Re: [Boost-users] Iterating over the seconds of a list of pairs

17 Aug 2009

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-----Original Message-----
From: boost-users-bounces@lists.boost.org [mailto:boost-users-
bounces@lists.boost.org] On Behalf Of Björn Karlsson
Sent: 08 August 2009 18:12
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] Iterating over the seconds of a list of pairs
You're right -- transform_iterator is the right tool for the job. Try
something like this.
1) Write a function object that extracts the second element of the pair.
2) Use the transform iterator adaptor to provide you with the type of
the iterators for your class.
3) Use the helper function make_transform_iterator in your begin() and
end() methods to create the iterators.
Here's an example that does the above:
#include <iostream>
#include <list>
#include <utility>
#include "boost/iterator/transform_iterator.hpp"
// Extract the second element of a pair
template <typename T> class pair_second_t {
public:
  typedef T result_type;
template <typename U> T operator()(const std::pair<U,T>& element)
const
  {
    return element.second;
  }
};
class some_class
{
public:
  typedef std::list<std::pair<double,float> > container_type;
  typedef boost::transform_iterator<pair_second_t<float>,
container_type::iterator> iterator_type;
Bjorn Karlsson
Hello bjorn,

I have a similar problem. I have a vector< pair<LeftT, RightT> >.
I'm very happy to use vector< pair<LeftT, RightT> >::iterator

The only extra is I'd like to name the functions GetLeft() and GetRight()
and make them callable as:

typedef vector< pair<LeftT, RightT> >::iterator stl_iterator;
stl_iterator i = ...;
i->GetLeft()  and  i->GetRight()  // I can't do this obviously

so:
typedef boost::transform_iterator<functor, stl_iterator> my_iterator_type;

Should functor provide GetLeft() and GetRight()?
or should my_iterator_type derive from the result of
transform_iterator<functor, stl_iterator> ?

regards,

Re: [Boost-users] Iterating over the seconds of a list of pairs

Hicham Mouline