Dear Experts,

Consider:

struct A {
  virtual ~A() = default;
  ....
};

struct B: A {
  ....
};


class C {
  struct D: B {
    ....
  };

public:
  D* d; // Or unique_ptr<D> etc.
  ....
};


Let's say that C is declared in C.hpp. I could forward-declare D in that header:

class C {
  struct D;
public:
  D* d;
};


Question: has the idea of a "partial forward declaration" ever been considered, where
D is declared as a struct that inherits from B, but with the body omitted? E.g.:

class C {
  struct D: B;
public:
  D* d;
};


The aim is to make it possible for a user of C to use the methods defined in A and B,
while keeping the details of D hidden.

Is this implementable? I could imagine that there could be issues with multiple inheritance.

Of course an alternative is:

class C {
  struct D;
public:
  B* d;
};

The disadvantage of this is that within the implementation of C, where I know that d is
actually a D*, I would constantly need to cast it.

This idea of partial forward declarations has occurred to me before with slightly
different motivations.

Thoughts anyone?

Phil.