Unable to link afx_msg in BEGIN_MESSAGE_MAP

Question

Hi, I'm trying to hook into the OnSysCommand function but I'm getting a confusing error.

In the header, I am declaring the function like:

afx_msg void OnSysCommand(UINT nID, LPARAM lParam );

And in the cpp the code is:

BEGIN_MESSAGE_MAP(CMFCTest1App, CWinAppEx)
 ON_COMMAND(ID_APP_ABOUT, &CMFCTest1App::OnAppAbout)
 // Standard file based document commands
 ON_COMMAND(ID_FILE_NEW, &CWinAppEx::OnFileNew)
 ON_COMMAND(ID_FILE_OPEN, &CWinAppEx::OnFileOpen)
 ON_WM_SYSCOMMAND()
END_MESSAGE_MAP()

void CMFCTest1App::OnSysCommand(UINT nID, LPARAM lParam )
{
 AfxMessageBox(L"System command recieved");
}

When compiling I get the following error message:

1> MFCTest1.cpp 1>c:\users\dell3\documents\visual studio 2010\projects\mfctest1\mfctest1\mfctest1.cpp(43): error C2440: 'static_cast' : cannot convert from 'void (__thiscall CMFCTest1App::* )(UINT,LPARAM)' to 'void (__thiscall CWnd::* )(UINT,LPARAM)' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast 1> 1>Build FAILED.

The same thing happens when I try to hook into the OnClose function. Any suggestions would be much appreciated. Thanks.

Answer 1

I think your CMFCTest1App has to inherit from CWnd. Anyhow, it is easier if you use the wizard to create message mappings.

Answer 2

Move the handlers out of your app class and into your window or frame class.

These messages are meant to be handled in a window class (derived from CWnd) and not in your app class (derived from CWinApp).

Answer 3

Your CMFCTest1App class does not have to inherit from CWnd. It's fine the way it is. Also, you can leave your ID_FILE_OPEN, etc command handlers in that class. It is a bit confusing for beginners, but it actually makes sense.

The OnSysCommand() message handler is not one of these exceptions. As others have already mentioned, it belongs to a certain window. The WM_SYSCOMMAND doc. explains in detail what it does.

So, move the command handler to the window it belongs you and you're good.