C++ nested class/forward declaration issue

Question

Is it possible to forward-declare a nested class, then use it as the type for a concrete (not pointer to/reference to) data member of the outer class?

I.E.

class Outer;

class Outer::MaybeThisWay   // Error: Outer is undefined
{
};

class Outer
{
 MaybeThisWay x;

 class MaybeThatOtherWay;

 MaybeThatOtherWay y;   // Error: MaybeThatOtherWay is undefined
};

Answer 1

You can't forward-declare a nested class like that.

Depending on what you're trying to do, maybe you can use a namespace rather than a class on the outer layer. You can forward-declare such a class no problem:

namespace Outer {
   struct Inner; 
};

Outer::Inner* sweets;  // Outer::Inner is incomplete so 
                       // I can only make a pointer to it

If your Outer absolutely must be a class, and you can't shoe-horn it into a namespace, then you'll need for Outer to be a complete type in the context where you forward declare Inner.

class Outer
{
   class Inner;  // Inner forward-declared
};  // Outer is fully-defined now

Outer yes;  // Outer is complete, you can make instances of it
Outer::Inner* fun;  // Inner is incomplete, you can only make 
                    // pointers/references to it

class Outer::Inner 
{
};  // now Inner is fully-defined too

Outer::Inner win;  // Now I can make instances of Inner too