"Error C2248" complier error when you declare a template parameter type as a friend class of a template class in Visual C++ (168384)

The information in this article applies to:

Microsoft Visual C++, 32-bit Enterprise Edition 5.0
Microsoft Visual C++, 32-bit Enterprise Edition 6.0
Microsoft Visual C++, 32-bit Professional Edition 5.0
Microsoft Visual C++, 32-bit Professional Edition 6.0
Microsoft Visual C++, 32-bit Learning Edition 6.0
Microsoft Visual C++ .NET (2003)
Microsoft Visual C++ .NET (2002)
Microsoft Visual C++ 2005 Express Edition

This article was previously published under Q168384

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 behavior.

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 ;
   }

Modification Type:	Major	Last Reviewed:	1/9/2006
Keywords:	kbtshoot kbCompiler kbCPPonly kberrmsg kbLangCPP kbprb KB168384 kbAudDeveloper