boost::bind/function and C function pointers

From: Thomas Pollinger (tpolling_at_[hidden])
Date: 2003-02-10 16:22:26

Apologies if this topic has already been discussed. I have looked around 
on various mailing lists, in addition I have tried to make it work with 
the current C++ STL standard with std::mem_fun and std::bind1st but with 
no success.

The problem is basically to bind a member function pointer with an 
instance of its declaring class and to assign the result to an existing 
C library function that expects a C style function pointer. This 
certainly works for ordinary C functions and static C++ functions, 
however fails for bound member function pointers.

Here is what I am trying to achieve:

#include <boost/bind.hpp>

// C function declared in some library
struct E;
extern void setCallBack(void (*callback_fct)(E*));

class A {
public:
  void init();
  void callBack(E *e);
  ....
};

void A::init() {setCallBack(boost::bind(&A::callBack, this));}
void A::callBack(E *e) {...}

I tried also using the STL calls, but without success either:
...
setCallBack(std::bind1st(std::mem_fun(&A::callBack), this));

I like the elegant approach of boost::bind and would like to use it for 
this case. The workaround would be to declare a static function and to 
store the instance somewhere to access the other members of the A 
instance - but I'd prefer the other way if there is a solution to make 
it work.

BTW, I am using gcc v 3.2 on Linux/i386 - but I guess that this is a 
more general problem than a specific compiler issue. The error I've 
gotten is

/opt/src/boost/boost/bind.hpp: In instantiation of 
`boost::_bi::result_traits<boost::_bi::unspecified, void (A::*)(E*)>':
/opt/src/boost/boost/bind/bind_template.hpp:16: instantiated from 
`boost::_bi::bind_t<boost::_bi::unspecified, void (A::*)(E*), 
boost::_bi::list1<boost::_bi::value<A*> > >'
controller.cpp:28: instantiated from here
/opt/src/boost/boost/bind.hpp:59: `void (A::*)(E*)' is not a class, 
struct, or union type

Chaning boost::bind(...) to boost::bind<void>(...) to explicitly 
delclare the return type yields:

controller.cpp:28: cannot convert `boost::_bi::bind_t<void, void 
(A::*)(E*),
boost::_bi::list1<boost::_bi::value<A*> > >' to `void (*)(E*)' for argument
`1' to `void setCallBack(void (*)(E*))'

which probably reveals the root of the problem, that of not being able 
to cast a boost function pointer type to a regular C function type.

I tried applying some cast operators, but that failed already in the 
syntactical stage of the parser, e.g. static_cast<void (*)(E*)>.

-Thomas