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

This would be a workaround (at least for the problem described in the question -- not for the actual problem, i.e., when not having control over the definition of C):

class C_base {
public:
    class D { }; // definition of C::D
    // can also just be forward declared, if it needs members of A or A::B
};
class A {
public:
    class B { };
    C_base::D *someField; // need to call it C_base::D here
};
class C : public C_base { // inherits C_base::D
public:
    // Danger: Do not redeclare class D here!!
    // Depending on your compiler flags, you may not even get a warning
    // class D { };
    A::B *someField;
};

int main() {
    A a;
    C::D * test = a.someField; // here it can be called C::D
}