[boost] boost::bind - cannot convert resulting function object to function pointer

From: Marc Jacobs (marcja_at_[hidden])
Date: 2003-03-04 10:02:42

I'm trying to use a member function as a callback to a C-style library. I've
got the bind working by itself, but the resulting function object does not
convert to the type required by the library. Clearly a boost::bind object is
not a simple function pointer and the type is being checked by the compiler.
Is there any way to coerce the types and still use boost::bind? I have no
control over the declaration or definition of the C-style library. Here's a
sample:

#include <boost/bind.hpp>
#include <iostream>
#include <string>

using namespace boost;
using namespace std;

typedef void (*callback_type)( int i );

// the C-style library function
void caller( callback_type callback )
{
    callback( 501 );
}

// normal usage
void simple_callback( int i )
{
    cout << i << endl;
}

// attempting to use member function as callback
struct member_callback
{
    explicit member_callback( const string & prefix ) :
        prefix_( prefix )
    {}

    void callback( int i )
    {
        cout << prefix_ << i << endl;
    }

private:
    string prefix_;

};

int main( int argc, char * argv[] )
{
    caller( simple_callback ); // works

    member_callback mc( "info: " );
    caller( bind( &member_callback::callback, ref( mc ), _1 ) ); // error!

    return 0;
}

It fails to compile with the following message:

callback.cpp: In function `int main (int, char **)':
callback.cpp:45: cannot convert `boost::_bi::bind_t<void,
boost::_mfi::mf1<void, member_callback, int>,
boost::_bi::list2<boost::reference_wrapper<member_callback>,
boost::arg<1> > >' to `void (*) (int)' for argument `1' to `caller
(void (*) (int))'
make: *** [/tmp/marcja/callback/1.0/dbg/obj/callback.o] Error 1

I'm using gcc 2.96 on Red Hat 7.1 with boost 1.29.0.
Marc