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.

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?