Question

Is it legal/advised to do this in C++

//Interface.h
#ifndef INTERFACE_H
#define INTERFACE_H
    #include "WinImplementation.h"
    #include "NixImplementation.h"
    class Interface {
        class WinImplementation;
        class NixImplementation;
    }
#endif

//WinImplementation.h
#ifndef WINIMPLEMENTATION_H
#define WINIMPLEMENTATION_H
    #include "Interface.h"
    class Interface::WinImplementation {}
#endif

//NixImplementation.h
#ifndef NIXIMPLEMENTATION_H
#define NIXIMPLEMENTATION_H
    #include "Interface.h"
    class Interface::NixImplementation {}
#endif

Answer 1

Yes, you can forward declare nested classes in C++. The following example is taken directly from the C++ standard (section 9.7.3):

class E
{
    class I1;     // forward declaration of nested class
    class I2;
    class I1 {};  // definition of nested class
};
class E::I2 {};   // definition of nested class