Re: [Boost-users] bind member function of a member of a class...

Subject: Re: [Boost-users] bind member function of a member of a class...
From: Archie14 (admin_at_[hidden])
Date: 2009-03-04 18:52:44

Alan M. Carroll <amc <at> network-geographics.com> writes:

> 
> That's not a good usage scenario for bind - you should just call the method 
directly. Bind is useful when you
> want to either (1) pass the functor to something else or (2) store the 
functor for future use.
> Conceptually, as a rough guide, you should think of the result of bind as a 
pointer, not a value. So,
> naturally, the result of bind of anything isn't going to be equal to 1, just 
like you won't have a pointer
> equal to one. Instead you want to compare the result of invoking / 
dereferencing the functor, which is done
> with the function operator (). So your example would work if you added that, 
e.g.
> 
> return (boost::bind (&B::getA::test, b)() ==1);
> 
> But a more standard use case would look like (untested)
> 
> template < typename F > bool check_property(F const& func, B const& b) {
>   return func(b);
> }
> 
> bool testbind() {
>   B b;
>   return check_property(boost::bind(&A::test, boost::bind(&B::getA, _1)) == 
1, b);
> }
> 
> The == operator of a bind result and a int creates a functor that compares 
the result of invoking the functor
> to that int. This passes that to check_property which invokes it with the 
function operator.
> 
> P.S. If you want to store a Bind result for later use, look at 
Boost.Function. These two libraries are duals
> of each other - Bind to *pass* function objects to somebody else, Function to 
*receive* function objects
> from somebody else.
> 
> At 04:33 PM 3/4/2009, you wrote:
> >I cannot figure out how to use boost::bind in following scenario:
> >
> >class A
> >{
> >  int i_;
> >public:
> >  int test() {return i;}
> >};
> >
> >class B
> >{
> >   A _a;
> >public:
> >   A& getA() {return _a;}
> >}
> >
> >bool testbind()
> >{
> >  B b;
> > 
> >  // here I am trying to find out if b.getA().test() equals 1
> >  // I understand that B::getA::test is incorrect, and that's my question-
> >  // how to use bind here?
> >  return (boost::bind (&B::getA::test, b) ==1);
> >}
> >
> >
> >
> >
> >_______________________________________________
> >Boost-users mailing list
> >Boost-users <at> lists.boost.org
> >http://lists.boost.org/mailman/listinfo.cgi/boost-users
> 

Thank you for the quick reply and comments that I agree with. In the meantime I 
managed to make it work with double bind. When I looked at your suggestion I 
tried to implement it. Unfortunately I am getting compiled errors. Here is the 
complete test example (vc2008):

#include "stdafx.h"
#include <sstream>
#include <algorithm>
#include <vector>
#include <boost\bind.hpp>

class A
{
        int _i;
public:
        A(int val) : _i(val){}
        A(const A& another) : _i(another._i){}

        int value () {return _i;}
};

class B
{
        A _a;
public:
        B (int val) : _a(val){};
        B (const B& another) : _a(another._a) {}
        A& getA () {return _a;}
};

int _tmain(int argc, _TCHAR* argv[])
{
        std::vector<B> lstb;
        lstb.push_back(B(1));

        std::vector<B>::iterator it = std::find_if (lstb.begin(), lstb.end(),
                boost::bind (&B::getA::value, _1)() == 1);
        if (it == lstb.end())
                return 1;

        return 0;
}

Compiler errors are:
error C3083: 'getA': the symbol to the left of a '::' must be a type
error C2039: 'value' : is not a member of 'B'
see declaration of 'B'