PRB: C2922 Error for Class Template Containing Friend Function (131066)

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.

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;
}

PRB: C2922 Error for Class Template Containing Friend Function (131066)

SYMPTOMS

CAUSE

RESOLUTION

STATUS

MORE INFORMATION

Sample Code to Demonstrate Behavior