Re: Is void* as key a bad idea?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Thu, 18 Feb 2010 19:28:45 +0100

Message-ID:

<hlk10r$a3p$1@news.eternal-september.org>

* DeMarcus:

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?

std::set<void*> regSet;

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

Throw an exception. For example, think about using this in a GUI program.

/* ... */
}

Or do you consider the example above fully ok?

The above is generally not OK, even with corrected failure handling.

There are two main cases.

One, you know the type of pointers in the set. Or at least a common interface
type. Then declare the set with that type (not done above).

Or two, the set will contain pointers to objects of different types. Generally
this is only meaningful for class types. Then void* might be suitable but you
need to be sure to store pointers to most derived objects (not done above).

You can use dynamic_cast<void*>(p) to obtain most derived object pointers if
every type is polymorphic. Otherwise you risk not detecting that a pointer to an
object is already in the set. Pointers to the same object but accessed via
different static types may /not/ necessarily yield the same void* pointers.

Cheers & hth.,

- Alf