Re: Compile time vs runtime?

From:

"Ole Nielsby" <ole.nielsby@tekare-you-a-spammer?logisk.dk>

Newsgroups:

comp.lang.c++

Date:

Thu, 17 May 2007 13:21:23 +0200

Message-ID:

<464c3aaf$0$1445$edfadb0f@dread14.news.tele.dk>

Juha Nieminen wrote:

Pointers and casts in general do not generate runtime code.
The only cast which does is a dynamic upcast. (There might
also be some multiple inheritance cases where downcasting
actually changes the value of the pointer,

You don't need multiple-inheritance to screw things up:

#include <iostream>
class a{char c;};
class b: public a{virtual void whatever(){};};
int main(int argc, char* argv[]) {
    b x;
    std::cout << &x << " " << (a*)&x << "\n";
    return 0;}

but by how much this is changed can, afaik,
be resolved at compile time.)

Except when downcasting to a virtual base class:

#include <iostream>
class a{char c;};
class b: public virtual a {};
class c1: public b {};
class c2: public b {};
class d: public c1, public c2 {};
int main(int argc, char* argv[]) {
    d x;
    b *b1 = (c1*)&x;
    b *b2 = (c2*)&x;
    std::cout << b1 << " " << (a*)b1 << "\n";
    std::cout << b2 << " " << (a*)b2 << "\n";
return 0;}

Here, b1 and b2 point to different objects sharing the same a, so the
offsets differ and have be stored in the vtable.