Q128344: FIX: Error C2593: ‘operator +’ Is Ambiguous

Article: Q128344
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,4.1,4.2
Operating System(s): 
Keyword(s): kbcode kbtool kbCompiler kbCPPonly kbVC kbVC500fix
Last Modified: 29-JUL-2001

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The information in this article applies to:

- Microsoft C/C++ Compiler (CL.EXE), included with:
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1, 4.0, 4.1, 4.2 
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SYMPTOMS
========

When enumerated types are used in some (+-*/|&^%) operator overloads, the
Microsoft compilers listed above may generate the following message:

  Error C2593: 'operator +' is ambiguous

In Visual C++, 32-bit edition, version 2.1, the following warnings are also
displayed:

  Warning C4387: 'enum Test __cdecl operator+(enum Test, enum Test)': was
  considered

  Warning C4388: and built-in global operator was considered

The "Sample Code" section in this article demonstrates this problem.

CAUSE
=====

Without determining that there is an exact operator overload match, the compiler
is considering the enumerator types promoted to int type. This causes the C2593
ambiguous error.

RESOLUTION
==========

Use one of the following three workarounds:

- With Visual C++ 32-bit Edition, versions 2.x and 4.x, you can work around the
  problem by using a class template. See the "Sample Code to Work Around
  Problem" section in this article. This workaround provides a class template
  that can be used for any enum type. One of the parameters for the overloaded
  operator needs to be of the class template type and the enums need to be
  declared as the class type.

-or-

- You can call the overloaded operator using normal function call syntax. For
  example:

     eTest = operator+( eTest1, eTest2 );

-or-

- You can just use the built-in global operator, and then convert the result
  back to enum type. For example:

     eTest = Test( (int)eTest1 + eTest2 );

  where eTest, eTest1 and eTest2 are of type enum Test{}.

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
================

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

     /* Compiler options needed: None.
     */ 

     enum Test { ONE, TWO, THREE };

     Test operator+(Test eTest, Test eTest2)
     {
        Test   eRet = Test(int(eTest) + int(eTest2));
        return eRet;
     }

     void main()
     {
        Test    eTest=ONE, eTest2=TWO;
        eTest = eTest + eTest2;     // C2593 error
     }

Sample Code to Work Around Problem
----------------------------------

     /* Compiler options needed: None.
     */ 

     enum Test { ONE=1, TWO, THREE };

     template<class T> class CFix
     {
        T m_fix;

      public:

        CFix(T eFix) { m_fix = eFix; }

        operator T() { return  m_fix; }
     };

     Test operator+(CFix<Test> eTest, Test eTest2)
     {
        Test   eRet = Test(int(eTest) + int(eTest2));
        return eRet;
     }

     void main()
     {
        CFix<Test>  eTest=ONE, eTest2=TWO;
        eTest = eTest + eTest2;
     }

REFERENCES
==========

More information about operator overloading can be found in "The Annotated C++
Reference Manual (Jan '94)" section 13.4.

Additional query words: kbVC400bug cpp 9.00 8.00 8.00c 10.00 10.10 10.20

======================================================================
Keywords          : kbcode kbtool kbCompiler kbCPPonly kbVC kbVC500fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,4.1,4.2
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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