` parameter. The `slot_callback` option must also be used to specify the name of the glue callback function to also pass on to the C function.

slot_callback <function_name>

  Used in conjunction with the `slot_name` option to specify the name of the glue callback function that handles extracting the slot and then calling it. The address of this callback is also passed on to the C function that the method wraps.

no_slot_copy

  Tells `gmmproc` not to pass a copy of the slot to the C function, if the method has one. Instead the slot itself is passed. The slot parameter name and the glue callback function must have been specified with the `slot_name` and `slot_callback` options respectively.

Selecting which C++ types should be used is also important when wrapping C API. Though it's usually obvious what C++ types should be used in the C++ method, here are some hints:

- Objects used via RefPtr: Pass the RefPtr as a const reference. For instance, `const Glib::RefPtr<Gtk::FileFilter>& filter`.

- Const Objects used via RefPtr: If the object should not be changed by the function, then make sure that the object is const, even if the RefPtr is already const. For instance, `const Glib::RefPtr<const Gtk::FileFilter>& filter`.

- Wrapping GList* and GSList* parameters: First, you need to discover what objects are contained in the list's data field for each item, usually by reading the documentation for the C function. The list can then be wrapped by a std::vector type. For instance, `std::vector<Glib::RefPtr<Gdk::Pixbuf>>`. You may need to define a Traits type to specify how the C and C++ types should be converted.

- Wrapping GList* and GSList* return types: You must discover whether the caller should free the list and whether it should release the items in the list, again by reading the documentation of the C function. With this information you can choose the ownership (none, shallow or deep) for the m4 conversion rule, which you should probably put directly into the .hg file because the ownership depends on the function rather than the type. For instance:

      #m4 _CONVERSION(`GSList*',`std::vector<Widget*>',`Glib::SListHandler<Widget*>::slist_to_vector($3, Glib::OWNERSHIP_SHALLOW)')

##### _WRAP_METHOD_DOCS_ONLY

This macro is like `_WRAP_METHOD()`, but it generates only the documentation for a C++ method that wraps a C function. Use this when you must hand-code the method, but you want to use the documentation that would be generated if the method was generated.

`_WRAP_METHOD_DOCS_ONLY(C function name)`

For instance, from `recentinfo.hg`:

    _WRAP_METHOD_DOCS_ONLY(gtk_recent_info_get_applications)

There are some optional extra arguments:

errthrow ["<exceptions>"]

  Excludes documentation of the last GError** parameter of the C function. The optional "<exceptions>" is a comma-separated list of exceptions that can be thrown. It determines which @throws Doxygen commands are added to the documentation. Default value is Glib::Error. If you want a comma in the description of an exception, precede it by a backslash. Example: `errthrow "Glib::OptionError Hello\, world, Glib::ConvertError"`

newin "<version>"

  Adds a @newin Doxygen command to the documentation, or replaces the @newin command generated from the C documentation.

voidreturn

  Don't include a @return Doxygen command in the documentation. Useful if the wrapped C function returns a value, but the corresponding C++ method returns void.

##### _IGNORE, _IGNORE_SIGNAL, _IGNORE_PROPERTY

`gmmproc` will warn you on stdout about functions, signals, properties and child properties that you have forgotten to wrap, helping to ensure that you are wrapping the complete API. But if you don't want to wrap some functions, signals, properties or child properties, or if you chose to hand-code some methods then you can use the _IGNORE(), _IGNORE_SIGNAL() or _IGNORE_PROPERTY() macro to make `gmmproc` stop complaining.

    _IGNORE(C function name 1, C function name 2, etc)
    _IGNORE_SIGNAL(C signal name 1, C signal name 2, etc)
    _IGNORE_PROPERTY(C property name 1, C property name 2, etc)

For instance, from `flowbox.hg`:

    _IGNORE(gtk_flow_box_set_filter_func, gtk_flow_box_set_sort_func)
    _IGNORE_SIGNAL(activate-cursor-child, toggle-cursor-child, move-cursor)

##### _WRAP_SIGNAL

This macro generates the C++ libsigc++-style signal to wrap a C GObject signal. It actually generates a public accessor method, such as `signal_clicked()`, which returns a proxy object. `gmmproc` uses the .defs file to discover the C parameter types and the .m4 convert files to discover appropriate type conversions.

