wxApp Overview

Table of Contents

• Application Shutdown

A wxWidgets application does not have a main function; the equivalent, i.e.

the entry point where the execution of the program begins, is the wxApp::OnInit() member function defined in a class derived from wxApp (this class is typically specified using wxIMPLEMENT_APP() macro).

OnInit usually creates the main application window and returns true. If it returns false, the application will exit immediately, without starting to run.

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

Note the use of wxIMPLEMENT_APP(), which defines the application entry point (either main() or WinMain() function, depending on the platform) and tells wxWidgets which application class should be used.

You can also use wxDECLARE_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;
}