PRB: C2922 Error for Class Template Containing Friend Function

• The Microsoft C/C++ Compiler (CL.EXE) included with: Microsoft Visual C++, 32-bit Edition, versions 2.00 2.10

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);

   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;