Q168384: PRB: C2248: No Access for Friend Class in a Template Class

Article: Q168384
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kberrmsg kbtool kbCompiler kbCPPonly kbLangCPP kbVC kbVC500 kbVC600kbbuglist
Last Modified: 13-JUN-2002

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The information in this article applies to:

- 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 
- Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

Declaring a template parameter type as a friend class of a template class,
causes the compiler error C2248:

  error C2248: 'xx' : cannot access private member declared in class
  'XX<class T>'

Member access is not granted to friend class T, where T is a template parameter
of the template class XX.

CAUSE
=====

The compiler treats typename T as a regular C++ class and not a template
parameter type. This implies the following:

- typename T, where T is a template parameter type is not granted access to
  private members of the template class.

- A regular C++ class named class T is granted access to private members of the
  template class.

RESOLUTION
==========

Do not declare a template parameter type as a friend of a template class.
Specify the friend class using the regular C++ class name.

STATUS
======

This behavior is by design as per C++ standard section 7.1.5.3 paragraph 2.

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

The following sample code demonstrates the problem.

Sample Code
-----------

    /*
     * Compiler Options: none
     */ 

     template <typename T>
     class Test
     {
        //template parameter T declared as friend class
        friend class T ;
        int m_i ;
     } ;

     class B
     {
     public:
        //Cannot access private member of template class Test
        //since class B is not recognized as friend class of
        //template class Test
        B() { tB.m_i = 0 ;} //C2248 here
        Test<B> tB ;
     } ;

     int main()
     {
        B bObj ;
        return 0 ;
     }

Additional query words:

======================================================================
Keywords          : kberrmsg kbtool kbCompiler kbCPPonly kbLangCPP kbVC kbVC500 kbVC600 kbbuglist
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :5.0,6.0
Issue type        : kbprb

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