`_WRAP_SIGNAL( C++ signal handler signature, C signal name)`

For instance, from `button.hg`:

    _WRAP_SIGNAL(void clicked(),"clicked")

Signals usually have function pointers in the GTK struct, with a corresponding enum value and a `g_signal_new()` in the .c file.

There are some optional extra arguments:

no_default_handler

  Do not generate an `on_something()` virtual method to allow easy overriding of the default signal handler. Use this when adding a signal with a default signal handler would break the ABI by increasing the size of the class's virtual function table, and when adding a signal without a public C default handler.

custom_default_handler

  Generate a declaration of the `on_something()` virtual method in the `.h` file, but do not generate a definition in the `.cc` file. Use this when you must generate the definition by hand.

custom_c_callback

  Do not generate a C callback function for the signal. Use this when you must generate the callback function by hand.

refreturn

  Do an extra `reference()` on the return value of the `on_something()` virtual method, in case the C function does not provide a reference.

deprecated ["<text>"]

  Puts the generated code in #ifdef blocks. Text about the deprecation can be specified as an optional parameter.

newin "<version>"

  Adds a @newin Doxygen command to the documentation, or replaces the @newin command generated from the C documentation.

ifdef <identifier>

  Puts the generated code in #ifdef blocks.

exception_handler <method_name>

  Allows to use custom exception handler instead of default one. Exception might be rethrown by user-defined handler, and it will be caught by default handler.

detail_name <parameter_name>

  Adds a const Glib::ustring& parameter to the signal_something() method. Use it, if the signal accepts a detailed signal name, i.e. if the underlying C code registers the signal with the `G_SIGNAL_DETAILED` flag.

two_signal_methods

  Used in conjunction with the `detail_name` option to generate two signal_something() methods, one without a parameter and one with a parameter without a default value. With only the `detail_name` option one method is generated, with a parameter with default value. Use the `two_signal_methods` option, if it's necessary in order to preserve ABI.

##### _WRAP_PROPERTY

This macro generates the C++ method to wrap a C GObject property. You must specify the property name and the wanted C++ type for the property. `gmmproc` uses the .defs file to discover the C type and the .m4 convert files to discover appropriate type conversions.

`_WRAP_PROPERTY(C property name, C++ type)`

For instance, from `button.hg`:

    _WRAP_PROPERTY("label", Glib::ustring)

There are some optional extra arguments:

deprecated ["<text>"]

  Puts the generated code in #ifdef blocks. Text about the deprecation can be specified as an optional parameter.

newin "<version>"

  Adds a @newin Doxygen command to the documentation, or replaces the @newin command generated from the C documentation.

##### _WRAP_VFUNC

This macro generates the C++ method to wrap a virtual C function.

`_WRAP_VFUNC( C++ method signature, C function name)`

For instance, from `widget.hg`:

    _WRAP_VFUNC(SizeRequestMode get_request_mode() const, get_request_mode)

The C function (e.g. `get_request_mode`) is described more fully in the `*_vfuncs.defs` file, and the `convert*.m4` files contain the necessary conversion from the C++ parameter type to the C parameter type. Conversions can also be written in the .hg file. Virtual functions often require special conversions that are best kept local to the .hg file where they are used.

There are some optional extra arguments:

refreturn

  Do an extra `reference()` on the return value of the `something_vfunc()` function, in case the virtual C function does not provide a reference.

refreturn_ctype

  Do an extra `reference()` on the return value of an overridden `something_vfunc()` function in the C callback function, in case the calling C function expects it to provide a reference.

keep_return

  Keep a copy of the return value in the C callback function, in case the calling C function does not expect to get its own reference.

errthrow

  Use the last GError** parameter of the C virtual function (if there is one) to throw an exception.

custom_vfunc

  Do not generate a definition of the vfunc in the `.cc` file. Use this when you must generate the vfunc by hand.

custom_vfunc_callback

  Do not generate a C callback function for the vfunc. Use this when you must generate the callback function by hand.

ifdef <identifier>

  Puts the generated code in #ifdef blocks.

slot_name <parameter_name>

  Specifies the name of the slot parameter of the method, if it has one. This enables `gmmproc` to generate code to copy the slot and pass the copy on to the C function in its final `gpointer user_data` parameter. The `slot_callback` option must also be used to specify the name of the glue callback function to also pass on to the C function.

slot_callback <function_name>

  Used in conjunction with the `slot_name` option to specify the name of the glue callback function that handles extracting the slot and then calling it. The address of this callback is also passed on to the C function that the method wraps.

