Microsoft KB Archive/148789

= FIX: No Error Given for Non-static Callback Member Function =

Article ID: 148789

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 4.1 Subscription
 * Microsoft Visual C++ 4.2 Enterprise Edition
 * Microsoft Visual C++ 4.2 Professional Edition

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This article was previously published under Q148789



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    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 {  }

Keywords: kbbug kbfix kbprogramming kbvc500fix kbcode kbcompiler KB148789

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