Q240862: FIX: C2555 On Virtual Functions with Covariant Return Types

Article: Q240862
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC kbVC500 kbVC500bug kbVC600 kbVC600bug kbNoUpdate
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 Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

You can receive the following error message if you try to override a virtual
function with a covariant return type:

  error C2555: 'B::function' : overriding virtual function differs
  from'A::function' only by return type or calling convention

Please refer to the sample code in the "More Information" section for details.

CAUSE
=====

The compiler does not support virtual functions with covariant return types as
specified in the C++ Standard, section 10.3, which is quoted below:

  5- The return type of an overriding function shall be either identical to the
  return type of the overridden function or covariant with the classes of the
  functions. If a function D::f overrides a function B::f, the return types of
  the functions are covariant if they satisfy the following criteria:

- both are pointers to classes or references to classes

- the class in the return type of B::f is the same class as the class in the
  return type of D::f or, is an unambiguous direct or indirect base class of
  the class in the return type of D::f and is accessible in D

- both pointers or references have the same cv-qualification and the class type
  in the return type of D::f has the same cv-qualification as or less
  cv-qualification than the class type in the return type of B::f.

STATUS
======

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

This problem was corrected in Microsoft Visual C++ .NET.

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

Steps to Reproduce Behavior
---------------------------

The following sample code demonstrates the error:

  //test.cpp
  // compiler option needed: none

  #include "stdafx.h"

  class A
       {
        public:
  	virtual A* function();
       };
  	
  class B : public A
       {
        public:
  	 B* function();
       };

Additional query words:

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC kbVC500 kbVC500bug kbVC600 kbVC600bug kbNoUpdate 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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