no_slot_copy

  Tells `gmmproc` not to pass a copy of the slot to the C function, if the method has one. Instead the slot itself is passed. The slot parameter name and the glue callback function must have been specified with the `slot_name` and `slot_callback` options respectively.

return_value <value>

  Defines a non-default return value.

err_return_value <value>

  Defines a non-default return value, used only if the C++ `something_vfunc()` function throws an exception which is propagated to the C callback function. If return_value is specified, but err_return_value is not, then return_value is used also when an exception is propagated.

exception_handler <method_name>

  Allows to use custom exception handler instead of default one. Exception might be rethrown by user-defined handler, and it will be caught by default handler.

A rule to which there may be exceptions: If the virtual C function returns a pointer to an object derived from GObject, i.e. a reference-counted object, then the virtual C++ function shall return a Glib::RefPtr<> object. One of the extra arguments refreturn or refreturn_ctype is required.

#### Other macros

##### _IMPLEMENTS_INTERFACE

This macro generates initialization code for the interface.

`_IMPLEMENTS_INTERFACE(C++ interface name)`

For instance, from `grid.hg`:

    _IMPLEMENTS_INTERFACE(Orientable)

There is one optional extra argument:

ifdef <identifier>

  Puts the generated code in #ifdef blocks.

##### _WRAP_ENUM

This macro generates a C++ enum to wrap a C enum. You must specify the desired C++ name and the name of the underlying C enum.

For instance, from `enums.hg`:

    _WRAP_ENUM(Orientation, GtkOrientation)

There are some optional extra arguments:

NO_GTYPE

  Use this option, if the enum is not a GType. This is the case when there is no `*_get_type()` function for the C enum, but be careful that you don't just need to include an extra header for that function. You should also file a bug against the C API, because all enums should be registered as GTypes.

  If you specify `NO_GTYPE`, don't use that enum as the type in _WRAP_PROPERTY. It would cause a runtime error, when the generated property_*() method is called.

  For example, from `icontheme.hg`:

      _WRAP_ENUM(IconLookupFlags, GtkIconLookupFlags, NO_GTYPE)

gtype_func <function_name>

  Specifies the name of the `*_get_type()` function for the C enum. Use this parameter if `gmmproc` can't deduce the correct function name from the name of the C enum type.

  For example, from `dbusproxy.hg` in glibmm:

      _WRAP_ENUM(ProxyFlags, GDBusProxyFlags, gtype_func g_dbus_proxy_flags_get_type)

CONV_TO_INT

  "Convertible to int." Generates a plain enum (not an enum class) within a class. Such an enum is scoped like an enum class, but unlike an enum class, it can be implicitly converted to int.

  For example, from `dialog.hg`:

      _WRAP_ENUM(ResponseType, GtkResponseType, CONV_TO_INT)

s#<from>#<to>#

  Substitutes (part of) the name of one or more enum constants. You can add any number of substitutions.

  For example, from `iochannel.hg` in glibmm:

      _WRAP_ENUM(SeekType, GSeekType, NO_GTYPE, s#^SEEK_#SEEK_TYPE_#)

deprecated ["<text>"]

  Puts the generated code in #ifdef blocks. Text about the deprecation can be specified as an optional parameter.

newin "<version>"

  Adds a @newin Doxygen command to the documentation, or replaces the @newin command generated from the C documentation.

##### _WRAP_ENUM_DOCS_ONLY

This macro just generates a Doxygen documentationn block for the enum. This is useful for enums that can't be wrapped with `_WRAP_ENUM()` because they are complexly defined (maybe using C macros) but including the generated enum documentation is still desired. It is used with the same syntax as `_WRAP_ENUM()` and also processes the same options (though NO_GTYPE, gtype_func <function_name> and CONV_TO_INT are ignored because they make no difference when just generating the enum's documentation).

##### _WRAP_GERROR

This macro generates a C++ exception class, derived from Glib::Error, with a Code enum and a code() method. You must specify the desired C++ name, the name of the corresponding C enum, and the prefix for the C enum values.

This exception can then be thrown by methods which are generated from _WRAP_METHOD() with the errthrow option.

For instance, from `pixbuf.hg`:

    _WRAP_GERROR(PixbufError, GdkPixbufError, GDK_PIXBUF_ERROR)

_WRAP_GERROR() accepts the same optional arguments as _WRAP_ENUM() (though CONV_TO_INT is ignored because all exception class enums are plain enums within a class).

