PRB: Problems Occur When Defining _AFX_NO_XXX

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, version 4.0

SYMPTOMS

The _AFX_NO_XXX macros are not intended to be defined or undefined by users of MFC. These macros exist only for the purpose of checking whether a particular target platform supports that feature or not. You can write your program to check these macros (for example, #ifndef _AFX_NO_OLE_SUPPORT), but your program should never define or undefine these macros.

If you define _AFX_NO_OLE_SUPPORT, the symptoms might include:

• Application encounters Access Violation on start-up.
• InitInstance is not called in a DLL.

_AFX_NO_AFXCMN_SUPPORT _AFX_NO_CTL3D_RESOURCES _AFX_NO_DAO_SUPPORT _AFX_NO_DB_SUPPORT _AFX_NO_MAPI_RESOURCES _AFX_NO_NESTED_DERIVATION _AFX_NO_OCC_SUPPORT _AFX_NO_OCX_SUPPORT _AFX_NO_OLE_SUPPORT _AFX_NO_RICHEDIT_SUPPORT _AFX_NO_SOCKET_SUPPORT _AFX_NO_SPLITTER_RESOURCES _AFX_NO_SYNC_SUPPORT _AFX_NO_TRACKER_RESOURCES

CAUSE

If you define or undefine any of these macros differently from the way MFC defines them, then your application's compile will be based on a different definition of MFC than the pre-compiled MFC library.

One example of how these macros are used by MFC can be found in AfxWin.h. CCmdTarget has the following section of code in its declaration:

class CCmdTarget : public CObject { ... #ifndef _AFX_NO_OLE_SUPPORT

  // called before dispatching to an automation handler function
  virtual BOOL IsInvokeAllowed(DISPID dispid);

If _AFX_NO_OLE_SUPPORT is defined in your project, then the declaration of CCmdTarget will not include this virtual function. However, the compiled version of CCmdTarget in the MFC library does contain this function. This will result in a v-table mismatch between your code and MFC's code.

RESOLUTION

Don't define or undefine these macros in your own code. They are intended to be tested only. For example, for maximum future portability, if MFC were to eventually target a platform that did not contain OLE support, then you could use _AFX_NO_OLE_SUPPORT as follows:

#ifndef _AFX_NO_OLE_SUPPORT

  DoSomeOLEStuff();

A common place to try to use these macros might be in your pre-compiled header file (for example, StdAfx.h). In this case, you don't need to define these symbols to prevent the including of unneccessary header files, you can simply remove the #include lines.

STATUS

This behavior is by design.