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

"Alf P. Steinbach" <>
Sat, 22 Sep 2007 19:38:42 +0200

On Sep 22, 11:48 am, "Alf P. Steinbach" <> wrote:


On Sep 22, 6:27 am, wrote:

  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

You could simple define a small structure with a type tag and a
struct C {
   int type;
   void *ptr; // to either an object of class A or class B
When you create the structure, modify the type field accordingly so
that it can be read back once you have to determine what class the
objects belong to. After that, it's just a simple static_cast<> of the
'ptr' member.

Uhm, well.

In C++ it's usually ungood to explicitly discriminate on type, when a
virtual function can do that type-discrimination.

Here's one way to Do Things (off the cuff, not compiled):

    typedef void (*CCallbackFunc)( void* );
    typedef void (*CFuncThatCallsBack)( CCallbackFunc, void* );
    void generalCallback( void* );

    struct AbstractCallbackHandler
        virtual void onCallback() = 0;
        void* pointer() { return this; }
        invoke( CFuncThatCallsBack f ) { f( generalCallback, pointer(); }

Seems like you forgot a paren here.



    CCallBackFunc generalCallBack( void* arg )

I think you meant "void", not "CCallBackFunc".


        static_cast<AbstractCallbackHandler*>( arg )->onCallback();

    template< class T > void callbackFor( T& o );

    template< class T >
    struct CallbackHandler
        T* p;
        CallbackHandler( T& o ): p( &o ) {}
        virtual void onCallback() { callbackFor( *p ); }

    // ... code specific to using classes A and B:

    template<> callbackFor<A>( A& aha ) { ... }
    template<> callbackFor<B>( B& boo ) { ... }

    extern "C" void someCFunc( CCallBackFunc callBackFunc, void* arg );

    struct A {};
    struct B {};

And here the struct declarations need to come textually before the
callbackFor specializations.

    void doThingsTo( A& a )
        CallbackHandler<A>( a ).invoke( someCFunc );

All of the code you gave took me a hell of a long time to parse... I
personally would opt for the simple and transparent structure.

As I wrote, discriminating explicitly on type is generally ungood.

For it means that as you introduce or use other types, you'll have to
(remember to) update every place that does such discrimination. And fix
bugs separately each place. Instead of in one central place.

And with explicit discrimination, every place you need the same
mechanism you'll have to code it anew, duplicating the earlier effort.

said, I couldn't have come up with the code you gave in a million
years... it's very clever.

Uh, thanks, but it isn't, really. :-)

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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