##### _MEMBER_GET / _MEMBER_SET

Use these macros if you're wrapping a simple struct or boxed type that provides direct access to its data members, to create getters and setters for the data members.

`_MEMBER_GET(C++ name, C name, C++ type, C type)`

`_MEMBER_SET(C++ name, C name, C++ type, C type)`

For example, in `rectangle.hg`:

    _MEMBER_GET(x, x, int, int)

##### _MEMBER_GET_PTR / _MEMBER_SET_PTR

Use these macros to automatically provide getters and setters for a data member that is a pointer type. For the getter function, it will create two methods, one const and one non-const.

`_MEMBER_GET_PTR(C++ name, C name, C++ type, C type)`

`_MEMBER_SET_PTR(C++ name, C name, C++ type, C type)`

For example, for Pango::Analysis in `item.hg`:

    // _MEMBER_GET_PTR(engine_lang, lang_engine, EngineLang*, PangoEngineLang*)
    // It's just a comment. It's difficult to find a real-world example.

##### _MEMBER_GET_GOBJECT / _MEMBER_SET_GOBJECT

Use these macros to provide getters and setters for a data member that is a GObject type that must be referenced before being returned.

`_MEMBER_GET_GOBJECT(C++ name, C name, C++ type, C type)`

`_MEMBER_SET_GOBJECT(C++ name, C name, C++ type, C type)`

For example, in Pangomm, `layoutline.hg`:

    _MEMBER_GET_GOBJECT(layout, layout, Pango::Layout, PangoLayout*)

#### gmmproc Parameter Processing

`gmmproc` allows processing the parameters in a method signature for the macros that process method signatures (like `_WRAP_METHOD()`, `_WRAP_CTOR()` and `_WRAP_CREATE()`) in a variety of ways:

##### Parameter Reordering

For all the macros that process method signatures, it is possible to specify a different order for the C++ parameters than the existing order in the C function, virtual function or signal. For example, say that the following C function were being wrapped as a C++ method for the Gtk::Widget class:

    void gtk_widget_set_device_events(GtkWidget* widget, GdkDevice* device,
      GdkEventMask events);

However, changing the order of the C++ method's two parameters is necessary. Something like the following would wrap the function as a C++ method with a different order for the two parameters:

    _WRAP_METHOD(void set_device_events(Gdk::EventMask events{events},
      const Glib::RefPtr<const Gdk::Device>& device{device}),
      gtk_widget_set_device_events)

The `{c_param_name}` following the method parameter names tells `gmmproc` to map the C++ parameter to the specified C parameter within the `{}`. Since the C++ parameter names correspond to the C ones, the above could be re-written as:

    _WRAP_METHOD(void set_device_events(Gdk::EventMask events{.},
      const Glib::RefPtr<const Gdk::Device>& device{.}),
      gtk_widget_set_device_events)

Please note that when reordering parameters for a `_WRAP_SIGNAL()` method signature, the C parameter names would always be `p0`, `p1`, etc. because the `generate_extra_defs` utility uses those parameter names no matter what the C API's parameter names may be. It's how the utility is written presently.

##### Optional Parameter Processing

For all macros processing method signatures except `_WRAP_SIGNAL()` and `_WRAP_VFUNC()` it is also possible to make the parameters optional so that extra C++ methods are generated without the specified optional parameter. For example, say that the following `*_new()` function were being wrapped as a constructor in the Gtk::ToolButton class:

    GtkToolItem* gtk_tool_button_new(GtkWidget* icon_widget, const gchar* label);

Also, say that the C API allowed NULL for the function's label parameter so that that parameter is optional. It would be possible to have `gmmproc` generate the original constructor (with all the parameters) along with an additional constructor without that optional parameter by appending a `{?}` to the parameter name like so:

    _WRAP_CTOR(ToolButton(Widget& icon_widget, const Glib::ustring& label{?}),
      gtk_tool_button_new)

In this case, two constructors would be generated: One with the optional parameter and one without it.

