Re: Is void* as key a bad idea?

From:

Kaz Kylheku <kkylheku@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 18 Feb 2010 18:25:55 +0000 (UTC)

Message-ID:

<20100218102018.590@gmail.com>

On 2010-02-18, DeMarcus <use_my_alias_here@hotmail.com> wrote:

Hi,

Would it for any reason be a bad idea to have void* as key in a std::set
to test if a variable has been registered already?

Converting an object pointer to void * is valid, and comparing two
void * pointers for exact equality is also valid. If such a comparison
yields true, it indicates that the pointers are aimed at the same
object.

The drawback (if it may even be considered one) is that you can't
use this mechanism to distinguish an object from a subobject
which is aligned to its base address.

std::set<void*> regSet;

template<typename T>
void add( T* i )
{
    if( regSet.insert( i ).second == false )
       std::cout << "ERROR!" << std::endl;

    /* ... */
}

So here, if you have an object struct s { int i; } so;
you can't add(&so) and add(&so.i): the pointers are to the same address.

If that's not a requirement, you're fine.

Also watch out for pointers going invalid, while remaining in the set,
e.g:

{ { int x; add(&x); } { int y; add(&y); } }

That of course is a separate issue from whether to use void *.