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

"Alf P. Steinbach" <>
Sat, 22 Sep 2007 18:48:41 +0200

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(); }

    CCallBackFunc generalCallBack( void* arg )
        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 {};

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

Cheers, & hth.,

- Alf

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