# wxApp Overview

Classes: wxApp

- Introduction
- Application Shutdown

## Introduction

A wxWidgets application does not have a main procedure; the equivalent is the wxApp::OnInit member defined for a class derived from wxApp.

OnInit will usually create a top window as a bare minimum. Unlike in earlier versions of wxWidgets, OnInit does not return a frame. Instead it returns a boolean value which indicates whether processing should continue (true) or not (false).

Note that the program's command line arguments, represented by argc and argv, are available from within wxApp member functions.

An application closes by destroying all windows. Because all frames must be destroyed for the application to exit, it is advisable to use parent frames wherever possible when creating new frames, so that deleting the top level frame will automatically delete child frames. The alternative is to explicitly delete child frames in the top-level frame's wxCloseEvent handler.

In emergencies the wxExit function can be called to kill the application however normally the application shuts down automatically, see Application Shutdown.

An example of defining an application follows:

    class DerivedApp : public wxApp
    {
    public:
        virtual bool OnInit();
    };

    IMPLEMENT_APP(DerivedApp)

    bool DerivedApp::OnInit()
    {
        wxFrame *the_frame = new wxFrame(NULL, ID_MYFRAME, argv[0]);
        ...
        the_frame->Show(true);

        return true;
    }

Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically create an instance of the application object at the appropriate point in wxWidgets initialization. Previous versions of wxWidgets used to rely on the creation of a global application object, but this is no longer recommended, because required global initialization may not have been performed at application object construction time.

You can also use DECLARE_APP(appClass) in a header file to declare the wxGetApp function which returns a reference to the application object. Otherwise you can only use the global `wxTheApp` pointer which is of type `wxApp*`.

## Application Shutdown

The application normally shuts down when the last of its top level windows is closed. This is normally the expected behaviour and means that it is enough to call wxWindow::Close() in response to the "Exit" menu command if your program has a single top level window. If this behaviour is not desirable wxApp::SetExitOnFrameDelete can be called to change it.

Note that such logic doesn't apply for the windows shown before the program enters the main loop: in other words, you can safely show a dialog from wxApp::OnInit and not be afraid that your application terminates when this dialog -- which is the last top level window for the moment -- is closed.

Another aspect of the application shutdown is wxApp::OnExit which is called when the application exits but before wxWidgets cleans up its internal structures. You should delete all wxWidgets object that you created by the time OnExit finishes.

In particular, do not destroy them from application class' destructor! For example, this code may crash:

    class MyApp : public wxApp
    {
    public:
        wxCHMHelpController m_helpCtrl;
        ...
    };

The reason for that is that `m_helpCtrl` is a member object and is thus destroyed from MyApp destructor. But MyApp object is deleted after wxWidgets structures that wxCHMHelpController depends on were uninitialized! The solution is to destroy HelpCtrl in OnExit:

    class MyApp : public wxApp
    {
    public:
        wxCHMHelpController *m_helpCtrl;
        ...
    };

    bool MyApp::OnInit()
    {
        ...
        m_helpCtrl = new wxCHMHelpController;
        ...
    }

    int MyApp::OnExit()
    {
        delete m_helpCtrl;
        return 0;
    }

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