Q102619: FIX: Member Conversion Function Calling Convention Incorrect

Article: Q102619
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVCkbbuglist kbfixlist
Last Modified: 26-JUL-2001

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The information in this article applies to:

- Microsoft C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, versions 1.0, 1.5 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 1.0 
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SYMPTOMS
========


After compiling an application with Microsoft C/C++ for MS-DOS, an attempt to
link the application fails and Microsoft LINK generates the following message:

  error L2029: 'public: __near __pascal CMyclass::operator`int const __near*'
  (void)const __near' : unresolved external

After compiling an application with Microsoft C/C++ 32-bit Edition, an attempt to
link the application fails and Microsoft LINK generates the following messages:

  warning LNK4016: unresolved external symbol "??BCMyclass@@QBGPBHXZ (public:
  __stdcall CMyclass::operator`int const *' (void)const )" error LNK1120: 1
  unresolved externals

CAUSE
=====

Microsoft C/C++ compiler version 8.0 and 8.0c for MS-DOS and Windows always uses
the __cdecl calling convention for functions declared as follows:

     <classname>::operator const <type-specifier> *() const

However, if the compiler command line includes the /Gc compiler option switch,
the function has the __pascal calling convention. Normally, every member
function of a class uses the __pascal calling convention without regard to the
presence of the /Gc compiler option switch.

For example, if you build an application with the Microsoft Foundation Class
Library and refer to the function CString::operator const char *() const, an
L2029 error occurs if you specify the /Gc compiler option switch. Because the
Class Library was built without the /Gc option, the conversion function in the
library has the __cdecl calling convention. If you build your application with
the /Gc option switch, it attempts to call the conversion function with the
__pascal calling convention.

A similar problem occurs in Microsoft C/C++ 32-bit Edition if you specify the /Gz
compiler option switch. Normally, each class member function uses the thiscall
calling convention without regard to the presence of the /Gz compiler option
switch. However, if the compiler command line specifies the /Gz compiler option
switch, the conversion function uses the __stdcall calling convention.

RESOLUTION
==========

In Microsoft C/C++ version 8.0 for MS-DOS and Windows, modify the compiler
command line to remove the /Gc compiler option switch. Doing so compiles the
conversion function with the __cdecl calling convention. In Microsoft C/C++
version 8.0 (32-bit), modify the compiler command line to remove the /Gz
compiler option switch. Doing so compiles the conversion function with the
thiscall calling convention.

STATUS
======

Microsoft has confirmed this to be a bug in the products listed at the beginning
of this article. This bug was corrected in Visual C++ version 2.0.

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

The code example below demonstrates this problem. Recall that because member
functions use the __pascal or thiscall calling convention, compiling MAIN.CPP
with the /Gc or /Gz switches specified or not should not make any difference.
However, the example shows that a problem does occur.

The code below demonstrates the problem. Recall that member functions use the
__pascal or thiscall calling convention, and therefore there should be no
difference between using /Gc or /Gz with MAIN.CPP and not using /Gc or /Gz.
However, the sample shows that there is a problem.

Sample Code
-----------

  // TEST.H

  class CMyclass
  {
     int * m_pinteger;
  public:
     operator const int *() const;
     operator int();
  };

  // MAIN.CPP

  /*
   * Compiler options needed: /Gc for C/C++ for MS-DOS and Windows
   *                          /Gz for C/C++ for Windows NT
   */ 

  #include "test.h"
  void __cdecl main(void)
  {
     CMyclass obj;
     const int *y = obj;
     const int z = obj;
  }

  // OTHER.CPP

  /*
   * Compiler options needed: None (omit /Gc and /Gz)
   */ 

  #include "test.h"

  CMyclass::operator const int *()const
  {
     return m_pinteger;
  }

  CMyclass::operator int()
  {
     return *m_pinteger;
  }

Additional query words: 1.00 1.50 8.00 8.00c

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC kbbuglist kbfixlist
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5
Issue type        : kbbug
Solution Type     : kbfix

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