[Boost-Users] boost::lambda::select2nd

From: Jaakko Jarvi (jajarvi_at_[hidden])
Date: 2003-06-24 14:35:17

> Hi!
> I would like to write code like
> int main()
> {
>  namespace l = boost::lambda;
>   std::map<int, double> M;
>   M[1] = 5.0;
>   M[3] = 7.0;
>   std::for_each(M.begin(), M.end(),
>                 l::select2nd(l::_1) *= 6.0);
> }
> Is it hard to implement this for boost::lambda?
> Markus

Accessing data members can be done, but it's unfortunately a bit more
complicated than:

  _1.second

I just wish . could be overloaded.
Example code below.

  Cheers, Jaakko

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#include "boost/lambda/lambda.hpp"
#include "boost/lambda/bind.hpp"
#include <algorithm>
#include <map>

#include <iostream>

// A function object class that takes returns a second argument of a pair.
// This can be used with bind.

struct select2nd {

  // Tell LL how to get the return type. Args will be tuple<select2nd, pair<int, double> >, 
  // so get the element at index 1.
  template <class Args> 
  struct sig {
    typedef typename 
      boost::tuples::element<1, Args>::type pair_t;
    typedef typename boost::add_reference<typename pair_t::second_type>::type type; 
  };

  template <class T>
  typename boost::add_reference<typename T::second_type>::type
  operator()(T& t) const { return t.second; }
};

int main()
{
  namespace l = boost::lambda;
  std::map<int, double> M;

  M[1] = 5.0;
  M[3] = 7.0;
  
  std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " ");
  std::cout << std::endl;

  std::for_each(M.begin(), M.end(), l::bind(select2nd(), l::_1) *= 6.0);

  std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " ");
  std::cout << std::endl;

  // alternatively, use bind to refer directly to the data member. You need to 
  // spell out the full type of the pair though.
  std::for_each(M.begin(), M.end(), 
                l::bind(&std::map<int, double>::value_type::second, l::_1) *= 6.0);

  std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " ");
  std::cout << std::endl;

}

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