Q116239: BUG: Function that Inherits Through Dominance Fails

Article: Q116239
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC kbVC100bug kbVC150bug kbVC200bug kbVC400bug kbVC410bug kbVC42
Last Modified: 17-FEB-2002

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), used with:
   - Microsoft C/C++ for MS-DOS 
   - Microsoft Visual C++, versions 1.0, 1.5 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
   - Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

Code that makes use of the C++ implicit invocation of a user-defined conversion
causes the C/C++ compiler to generate the following error message:

  error C2594: '=' : ambiguous conversions from 'class ::E ' to 'int '

C/C++ 9.00 gives the following level 2 warning and errors from the implicit cast
and only the warning from the explicit call:

  warning C4250: 'E' : inherits 'B::operator`int'' via dominance error C2679:
  binary '=' : no operator defined which takes a right- hand operand of type
  'class E' (or there is no acceptable conversion)
  error C2594: '=' : ambiguous conversions from 'class E' to 'int'

NOTE: In Visual Studio .NET, the error message is:

  warning C4250: 'E' : inherits 'B::B::operator`int'' via dominance. error
  C2679: binary '=' : no operator found which takes a right-hand operand of
  type 'E' (or there is no acceptable conversion)

RESOLUTION
==========

Explicitly invoke the user-defined conversion, as demonstrated in the sample
code in the "MORE INFORMATION" section below, rather then depending on the
implicit invocation.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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

Suppose a virtual-base class A is defined and classes B and C inherit from class
A. Furthermore, a class E inherits from both B and C. The two classes, A and B,
each provide a virtual user-defined conversion function to "int". The
relationship may be represented by the following directed acyclic graph:

                        A int () }
                         ^      ^
                        /        \ 
                       /          \ 
                 B { int () }    C {}
                      ^           ^
                        \        / 
                         \      / 
                          E { }

The dominance rule requires that when you invoke the virtual user-defined
conversion function to int from an instance of an object of type E, the version
provided by B should be used. This conversion function may be invoked implicitly
by assigning an instance of an object of type E to an int. It may also be
invoked explicitly through the use of the "operator int" function.

As demonstrated by the following sample code, the compiler accurately identifies
and invokes the correct user-defined conversion function when using the explicit
call syntax. However, when attempting to make use of the implicit conversion,
the compiler produces an error, citing an ambiguity in its search for the
correct user-defined conversion function.

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

  /* Compile options needed: /W3
  */ 
  #include <iostream.h>

  class A
  {
  public:
      virtual operator int () { return 2; }
  };

  class B : virtual public A
  {
  public:
      virtual operator int () { return 3; }
  };

  class C : virtual public A {};

  class E : public B, public C {};

  void main(void)
  {
     int i;
     int error=0;

     E eObject;

     // Both of the invocations below should call B's
     // operator int ().

     if ((i = eObject) != 3)                // Fails to compile
        error = 1;

     if ((i = eObject.operator int()) != 3) // Compiles OK
        error = 1;

     if (!error)
        cout << "PASSED" << endl;
     else
        cout << "FAILED" << endl;
  }

REFERENCES
==========

The Annotated C++ Reference Manual (ARM), Ellis and Stroustrup, section 10.1.1,
"Ambiguities," section 10.2, "Virtual Functions," and section 12.3,
"Conversions."

Additional query words: kbVC400bug 8.00 8.00c 9.00

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Keywords          : kbCompiler kbCPPonly kbVC kbVC100bug kbVC150bug kbVC200bug kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600bug 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,2.0,4.0,4.1,4.2,5.0,6.0
Issue type        : kbbug

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