Re: pointer equality and inheritance

From: Nick Hounsome (nh002_at_blueyonder.co.uk)
Date: 04/03/04 

Date: Sat, 3 Apr 2004 15:28:51 +0100

"sb" <spam_bait101@yahoo.com> wrote in message
news:221dd125.0404011328.4f48de15@posting.google.com...
> Given this relationship,
>
> class Base {
> //...
> };
>
>
> class Derived : public Base {
> //...
> bool myself(const void* p) { return p == this}
> };
>
>
>
> Is there a guarantee in the standard that
>
> Derived d;
> Base* p = &d;
> d.myself((void*)p); // - ?

You dont need the cast

>
> will be always true?

No for the reasons others have given but why would you compare a void
pointer anyway?
Why not do the sensible thing:

d == p;

If you don't actually have a Base* (which makes your example incorrect) then
you MUST
make sure that the void pointer always pointed to the same sort of thing
then you can cast back and compare:

Derived d; // general case derived in complex way
Base b;
Unrelated u; // If you are not storing some unrelated class
            //as well then you should be using Base* not void*
void* p[3] = {static_cast<Base*>(&d), &b, &u };

assert(&d == static_cast<Base*>(p[0]));
assert(&b == static_cast<Base*>(p[1]));
assert(&d != static_cast<Base*>(p[2]));

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