Re: [Boost-Users] boost::bind/function and C function pointers

11 Feb 2003


      On Monday 10 February 2003 04:22 pm, Thomas Pollinger wrote:
...
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 
// 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));
Unfortunately, the major problem with function pointers is that you can't send 
any extra data with them, so there isn't a good way to package up the A 
pointer and send it to setCallback. However, most C callback interfaces also 
store a user-defined void* for callback data, which I'm hoping you omitted 
for brevity. With such a void*, you can do something like this:

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

typedef boost::function callback;

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

private:
  callback* cb;

  // Trampoline function
  static void do_callback(E* e, void* data) 
    { (*static_cast(data))(e); }
};

void A::init() 
{
  cb = new callback(boost::bind(&A::callBack, this, _1));
  setCallback(&do_callback, cb);
}

	Doug

Re: [Boost-Users] boost::bind/function and C function pointers

Douglas Gregor