##### Output Parameter Processing

With `_WRAP_METHOD()` it is also possible for the return of the wrapped C function (if it has one) to be placed in an output parameter of the C++ method instead of having the C++ method also return a value like the C function does. To do that, simply include the output parameter in the C++ method parameter list appending a `{OUT}` to the output parameter name. For example, if `gtk_widget_get_request_mode()` is declared as the following:

    GtkSizeRequestMode gtk_widget_get_request_mode(GtkWidget* widget);

And having the C++ method set an output parameter is desired instead of returning a SizeRequestMode, something like the following could be used:

    _WRAP_METHOD(void get_request_mode(SizeRequestMode& mode{OUT}) const,
      gtk_widget_get_request_mode)

The `{OUT}` appended to the name of the mode output parameter tells `gmmproc` to place the return of the C function in that output parameter. In this case, however, a necessary initialization macro like the following would also have to be specified:

    _INITIALIZATION(`SizeRequestMode&',`GtkSizeRequestMode',`$3 = (SizeRequestMode)($4)')

Which could also be written as:

    _INITIALIZATION(`SizeRequestMode&',`GtkSizeRequestMode',`$3 = ($1)($4)')

`_WRAP_METHOD()` also supports setting C++ output parameters from C output parameters if the C function being wrapped has any. Suppose, for example, that we want to wrap the following C function that returns a value in its C output parameter rect:

    gboolean gtk_icon_view_get_cell_rect(GtkIconView* icon_view,
      GtkTreePath* path, GtkCellRenderer* cell, GdkRectangle* rect);

To have `gmmproc` place the value returned in the C++ rect output parameter, something like the following `_WRAP_METHOD()` macro could be used:

    _WRAP_METHOD(bool get_cell_rect(const TreeModel::Path& path,
      const CellRenderer& cell, Gdk::Rectangle& rect{>>}) const,
      gtk_icon_view_get_cell_rect)

The `{>>}` following the rect parameter name indicates that the C++ output parameter should be set from the value returned in the C parameter from the C function. `gmmproc` will generate a declaration of a temporary variable in which to store the value of the C output parameter and a statement that sets the C++ output parameter from the temporary variable. In this case it may be necessary to have an `_INITIALIZATION()` describing how to set a Gdk::Rectangle& from a GdkRectangle* such as the following:

    _INITIALIZATION(`Gdk::Rectangle&',`GdkRectangle',`$3 = Glib::wrap(&($4))')

##### String Parameter Processing

A string-valued input parameter in a C++ method is usually a const Glib::ustring& or a const std::string&. In C code it's a const gchar*. When an empty string is converted to const gchar*, it can be converted either to `nullptr` or to a pointer to an empty string (with c_str()). Some parameters in some C functions accept a `nullptr`, and interpret it in a special way. Other parameters must not be `nullptr`.

The default conversion in `_WRAP_METHOD()` and similar macros is

- for mandatory parameters (with or without default values): empty string to empty string,

- for optional parameters (with appended `{?}`): empty string to `nullptr`.

If the default conversion is not the best conversion, append `{NULL}` to a mandatory parameter or `{?!NULL}` to an optional parameter (`!NULL` = not `NULL`). If you append both a C parameter name and `NULL`, separate them with a space: `{c_param_name NULL}`.

#### Basic Types

Some of the basic types that are used in C APIs have better alternatives in C++. For example, there's no need for a gboolean type since C++ has bool. The following list shows some commonly-used types in C APIs and what you might convert them to in a C++ wrapper library.

Basic Type equivalents

C type C++ type
- gboolean bool
- gint int
- guint guint
- gdouble double
- gunichar gunichar
- gchar*Glib::ustring (or std::string for filenames)

### Hand-coded source files

You might want to include additional source files that will not be generated by `gmmproc` from `.hg` and `.ccg` files. You can simply place these in your `libsomething/libsomethingmm` directory and mention them in the `meson.build` in the `extra_h_files` and `extra_cc_files` variables.

### Initialization

Your library must be initialized before it can be used, to register the new types that it makes available. Also, the C library that you are wrapping might have its own initialization function that you should call. You can do this in an `init()` function that you can place in hand-coded `init.h` and `init.cc` files. This function should initialize your dependencies (such as the C function, and >kmm;) and call your generated `wrap_init()` function. For instance:

    void init()
    {
      Gtk::init_gtkmm_internals(); //Sets up the g type system and the Glib::wrap() table.
      wrap_init(); //Tells the Glib::wrap() table about the libsomethingmm classes.
    }

The implementation of the `wrap_init()` method in `wrap_init.cc` is generated by `generate_wrap_init.pl`, but the declaration in `wrap_init.h` is hand-coded, so you will need to adjust `wrap_init.h` so that the `wrap_init()` function appears in the correct C++ namespace.

