Re: c++ casting issues

James Kanze <>
Thu, 20 Mar 2008 02:49:14 -0700 (PDT)
On Mar 20, 2:10 am, Christopher <> wrote:

On Mar 19, 4:48 pm, Juha Nieminen <nos...@thanks.invalid> wrote: wrote:
  Don't use void* at all. You are only asking for problems.

I disagree. Sometimes it is now avoidable to use a void *,
especially when using the Windows API as they force you to in
some of thier function calls.

Not just Windows. A lot of the Posix API uses void* as well.
Whenever there is a callback in a C API, it is traditional to
add void* for user data---typically, in C++, this will be a
pointer to a class, and the `extern "C"' function which we pass
to the callback will convert the pointer back to the class
type, and call a member function on it.

In such cases, it is very, very important to explicitly cast the
actual pointer to the type used in the callback function before
passing it. A typical scenario might be (using pthread_create,
from the Posix interface):

    extern "C" void* threadStarter( void* p )
        return static_cast< ThreadBase* >( p )->run() ;

    // ...

    class MyThread : public ThreadBase
        // ...
    } ;

    // ...
        MyThread newThread ;
        pthread_create( &threadId, NULL, threadStarter,
                        static_cast< ThreadBase* >( &newThread ) ) ;

Without the static_cast in the call, the code has undefined
behavior. (In this particular case, of course, and even better
solution would be to use boost::threads:-). But it serves as a
good example.)

It is often necessary to use void * and a reinterpret_cast to
and from.

Not reinterpret_cast, static_cast.

It is frowned upon and should be avoided _if possible_.

Obviously, if you don't have a choice, you don't have a choice.

James Kanze (GABI Software)
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