Re: Is void* as key a bad idea?

From:

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

Newsgroups:

comp.lang.c++

Date:

Fri, 19 Feb 2010 05:07:27 +0100

Message-ID:

<hll2tt$ujp$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.

Note that the introduction of a virtual destructor in the derived
class is not necessary in order to have this problem, except that with
that it's easier to convince folks since then the assertion fails with
two popular compilers.

How can I solve this to always be safe? Would a base class solve
everything?

struct A
{
   int a;
};

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

struct C : A
{
   virtual ~C() {}
   int c;
}

struct D : B, C
{
   virtual ~D() {}
   int d;
}

int main()
{
   B* b = new B;
   A* a = b;

   assert( a == b );

   C* c = new C;
   a = c;

   assert( a == c );

   D* d = new D;
   a = d;

   assert( a == d );

}

Yes, it's one solution. Here the pointers are all converted up to A*, and
adjusted appropriately (which is one important difference between static_cast
and reinterpret_cast; the latter is not guaranteed to adjust). Another solution
is as mentioned to have all classes polymorphic and use dynamic_cast<void*> to
obtain pointer to most derived object.

Cheers & hth.,

- Alf