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          


        

Answer 1

If you have access to change the source code of classes C and D, then you can take out class D separately, and enter a synonym for it in class C:

class CD {

};

class C {
public:

    using D = CD;

};

class CD;