static_cast overhead to base class and member of that class

Question

I've got this classes:

class A{};
class B
{
public:
    A* ptr_obj;
    A obj;
    operator A*&()
    {
         return ptr_obj;
    }
    operator A&()
    {
         return obj;
    }
};
class C : public B {};

Now I have a pointer to a obj of type C

C* ptr_c;

And two other pointers, one of type B and one of type A

B* ptr_b;
A* ptr_a;

I want to cast ptr_c and assign it to ptr_a and ptr_b:

ptr_b = static_cast<B*>(ptr_c);
ptr_a = static_cast<A*>(*ptr_c); //calls B::operator A*&()
ptr_a = &static_cast<A>(*ptr_c); //calls B::operator A&()

I cant figure out which is the runtime overhead of the above expressions. I think the compiler can easily optimize this, but can it always do it?

In the first case I just point to the base class so I think there isn't any overhead (maybe the compiler can add some offset to the pointer) but in the others I don't have any single idea of what is really happening.

Why do you believe there should be a _runtime overhead_? `static_cast` is resolved at compile time. — πάντα ῥεῖ, Aug 31 '15 at 09:08
http://stackoverflow.com/questions/6445841/c-static-cast-runtime-overhead — Liuka, Aug 31 '15 at 09:09
Look at the comments below the answer: _"You are casting pointers. A copy of a pointer (which may be optimized away by the compiler anyway) is a negligable operation. I wouldn't try to optimize it"_ — πάντα ῥεῖ, Aug 31 '15 at 09:12
`ptr_a = &static_cast(*ptr_c); //calls B::operator A*&()`This does not compile, what are you trying to achieve with that ? — Drax, Aug 31 '15 at 09:14
@πάντα: The cast itself doesn't have a runtime overhead, the conversion that is triggered by this cast might - especially in cases of user defined conversion operators. — MikeMB, Aug 31 '15 at 09:16
the class C is a resource with a reference count. I want to be able to assign a ptr of type C to one of type A or B and increment the reference count anyway. Of course there aren't raw pointers,, but some kind of handle with template operaotrs, may i post the whole code? — Liuka, Aug 31 '15 at 09:23
@πάνταῥεῖ I know i'm casting between pointers, but in the second assignement I first dereference the pointer and then call a conversion operator. Will this be optimized? — Liuka, Aug 31 '15 at 09:31
@Liuka As already mentioned `ptr_a = static_cast(*ptr_c);` doesn't compile. If so you would need something like `ptr_a = &static_cast (*ptr_c);` . And this would be optimized, yes. — πάντα ῥεῖ, Aug 31 '15 at 09:33
@πάνταῥεῖ on microsoft visual studio 2015 it is compiled correctly — Liuka, Aug 31 '15 at 09:38

static_cast overhead to base class and member of that class

0 Answers0