FIX: C2678 on Overloaded Functions When Using a Namespace

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

SYMPTOMS

The wrong version of an overloaded function is called causing the following compiler two errors:

   error C2678: binary '!=' : no operator defined which takes a left-hand
   operand of type 'const struct Test' (or there is no acceptable
   conversion) (new behavior; please see help)

   error C2664: 'Tester' : cannot convert parameter 1 from 'const struct
   Test' to 'const float &' (new behavior; please see help)

1. The function has a global definition.

2. The function (with the same name and different argument types compared to the global function) is defined in a named namespace.

3. The named Namespace function is included in the global namespace by the “using” declarative.

RESOLUTION

1. Put the function in a named namespace (within the global namespace) and then use the “using” declaration. See workaround #1 below.

2. Specify the “using” declaration at the global level. See workaround #2 below.

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

The following sample code demonstrates the problem of incorrect overloaded function call causing the C2678 and the C2664 compiler errors.

Sample Code

/* Compile options needed: none
*/
      struct Test
      {
      } ;
// Uncomment the following 2 lines for workaround #1
//      namespace Space2
//      {
          int Tester(const Test& rx, const Test& ry)
          {
             return 0 ;
          }

          int operator!=(const Test& rx, const Test& ry)
          {
             return 0 ;
          }
// Uncomment the following line for workaround #1
//      }

      namespace Space1
      {
         int operator!=(const float& rx, const Test& rt)
         {
             return 1 ;
         }

         int Tester(const float& rx, const int& ri)
         {
             return 1 ;
         }
      }

// Uncomment the following line for workaround #2
//      using namespace Space1 ;
      void main()
      {
         const Test a ;
         const Test b ;
         using namespace Space1 ;
         if (a != b) ;  // This line causes the C2678
         Tester(a, b);  // This line causes the C2664
      }

Keywords          : CLIss kbprg
Version           : 4.0 4.1 4.2
Platform          : NT WINDOWS
Issue type        : kbbug
Solution Type     : kbfix