Re: distinguishing wheter void* ptr points to class A or class B?

From:

"Heinz Ozwirk" <SPAMhozwirk@arcor.de>

Newsgroups:

comp.lang.c++

Date:

Sun, 23 Sep 2007 11:37:53 +0200

Message-ID:

<46f633f4$0$16103$9b4e6d93@newsspool1.arcor-online.net>

"Daniel Kraft" <d@domob.eu> schrieb im Newsbeitrag
news:fd31nq$ctb$1@newsreader2.utanet.at...

  I have some legacy C code which expects a pointer to a function to
be evaluated at one point.
Because of the pointer-to-function vs. pointer-to-member
incompatibility, this needs to be a
global function. To aid the passing of some extra data to the
function, it takes an extra
parameter of type void* which is passed uninterpreted to it.

  I am in a situation where this void* pointer can point either to
class A or to a class B,
which are not related. Is there a way to perform a reliable cast in
the function or otherwise
distinguish which the void* pointer actually points to? This is
compiled as C++.
I can static_cast<> to whatever, but, obviously, if I get the class
wrong, this segfaults
at the first dereference of a member. dynamic_cast<> does not work on
void*.

  Is there a way out? I can modify classes A and B at will and the
function in question,
but its signature must remain intact.

I'm not really sure whether this is valid, but I would try to define some
class C as:

class C
{
public:
virtual int getType() =0;
};

And have A and B inherit from C; then, cast your void* to C* (this is what
I'm not sure about whether it is valid to do), query for the type, and
proceed accordingly.

In practice it probably works, but the original pointer, either to an
instance of A or B, must be cast to C* before the result of this cast is
converted to void*. Otherwise problems might occure when multiple
inheritance is used.

Heinz