Template Parameter friends

(Reader Level : Intermediate)
(Knowledge assumptions : templates, friend)

The problem:
Currently, the C++ standard does not allow you to write code like this:

template<class T1, class T2>

class MyClass
{
  friend class T1;
  friend class T2;
};

You can’t make the template parameter of a class be a friend of that particular class. This might seem a reasonable requirement but the language does not allow you to do this, at least not directly.

The workaround:
In order to accomplish what we require here’s a quick workaround.

For the GCC compiler:-

#define MAKE_FRIEND(T) friend class MakeFriend<T>::Result

template<class T>
class MakeFriend
{
public:
  typedef T Result;
};

template<class T1, class T2>
class MyClass
{
 MAKE_FRIEND(T1);
 MAKE_FRIEND(T2);
};

For Microsoft's compiler:-

#define MAKE_FRIEND(T) friend MakeFriend<T>::Result

template<class T>
class MakeFriend
{
public:
  typedef T Result;
};

template<class T1, class T2>
class MyClass
{
 MAKE_FRIEND(T1);
 MAKE_FRIEND(T2);
};

Note the use of the 'class' keyword in the MAKE_FRIEND macro for the GCC compiler but not for the Microsoft compiler. If either T1 or T2 is not a class, then your compiler will complain. For example, if you try to instantiate an object with:

MyClass<SomeClass, int> myClassObj;

then the GCC compiler will say error: invalid type ‘int’ declared ‘friend’. But that makes sense, doesn’t it?

The code above has been tested on GCC 3.4.5 and on Microsoft Visual Studio 2008 (using the Microsoft Compiler).