Creating a console for your MFC app's debug output

Sample Image - MFCConsole.gif

It's simply a matter of three function calls to print a message to a console in your MFC application. There are several reasons why you might want to send your debugging output to a console. Mine were, that 1st, I didn't have an IDE on the PC where I tested the debug-builds of my app and 2nd, the Mainframe was not always visible, which made matters worse.

A last word of warning beforehand: Closing the console window will exit your application! Let's go:

Creating the console

To create a console window you need to call AllocConsole(). This function does not take any parameters and returns a BOOL, indicating whether the console has been created or not. You should create the console sometime in the beginning of your code. In the sample I've put the code into the CWinApp::OnInitInstance() override. Like this:

BOOL CSmplConsoleApp::InitInstance()
{
	// details omitted

	// allocate a console
#ifdef _DEBUG
	if (!AllocConsole())
		AfxMessageBox("Failed to create the console!", MB_ICONEXCLAMATION);
#endif
	return TRUE;
	
	//	and then create the frame
	pFrame->LoadFrame(IDR_MAINFRAME,
		WS_OVERLAPPEDWINDOW | FWS_ADDTOTITLE, NULL,
		NULL);

	// The one and only window has been initialized, so show and update it.
	pFrame->ShowWindow(SW_SHOW);
	pFrame->UpdateWindow();
}

It is important that you create your console before you create the main window. If you don't, the console will still be created, but your debugging messages send with _cprintf() will not arrive in your console. I couldn't figure out why it is like this, only that it is like this. If somebody has an explanation, please let me know.

Writing to the console

Send your output to the console via _cprintf(), which is prototyped like this:

int _cprintf( const char *format [, argument] ... );

The format has the same form and function as the format parameter for the printf() function. Please look up the details in your documentation. And don't forget to #include <conio.h> wherever you use _cprintf().

The CChildView::OnPaint() override in the demo application looks like this:

void CChildView::OnPaint() 
{
	CPaintDC dc(this); // device context for painting
	
	// TODO: Add your message handler code here
#ifdef _DEBUG
	static int nCallCounter = 0;
	nCallCounter++;
	_cprintf("Window painted now %i time(s)\n", nCallCounter);
#endif
	// Do not call CWnd::OnPaint() for painting messages
}

Getting rid of the console

Somewhere near the end of your program you should call FreeConsole() to free the console <g>. Again, this function does not take any parameters and returns a BOOL indicating success or failure. Here is what the CWinApp::ExitInstance() override in the demo project looks like:

int CSmplConsoleApp::ExitInstance() 
{
	//	deallocate console
#ifdef _DEBUG
	if (!FreeConsole())
		AfxMessageBox("Could not free the console!");
#endif
	return CWinApp::ExitInstance();
}

Hope that helps!

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