allow handling events in a function taking the base class of the event class correspo...

[wxWidgets.git] / docs / doxygen / overviews / app.h

/////////////////////////////////////////////////////////////////////////////
// Name:        app.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/**

@page overview_app wxApp Overview

Classes: wxApp

@li @ref overview_app_intro
@li @ref overview_app_shutdown


<hr>


@section overview_app_intro Introduction

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

@e 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). You call wxApp::SetTopWindow to let wxWidgets know about the top
window.

Note that the program's command line arguments, represented by @e argc and
@e 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
@ref overview_app_shutdown.

An example of defining an application follows:

@code
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);
    SetTopWindow(the_frame);

    return true;
}
@endcode

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 @c wxTheApp pointer which is of type @c wxApp*.


@section overview_app_shutdown 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 @e before wxWidgets cleans up its internal
structures. You should delete all wxWidgets object that you created by the time
OnExit finishes.

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

@code
class MyApp : public wxApp
{
public:
    wxCHMHelpController m_helpCtrl;
    ...
};
@endcode

The reason for that is that @c 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 @e OnExit:

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

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

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

*/