Appendix B. Signals

# Appendix B. Signals

## Connecting signal handlers

gtkmm 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 gtkmm, 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;
    }

    class some_class
    {
    public:
      some_class
      {
        button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
      }
    private:
      Gtk::Button button {"Hello World"};
    };

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.

A C++ lambda expression is a functor which can be implicitly converted to a `sigc::slot` in the call to `connect()`. A lambda expression can be used instead of `sigc::ptr_fun()`. It's also possible to use a lambda expression instead of `sigc::mem_fun()`, but then you won't get automatic disconnection of the signal handler when a `sigc::trackable`-derived object goes out of scope.

Here's a slightly larger example of slots in action:

    #include <gtkmm/button.h>

    void on_button_clicked()
    {
      std::cout << "Hello World" << std::endl;
    }

    class some_class
    {
    public:
      some_class
      {
        button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked));
        button.signal_clicked().connect(sigc::mem_fun(*this, &some_class::on_button_clicked));
      }
      void on_button_clicked();
    private:
      Gtk::Button button {"Hello World"};
    };

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 `*this`, 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.

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.

Constness |  | Writing signal handlers