Q131066: PRB: C2922 Error for Class Template Containing Friend Function

Article: Q131066
Product(s): Microsoft C Compiler
Version(s): 2.00 2.10
Operating System(s): 
Keyword(s): kberrmsg kbCompiler kbCPPonly kbVC kbprb
Last Modified: 29-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

The compiler generates the following error message when the friend function in a
template class is not prototyped:

  

  Error C2922: 'symbol' a friend function of a template
                class must have been previously defined

CAUSE
=====

Friend functions must be declared before they are used in a template class. You
cannot have a friend function defined within a class definition. This is a
Microsoft Specific requirement. Templates are not officially standardized and,
as a result, different C++ compiler vendors have implemented them differently,
so this requirement may change in future versions of the compiler.


RESOLUTION
==========

Declare the friend function before using it in the template definition.

STATUS
======

This behavior is by design.

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

For more information about the Microsoft-specific implementations on the
templates, please see the Books Online under C/C++, C++ Language Reference,
Chapter 6 Declarations, Template Specifications, Differences from Other
Implementations.

Sample Code to Demonstrate Behavior
-----------------------------------

  /* Compile options : /Ze ( default option )
  */ 

  #include <iostream.h>

  template <class T> class A;   // Forward declaration

  // Change the following line from a comment into an executed line to
  // prevent the compiler error with Visual C++, 32-bit edition, version 2.0
  // or 2.1:

  // template <class T> ostream& operator<< (ostream &s, const A<T> &b);

  template <class T>
  class A {
  public:
     friend ostream & operator << (ostream &s, const A<T> &b);
  };

  template <class T> ostream& operator<< (ostream &s, const A<T> &b)
  {
     return s << "testing";
  }

  void main()
  {
     A<int> fa;
     cout << fa;
  }


Additional query words: 2.00 2.10 8.0 8.00 8.0c 8.00c 9.0 9.00 9.1 9.10

======================================================================
Keywords          : kberrmsg kbCompiler kbCPPonly kbVC kbprb 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 2.00 2.10
Issue type        : kbprb

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

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