What is the difference between "friend struct A;" and "friend A"; syntax?

Question

What is the difference between execution:

struct A; struct B { friend struct A; }; 

and

 struct A; struct B { friend A; }; 

What does it mean to leave a struct in the second part?

Answer 1

The difference is that if you write friend A; , A must be a known type name, that is, it must be declared earlier.

If you write friend struct A; , this is itself an A declaration, so a preliminary declaration is not required:

 struct B { friend struct A; }; // OK 

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However, there are several subtleties. For example, friend class/struct A declares class A in the innermost namespace of class B (thanks to Captain Obvlious ):

 class A; namespace N { class B { friend A; // ::A is a friend friend class A; // Declares class N::A despite prior declaration of ::A, // so ::A is not a friend if previous line is commented }; } 

There are also several other cases where you can only write friend A :

• A is the name of typedef:

   class A; typedef A A_Alias; struct B { // friend class A_Alias; - ill-formed friend A_Alias; }; 

• A is a template parameter:

   template<typename A> struct B { // friend class A; - ill-formed friend A; };