Re: [boost] typeless trampoline

From: Rob Stewart (stewart_at_[hidden])
Date: 2004-09-04 18:21:42

From: Bronek Kozicki <brok_at_[hidden]>
> 
> #include <cassert>
> #include <cstdio>
> #include <utility>
> 
> //  Copyright Bronislaw Kozicki 2003-2004. Use, modification and
> //  distribution is subject to the Boost Software License, Version
> //  1.0. (See http://www.boost.org/LICENSE_1_0.txt )
> 
> #ifndef __COMO__
> #  if defined(_MSC_VER) && (_MSC_VER <= 1310)
> #    define FAULTY
> #  endif    // _MSC_VER
> #endif      // __COMO__
> 
> template <bool> struct static_assert;
> template <> struct static_assert<true> {};
> 
> class trampoline
> {
> private:
>   typedef void(*_pf_t)(void *);
> 
>   template <typename Functor>
>   struct _lever_t;
> 
>   template <typename Functor>
>   struct _lever_t<Functor *>
>   {
>     static_assert<sizeof(Functor*) == sizeof(void *)> size;
> 
>     union convert
>     {
>       Functor* f;
>       void*    p;
>     };

What if a void * and a pmf are not the same size?  You at least
need a static assertion to that effect, right?

>     static void exec(void * p)
>     {
>       convert c;
>       c.p = p;
>       (*c.f)();
>     }
> 
>     static std::pair<_pf_t, void *> init(Functor* f)
>     {
>       convert c;
>       c.f = f;
>       return std::pair<_pf_t, void *>(&exec, c.p);
                                        ^^^^^
&_lever_t::exec

Unfortunately, the type of exec is

   void (_lever_t<Function *>::*)(void *)

not

   void (*)(void *)

as you've declared _pf_t.  Thus, on any compiler that uses a
different function pointer type for pointers to member functions
and regular function pointers, will fail to compile this code.
(I haven't tested to learn if there are any, but even if there
aren't any today, there could be tomorrow.)

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;