FIX: Member Conversion Function Calling Convention Incorrect

WINDOWS   | WINDOWS NT

The information in this article applies to:

• The Microsoft C/C++ Compiler included with:

      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
      - Microsoft Visual C++ 32-bit Edition, version 1.0
  

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

   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

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

   int * m_pinteger;

   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"

   return m_pinteger;

CMyclass::operator int() {

   return *m_pinteger;