Q148688: FIX: C2243 on Friend Overload o

Article: Q148688
Product(s): Microsoft C Compiler
Version(s): 4.0 4.1 4.2
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbLangCPP kbVC kbVC500fix
Last Modified: 03-AUG-2001

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The information in this article applies to:

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

SYMPTOMS
========

The following error is generated:

  

  error C2243: 'abstract declarator' : conversion from 'class Derived *'
  to 'const class Base<Type> &' exists, but is inaccessible

in code that contains all of the following:

- A template base class that has a friend function that overloads operator
  << for ostream.

- A derived class that privately inherits its base class and has a friend
  function that overloads operator << for ostream where the second
  argument to the operator << is a reference to const type (const Derived
  &).

- A call to the operator << passing a non-const derived type object.

RESOLUTION
==========

Use one of the following two workarounds to resolve the problem:

- Typecast the derived object to a reference to const type when using the
  operator <<.

-or-

- Make the base class public instead of private.

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
-----------

     /* Compile options needed: None
     */ 
     #include <iostream.h>
     template<class T>
     class Base;
     template<class T>
     ostream& operator<<(ostream& s, const Base<T> & a)
     {
        return s;
     }
     template<class T>
     class Base
     {
     public:
        friend ostream& operator<<(ostream& s, const Base<T>& a);
     };
     class Derived;
     ostream& operator<<(ostream& s, const Derived& a)
     {
        return s;
     }
     // Replace 'private' by 'public' in the following line
     // to avoid the error (work around # 2)
     class Derived : private Base<char> {
     public:
        friend ostream& operator<<(ostream& s, const Derived& a);
     };
     void main(void)
     {
        Derived objDerived;
        // The C2243 error is generated for the following line.
        // Uncomment the typecast to avoid the error (work around # 1)
        cout << /*(const Derived&)*/ objDerived;
     }

Additional query words: kbVC400bug

======================================================================
Keywords          : kbCompiler kbCPPonly kbLangCPP kbVC kbVC500fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 4.0 4.1 4.2
Issue type        : kbbug
Solution Type     : kbfix

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