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

From: Thomas Pollinger (tpolling_at_[hidden])
Date: 2003-02-11 01:41:04

>
>
>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:
>
In this particular case, there was unfortunatelly no additional 
parameter to pass private data to the C callback registration function.

>
>// C function declared in some library
>struct E;
>extern void setCallBack(void (*callback_fct)(E*), void* data);
>
>typedef boost::function<void (E*)> 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<callback*>(data))(e); }
>};
>
>void A::init() 
>{
>  cb = new callback(boost::bind(&A::callBack, this, _1));
>  setCallback(&do_callback, cb);
>}
>
>	Doug
>
>  
>
This is a very useful solution that I am likely going to employ if I 
have C function pointers that take also private data. Thanks for the 
suggestion.

Probably more as a site note (curiosity if you will on a language level):

#include <boost/function.hpp>

int someFoo(int val) {return val + 5;}

void someMain() {
    boost::function<int (int)> bptr = someFoo;
    int (*cptr)(int) = someFoo;
    bptr(5);
    cptr(5);
    if (bptr == cptr) printf("equal\n");
}

The above code snipet does not compile as the types of bptr and cptr are 
not the same and thus their values cannot be compared although they are 
identical.

I have not followed the C++ Spec evolution closely and am only familar 
with what appears to be the 98 spec. Are there any plans to uniformize 
this on the language level? This would allow interfacing C style code 
with the nice boost function pointers in a much cleaner way.

Thanks again for your answer,
-Thomas