How to separate a template class from its friend template class into different header files?

Question

A file contains template class A and template class B. A is friend of B.

I want to separate them into different files. How to deal with it?

Answer 1

A.h

#if !defined(FILE_A_H)
#define      FILE_A_H

template<class T>
class A
{
  template<class> friend class B;

  // ...
};

#endif

B.h

#if !defined(FILE_B_H)
#define      FILE_B_H

template<class T> class B { /* ... */ };

#endif

Note that if the name of the class that is used in the friend declaration is not yet declared, it is forward declared on the spot (see http://en.cppreference.com/w/cpp/language/friend).

Further details:

• Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
• Why do I get linker errors when I use template friends?
• Templates, C++FAQ