Re: [Boost-users] Annoying coding

Subject: Re: [Boost-users] Annoying coding
From: Robert Jones (robertgbjones_at_[hidden])
Date: 2009-01-20 09:29:43

On Tue, Jan 20, 2009 at 2:21 PM, Alan M. Carroll <
amc_at_[hidden]> wrote:

> Why not use Boost.Bind? It will bind to data members, returning a functor
> that accesses that member. E.g.
>
>    boost::bind(&A::a, _1);
>
> will create a functor that takes an instance of A and returns a reference
> to the a member. You can then pass that to whatever other functor is being
> used. Something like (not tested!)
>
>    std::accumulate(m.begin(), m.end(), bind(std::add, _1,
> bind(&std::map<std::string, int>::value_type::first, _1));
>
> Boost.Lambda can do this as well, but I am not familiar enough with it to
> provide an example.

The Lambda version would be similar, just replacing the outer bind with
infix notation

#include "boost/lambda/lambda.hpp"
#include "boost/lambda/bind.hpp"

using namespace boost::lambda;

std::accumulate( begin, end, _1 + bind(&A::a, _2 ));

Again untested.

>
> At 07:22 AM 1/20/2009, you wrote:
> >Hi again,
> >
> >I have a problem. Not really big one but annoying.
> >Often we need to write code like this:
> >
> >struct A{
> >  int a;
> >  std::string b;
> >};
> >
> >int a_sum(int i, const A &v){ return v.a + i; };
> >
> >std::vector<A> v;
> >//fill this vector somewhere
> >
> >std::accumulate( v.begin(), v.end(), 0, a_sum );
> >
> >or something like
> >
> >int m_sum(int i, const std::pair<int, std::string> p) { return p.first +
> i; };
> >
> >std::map  <std::string, int> m;
> >
> >std::accumlate( m.begin(), m.end(), 0, m_sum);
> >
> >Or in common words we need to write some kind functor which just get
> >member from element and apply it to another functor.
> >
> >Of course I can write something like:
> >
> >template<typename T_, typename V_, V_ T_::*Ptr_>
> >struct get_mem{
> >   const V_ &operator()(const T_ &elem){
> >       return elem->*Ptr_;
> >   };
> >};
> >
> >but may be there is library way to do this?
>