Forward declaration of nested types/classes in C++

Question

I recently got stuck in a situation like this:

class A
{
public:
    typedef struct/class {...} B;
...
    C::D *someField;
}

class C
{
public:
    typedef struct/class {...} D;
...
    A::B *someField;
}

Usually you can declare a class name:

class A;

But you can't forward declare a nested type, the following causes compilation error.

class C::D;

Any ideas?

Answer 1

You can't do it, it's a hole in the C++ language. You'll have to un-nest at least one of the nested classes.

Answer 2

I would not call this an answer, but nonetheless an interesting find: If you repeat the declaration of your struct in a namespace called C, everything is fine (in gcc at least). When the class definition of C is found, it seems to silently overwrite the namspace C.

namespace C {
    typedef struct {} D;
}

class A
{
public:
 typedef struct/class {...} B;
...
C::D *someField;
}

class C
{
public:
   typedef struct/class {...} D;
...
   A::B *someField;
}