Polymorphism: Runtime-Casting Pointers

Question

I am curious as to how casting pointers to derived and base-classes actually works. Here is an example:

struct A {};
struct B : A {};

// Boxing a pointer to an instance of B
void* p = new B();

Now, let´s say I want to access possible members or methods of A through the pointer p.

A* a1 = (A*)p;
A* a2 = (A*)((B*)p);

Which one is correct? Is there a difference at all?
Could you tell me where I can get further information on this subject?

The only subtle diff I see is that in the second cast there is creation of two temp pointer objects whereas in first it's only one pointer object :P — Mr.Anubis, Aug 12 '12 at 13:11
@Mr.Anubis The difference is that the one with "two temp pointer objects" is the correct one. — curiousguy, Aug 13 '12 at 23:53

score 2 · Accepted Answer · edited May 23 '17 at 11:56

In this case, no difference in practice.

But there could be difference if there is multiple inheritance:

#include <cstdio>

struct A { int x; };
struct B { int y; };

struct C : B, A {};

int main() {
    void* c = new C();
    printf("%p\n", c);             // 0x1000
    printf("%p\n", (A*) c);        // 0x1000
    printf("%p\n", (A*) ((C*) c)); // 0x1004
    return 0;
}

or the subclass has a virtual method but the parent does not^[1], including using virtual inheritance^[2].

In terms of the standard, as OP uses C-style cast, which in this case is equivalent to static_cast.

The cast sequence B* → void* → B* → A* is valid, where the first two cast will return back the same pointer as required by §5.2.9[expr.static.cast]/13, and the last cast works as a pointer conversion §4.10[conv.ptr]/3.

However, the cast sequence B* → void* → A* will actually give undefined result, because, well, the result is not defined in §5.2.9/13 ☺.

[1]:

#include <cstdio>

struct A { int x; };
struct C : A { virtual void g() {} };

int main() {
    void* c = new C();
    printf("%p\n", c);              // 0x1000
    printf("%p\n", (A*) c);         // 0x1000
    printf("%p\n", (A*) ((C*) c));  // 0x1008
    return 0;
}

[2]:

#include <cstdio>

struct A { int x; };
struct C : virtual A {};

int main() {
    void* c = new C();
    printf("%p\n", c);              // 0x1000
    printf("%p\n", (A*) c);         // 0x1000
    printf("%p\n", (A*) ((C*) c));  // 0x1008
    return 0;
}

Thanks for your answer. I think I understand what you describe in your post. For my purposes, it will do. — Beta Carotin, Aug 12 '12 at 13:14

Polymorphism: Runtime-Casting Pointers

1 Answers1