Re: Is void* as key a bad idea?

From:

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

Newsgroups:

comp.lang.c++

Date:

Fri, 19 Feb 2010 05:03:50 +0100

Message-ID:

<hll2n4$tkj$1@news.eternal-september.org>

* DeMarcus:

Alf P. Steinbach wrote:

* Leigh Johnston:

All that's needed is inheritance.

Example? And don't say virtual inheritance.

Please quote enough of the article you're responding to to establish
the necessary context for your response. Not all readers have easy
access to the thread history.

Anyway, you're asking for and doubting the existence of this problem:

  #include <assert.h>

  struct A
  {
      int blah;
  };

  struct B: A
  {
      virtual ~B() {}
      int doh;
  };

  int main()
  {
      B* p1 = new B;
      A* p2 = p1;
      void* pv1 = p1;
      void* pv2 = p2;

      assert( pv1 == pv2 ); // Uh oh, not guaranteed.
  }

To some C++ programmers it comes as a surprise.

I had no idea this was the case! I've spent a while trying to find more
to read about this, both in Stroustrup's book and on C++-faq-lite
without luck.

Since the problem is the absence of a guarantee it's pretty difficult to find it
-- looking for what's not there...

And the guarantee that /is/ given is one of the most unknown in C++ programming
because it's designed only for support of C code emulation of inheritance and
because it's flagrantly in contradiction with some earlier general discussion of
reinterpret_cast, namely ?9.2/17, "A pointer to a POD struct, suitably converted
using reinterpret_cast, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides) and vice versa".

As soon as you have inheritance you're leaving POD-land.

I would appreciate if you could direct me to more information.

Except for the above I don't know about sources discussing this, sorry.

To some extent dynamic_cast<void*> is designed as a kind of solution, ?5.2.7/7.

Cheers & hth.,

- Alf