Re: Safely casting pointer types, purpose of static_cast, etc.

James Kanze <>
Thu, 5 Jun 2008 02:27:56 -0700 (PDT)
On Jun 5, 1:21 am, ""
<> wrote:


3. If c/d are not guaranteed to be valid pointers,

As I said, they are guaranteed to be valid, but I missed some
important issues in your example.

what is the correct way to do that conversion in a situation
where a void* must be used as an intermediate variable to hold
a pointer to an object (e.g. when passing through a layer of C
code)? For example, when creating a thread with
pthread_create, a void* parameter can be passed to the thread
function. So, then, is the following code guaranteed to always
do what I want on any platform:


class A {
  void CreateThread ();
  void * MyThreadProc_ ();
  static void * SThreadProc_ (void *);

// creates a thread
void A::CreateThread () {
  pthread_t tid;
  // 4th param is void* param to pass to SThreadProc_.
  pthread_create(&tid, NULL, &SThreadProc_, this);

// static thread function calls ((A*)va)->MyThreadProc_();
void * A::SThreadProc_ (void *va) {
  A *a = (A *)va; // <--- is this always safe?
  return a->MyThreadProc_();


First, this won't compile with a compliant compiler. The type
of the third parameter to pthread_create is ``extern "C" void*
(*) (void*)'', and a member function, even static, can never
have a type with ``extern "C"''. You *must* use a free function
for this.

Secondly, as I said in my previous answer, the type you get from
the void* *must* be the same type as you used to create it.
That's not a problem here, but it very much could be if you
derive. A common mistaken idiom is something like:

    class ThreadBase
        virtual ~ThreadBase() {}
        virtual void run() = 0 ;
    } ;

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

    class MyThread : public ThreadBase
        virtual void run() ;
        // ...
    } ;

and then somewhere:

        MyThread t ;
        pthread_t ti ;
        pthread_create( &ti, NULL, &threadStarter, &t ) ;

This does *not* work. Or rather, it is undefined behavior,
which may seem to work in some frequent cases. The last line
must be:

        pthread_create( &ti, NULL, &threadStarter,
                        static_cast< ThreadBase* >( &t ) ) ;

for the behavior to be guaranteed---the cast from void* is to
ThreadBase*, so the void* must have been created from a
ThreadBase*, and not a MyThread*.

James Kanze (GABI Software)
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