Question

Say we have this code

class A {
public:
    A() : x(1) {}
    virtual ~A() {}

    int x;
};

class B {
public:
    B() : y(2) {}
    virtual ~B() {}

    void g()
    {
        cout << "B::" << y << endl;
    }

    int y;
};

class C : private A, private B {
public:
    void f()
    {
        B* p = static_cast<B*>( this );
        p->g();
    }
};

int main()
{
    C c;
    ((B*)&c)->g();

    return 0;
}

The C style cast in the main function cannot be correctly expressed in terms of the C++ casts (static_cast, dynamic_cast, reinterpret_cast). But what is the reason to allow this in the first place? Doesn’t it hurt encapsulation?

UPDATE
This is not a duplicate of the linked question, because this question is about design decisions in C++. It does not ask what I can or cannot do with the language, it asks why certain decisions might have been made.

Answer 1

It’s because in C it was allowed to convert any pointer to any other pointer using this cast and C++ tries to be C-compatible as much as possible, but tries to do a good job to be correct when it comes to classes, so C style cast is stronger than reinterpret_cast in this situation.