### Problems in the C API.

You are likely to encounter some problems in the library that you are wrapping, particularly if it is a new project. Here are some common problems, with solutions.

#### Unable to predeclare structs

By convention, structs are declared in glib/GTK-style headers like so:

    typedef struct _ExampleWidget ExampleWidget;

    struct _ExampleWidget
    {
      ...
    };

The extra typedef allows the struct to be used in a header without including its full definition, simply by predeclaring it, by repeating that typedef. This means that you don't have to include the C library's header in your C++ header, thus keeping it out of your public API. `gmmproc` assumes that this technique was used, so you will see compiler errors if that is not the case.

This compiler error might look like this:

    example-widget.h:56: error: using typedef-name 'ExampleWidget' after 'struct'
    ../../libexample/libexamplemm/example-widget.h:34: error: 'ExampleWidget' has a previous declaration here
    make[4]: *** [example-widget.lo] Error 1

or this:

    example-widget.h:60: error: '_ExampleWidget ExampleWidget' redeclared as different kind of symbol
    ../../libexample/libexamplemm/example-widget.h:34: error: previous declaration of 'typedef struct _ExampleWidget ExampleWidget'

This is easy to correct in the C library, so do send a patch to the relevant maintainer.

#### Lack of properties

By convention, glib/GTK-style objects have `*_new()` functions, such as `example_widget_new()` that do nothing more than call `g_object_new()` and return the result. The input parameters are supplied to `g_object_new()` along with the names of the properties for which they are values. For instance,

    GtkWidget* example_widget_new(int something, const char* thing)
    {
            return g_object_new (EXAMPLE_TYPE_WIDGET, "something", something, "thing", thing, NULL);
    }

This allows language bindings to implement their own equivalents (such as C++ constructors), without using the `*_new()` function. This is often necessary so that they can actually instantiate a derived GType, to add their own hooks for signal handlers and vfuncs.

At the least, the `_new()` function should not use any private API (functions that are only in a .c file). Even when there are no functions, we can sometimes reimplement 2 or 3 lines of code in a `_new()` function as long as those lines of code use API that is available to us.

Another workaround is to add a `*_construct()` function that the C++ constructor can call after instantiating its own type. For instance,

    GtkWidget* example_widget_new(int something, const char* thing)
    {
            ExampleWidget* widget;
            widget = g_object_new (EXAMPLE_TYPE_WIDGET, NULL);
            example_widget_construct(widget, "something", something, "thing", thing);
    }

    void example_widget_construct(ExampleWidget* widget, int something, const char* thing)
    {
            //Do stuff that uses private API:
            widget->priv->thing = thing;
            do_something(something);
    }

Adding properties, and ensuring that they interact properly with each other, is relatively difficult to correct in the C library, but it is possible, so do file a bug and try to send a patch to the relevant maintainer.

### Documentation

In general, gtkmm-style projects use Doxygen, which reads specially formatted C++ comments and generates HTML documentation. You may write these doxygen comments directly in the header files.

#### Reusing C documentation

You might wish to reuse documentation that exists for the C library that you are wrapping. GTK-style C libraries typically use gtk-doc and therefore have source code comments formatted for gtk-doc and some extra documentation in .sgml and .xml files. The docextract_to_xml.py script, from glibmm's `tools/defs_gen` directory, can read these files and generate an .xml file that `gmmproc` can use to generate doxygen comments. `gmmproc` will even try to transform the documentation to make it more appropriate for a C++ API.

For instance,

    ./docextract_to_xml.py -s ~/checkout/gnome/gtk/gtk/ > gtk_docs.xml

Because this automatic transformation is not always appropriate, you might want to provide hand-written text for a particular method. You can do this by copying the XML node for the function from your `something_docs.xml` file to the `something_docs_override.xml` file and changing the contents. Alternatively you can write your own documentation in the `.hg` file.

#### Documentation build structure

If you copied the skeleton source tree in mm-common and substituted the placeholder text, then you will already have suitable `meson.build` and `Doxyfile.in` files in the `doc/reference/` directory. You probably need to modify the `tag_file_modules` variable in `meson.build`, though. With the mm-common build setup, the list of Doxygen input files is not defined in the Doxygen configuration file, but passed along from `meson/ninja` to the standard input of `doxygen`.