DOCUMENT:Q148789  03-AUG-2001  [visualc]
TITLE   :FIX: No Error Given for Non-static Callback Member Function
PRODUCT :Microsoft C Compiler
PROD/VER:4.1 4.2
OPER/SYS:
KEYWORDS:kbcode kbProgramming kbCompiler kbVC410bug kbVC420bug kbVC500fix

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The information in this article applies to:

 - The C/C++ Compiler (CL.EXE), included with:
    - Microsoft Visual C++, 32-bit Editions, version 4.1 
    - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
    - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

The compiler does not generate any compiler errors when a non-static member
function is used as a callback function and the calling convention for the
function is explicitly specified. For example, the sample code shown in this
article will not cause the compiler to generate any compiler errors although it
should.

Debugging the non-static member callback function will reveal the stack is
corrupted. Any parameters to the function, including the implied this pointer
may be invalid.

RESOLUTION
==========

By design, a member function used as a callback function must be static. A
non-static member function cannot be used as a callback function. You can also
use file scope global functions for callback functions.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ version 5.0.

MORE INFORMATION
================

Using a non-static member function as a callback function should trigger the
compiler to generate the following error when the callback function is invoked:

   test.cpp(line#): error C2643: illegal cast from pointer to member.

In C++ every callback function has its own prototype, which determines the
parameters that the calling function can pass to the function. However, C++
member functions have a hidden parameter known as the 'this' pointer. C++ is
able to associate a function with a particular instance of an object by means of
the 'this' pointer. Therefore, if a callback function, defined as a non-static
member function, is called, it will expect a particular instance of an object.
On the first line of the Test::Test_Func() member function in the following
sample code, there is no object instance when the callback function is invoked,
so a C2643 compiler error should be generated.

For more information on writing C++ callback functions, please see the article
"Calling All Members: Member Functions as Callbacks" on the Microsoft
Development Network (MSDN) CD-ROM.

Sample Code to Reproduce Problem
--------------------------------

      /* Compile options needed: default console application
      */ 
      #include <stdio.h>
      typedef void (__cdecl *lpCallback)();
      void Function1(lpCallback)
      {
      }
      class Test
      {
       public:
         void _cdecl BadCallBack();       // Non-static callback function
         static void _cdecl GoodCallBack(); // Static callback function
         void Test_Func();
      };
      void Test::Test_Func()
      {
        Function1(BadCallBack); // Line 20 : compiler doesn't generate C2643
        Function1(GoodCallBack);
      }
      void Test_Func()
      {
        Function1(Test::BadCallBack);  // Compiler correctly generates C2643
        Function1(Test::GoodCallBack);
      }
      void main()
      {
      }

Additional query words:

======================================================================
Keywords          : kbcode kbProgramming kbCompiler kbVC410bug kbVC420bug kbVC500fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 4.1 4.2
Issue type        : kbbug
Solution Type     : kbfix

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