Why does a C++ friend class need a forward declaration only in other namespaces?

问题

Suppose I have a class F that should be friend to the classes G (in the global namespace) and C (in namespace A).

to be friend to A::C, F must be forward declared.
to be friend to G, no forward declaration of F is necessary.
likewise, a class A::BF can be friend to A::C without forward declaration

The following code illustrates this and compiles with GCC 4.5, VC++ 10 and at least with one other compiler.

class G {
    friend class F;
    int g;
};

// without this forward declaration, F can't be friend to A::C
class F;

namespace A {

class C {
    friend class ::F;
    friend class BF;
    int c;
};

class BF {
public:
    BF() { c.c = 2; }
private:
    C c;
};

} // namespace A

class F {
public:
    F() { g.g = 3; c.c = 2; }
private:
    G g;
    A::C c;
};

int main()
{
    F f;
}

To me this seems inconsistent. Is there a reason for this or is it just a design decision of the standard?

回答1:

C++ Standard ISO/IEC 14882:2003(E)

7.3.1.2 Namespace member definitions

Paragraph 3

Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class or function (this implies that the name of the class or function is unqualified) the friend class or function is a member of the innermost enclosing namespace.
// Assume f and g have not yet been defined.
void h(int);
template <class T> void f2(T);
namespace A {
   class X {
   friend void f(X);  //  A::f(X) is a friend
      class Y {
         friend void g();  //  A::g is a friend
         friend void h(int);  //  A::h is a friend
         //  ::h not considered
         friend void f2<>(int);  //  ::f2<>(int) is a friend
      };
   };
   //  A::f, A::g and A::h are not visible here
   X x;
   void g() { f(x); }  // definition of A::g
   void f(X) { /* ... */}  // definition of A::f
   void h(int) { /* ... */ }  // definition of A::h
   //  A::f, A::g and A::h are visible here and known to be friends
}

Your friend class BF; is a declaration of A::BF in namespace A rather than global namespace. You need the global prior declaration to avoid this new declaration.

回答2:

Let's take into account these 3 code lines from your sample:

1. friend class F; // it creates "friend declaration", (that's not the same as ordinary forward declaration

2. class F; // without this forward declaration, F can't be friend to A::C <-- this is ordinary forward declaration

3. friend class ::F; // this is qualified lookup (because of ::), so it needs previous declaration, which you provide in line 2.

C++ standard in paragraph 7.3.1.2, point 3 ( Namespace member definitions) says:

The friend declaration does not by itself make the name visible to unqualified lookup (3.4.1) or qualified lookup (3.4.3). [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship). —end note ]

And line 2 follows exactly what standard requires.

All confusion is because "friend declaration" is weak, you need to provide solid forward declaration for further usage.

回答3:

Because it wouldn't make sense to be able to declare something in the global namespace if you're inside a namespace {} block. The reason friend class BF; works is that it acts like an implicit forward declaration.

来源：https://stackoverflow.com/questions/4492062/why-does-a-c-friend-class-need-a-forward-declaration-only-in-other-namespaces

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c++

namespaces

friend

forward-declaration