Q116487: BUG: No Compiler Error for Ambiguous C++ Conversion

Article: Q116487
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 kbVC151bug kbVC200bug kbVC400bug kbVC410bug kbVC42
Last Modified: 11-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++ for Windows, 16-bit edition, versions 1.0, 1.5 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 4.1, 4.2, 5.0, 6.0 
   - Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

Class A has a member function that converts an instance of class B to an
instance of class A. Class B also has a member function that converts an
instance of class B to an instance of class A. Therefore, when you assign an
instance of class B to an instance of class A, the compiler could use both
conversion methods, resulting in an ambiguity. However, the C/C++ compiler does
not generate an error message in this situation, as demonstrated by the sample
code in the "MORE INFORMATION" section, below.

STATUS
======

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

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

You can use the following sample code to reproduce this problem:

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

  /* Compile options needed: none
  */ 

  #include <iostream.h>

  class B;

  class A
  {
  public:
      // This is the function the compiler
      // chooses to call.
       A(B&) { cout << "called A::A(B&)" << endl; }
  };

  class B
  {
  public:
       operator A()  // This function does not get called.
       {
            B b;
            cout << "called operator B::A()" << endl;
            return b;
       }
  };

  void main()
  {
          B b;
          A a = b; // A(b) or b.operator A()?
                   // The compiler should issue an
                   // error here but instead chooses
                   // to call A::A(B&).

          cout << "failed: should not compile- see ARM 12.3.2" << endl;
  }

REFERENCES
==========

"The Annotated C++ Reference Manual" (ARM), Ellis and Stroustrup, Section
12.3.2, "Conversion Functions."

Additional query words: kbVC400bug 8.00 8.00c 9.00

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Keywords          : kbCompiler kbCPPonly kbVC kbVC100bug kbVC151bug 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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