Re: Pointer to member function

James Kanze <>
Fri, 27 Jun 2008 03:11:37 -0700 (PDT)
On Jun 26, 4:12 pm, Dan Smithers <> wrote:

I want to implement a C++ wrapper to C code that requires a
function pointer to be passed in. Specifically, I want a
wrapper for pthread that clients can use as a base class.

Is there a way of passing a non-static member function in that
will do this?

No. There's not even a way of passing a static member function.

First, of course, the calling conventions, and even the calling
syntax, of a non-static member function is necessarily different
from that of a static member or a free function; when calling a
non-static member, the compiler must associate an object with
the call. Obviously, there's no way that a C function can do
this. But there's also no guarantee that C and C++ use the same
calling conventions (and I've used compilers where they didn't).
And since the ``extern "C"'' function is going to call the
function whose address you pass it using the C calling
conventions, you can only pass it a pointer to a function which
uses the C calling conventions: a function declared ``extern
"C"'' in C.

In the case of functions like pthread_create, which also take a
void*, be very careful about your conversions to and from void*
as well. Once you've got the void*, the only thing you can do
with it is cast it back to the exact type which was converted to
get it. In particular, if you call pthread_create with a
Derived* as the last argument, then in the start up function,
you must cast it back to Derived*; casting it to Base* results
in undefined behavior.

In the following code, if I use the static k_main then it
builds and runs, but the derived class uses the implementation
in CMyThread.

If I use a virtual member function, I get a compile error:
thread2.cpp:21: error: argument of type ?void*
(CMyThread::)(void*)? does not match ?void* (*)(void*)?

Presumably this is because the member function pointer still
expect the this parameter when it is called.

Member function pointers typically have a completely different
layout from non-member function pointers, precisely because they
must be able to handle virtual functions. (If you take the
address of a virtual &Base::f, and call it through a Derived* p,
(p->*f)(), or through a Base* which actually points to a
Derived, it is Derived::f() which will be called.)

I have thought of an alternative way that requires the
CMyThread constructor to pass the this pointer as an
additional argument that is then used in k_main to call the
member function. It just looks really horrible.

It's the standard procedure. It is, in fact, the exact reason
why the void* is there in pthread_create (and most other
functions of its ilk). Calling a member function requires a
user supplied object. The purpose of the void* is to allow the
user to pass a pointer to an "object" (which, of course, can be
anything the user wants; boost::thread passes the address of a
Boost functional object, for example, and in some very special
cases, I've used it to pass a small integer, with some pretty
hairy reinterpret_casting).

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

class CMyThread

(Just an aside, but a single capital C prefix is the convention
for MFC classes. I'd avoid it unless I was actually
implementing MFC.)

  pthread_t m_thread;
  void *m_args;

  static void *k_main( void *ptr );
  virtual void *m_main( void *ptr );
  CMyThread(char *name);
  virtual ~CMyThread();

CMyThread::CMyThread(char *args)
  : m_args(args)
  int rc = pthread_create( &m_thread, NULL, m_main, (void *)args);

If this compiles, your compiler is broken. And replacing m_main
with k_main shouldn't change anything here. You need a
separate free function:

    extern "C" void*
    threadStarter( void* args )
        static_cast< CMyThread* >( args )->m_main() ;

And since you're dealing with an object, you don't need the
additional args---they can just be member variables of the

Finally, of course, you can't call pthread_create from a
constructor of a base class without encuring race conditions; as
a general rule, you should never call it from the constructor of
a "thread" object which you expect run.


  pthread_join( m_thread, NULL);

pthread_join can block. I'm not sure its a good idea to call it
from a destructor. (Destructors are called during stack
walkback, in case of an exception. Which is generally a context
where you don't want to block for an indefinite time.)


The usual solution for this sort of thing is to separate the
thread from what it does. Not only does it work, but it's a lot
cleaner in the long run. Thus, you might end up with something

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

                            Executable( Executable const& ) ;
        Executable& operator=( Executable const& ) ;

    class Thread
        explicit Thread( Executable* exec ) ;
                            ~Thread() ;
        Executable* join() ;

                            Thread( Thread const& ) ;
        Thread& operator=( Thread const& ) ;

        Executable* myExecutable ;
        pthread_t threadId ;
        bool isJoined ;
    } ;

    // ...
    extern "C"
        void* theExecutable )
        static_cast< Executable >( theExecutable )->run() ;
        return theExecutable ;

        Executable* exec )
        : myExecutable( exec )
        , isJoined( false )
            &threadId, NULL, &executableThreadStarter,
myExecutable ) ;

        assert( isJoined ) ;

        assert( ! isJoined ) ;
        void* dummy ;
        pthread_join( threadId, &dummy ) ;
        return myExecutable ;

(This obviously needs far more error handling.)

Client code then derives from Executable.

James Kanze (GABI Software)
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