Question

How to implement casting to a private base class in C++? I don’t want to use hacks such as adding a friend etc. Defining public casting operator does not work.

EDIT :

For example I have:

class A {
//base class
}

class AX : private A {
//a child
}

class AY : private A {
//another specialized child
}

class B {
//base class
void do (A a) {//do
    }
}

class BX : private B {
//a child
void do (AX a) {
     B::do( static_cast <A> (a) );
    }
}

class BY : private B {
//another specialized child
void do (AY a) {
    B::do( static_cast <A> (a) );
    }
}

EDIT2

Why do I do this?

Suppose I have to define some property which is quite heavyweight and can be of several similar types (like VelocityX VelocityY etc). Then I want to be able to have classes which can have any set of these properties. If I want to process these properties, it is obvious, that I’d rather cast them to their base type than to add an implementation for each variation. I do not use public inheritance because it’s better to explicitly cast where needed than to have the private interface implicitly visible. Not a real problem but I’d like to have a solution 🙂

Answer 1

If defining a public casting operator does not work, you can try with a regular function:

class D: private B {
    public:
        B& asB() { return static_cast<B&>(*this); }
};
...
D d;
d.asB().methodInB();
...

Anyway, what is the point? If D derives privately from B, then you are not supposed to use a D as a B from the outside.