STLAlgorithmExtensions/Select1st

The [select1st] struct template and its counterpart [select2nd], are function objects that can be used to select the first or second attribute of a pair, respectively. They are both very convenient when using standard algorithms to copy map keys or values into non-associative (i.e. sequence) containers. Unfortunatelly, both of these non-standard algorithm extensions are absent from many, if not all, non-SGI implementations of the STL. So, without further ado, I present these two struct templates for use in your own applications:

Definitions

#include <functional>

template <typename pair_type>
struct select1st: public std::unary_function<const pair_type &, typename const pair_type::first_type &>
{
  const typename pair_type::first_type &operator()(const pair_type &v) const
  {
    return v.first;
  }
};

template <typename pair_type>
struct select2nd: public std::unary_function<const pair_type &, typename const pair_type::second_type &>
{
  const typename pair_type::second_type &operator()(const pair_type &v) const
  {
    return v.second;
  }
};

Sample usage

Code

#include <algorithm>
#include <boost/assign/list_of.hpp>
#include <functional>
#include <iostream>
#include <map>
#include <string>

using namespace std; // For brevity in this example

// Here are the select1st and select2nd definitions. In a real application, you would want to
// include these from some header file that contained these and other like minded definitions
template <typename pair_type>
struct select1st: public std::unary_function<const pair_type &, typename const pair_type::first_type &>
{
  const typename pair_type::first_type &operator()(const pair_type &v) const
  {
    return v.first;
  }
};

template <typename pair_type>
struct select2nd: public std::unary_function<const pair_type &, typename const pair_type::second_type &>
{
  const typename pair_type::second_type &operator()(const pair_type &v) const
  {
    return v.second;
  }
};

int main()
{
  // Create a map of ordinal values to their names, with three associations
  typedef map<int,string> IntStringMap;
  IntStringMap ordinalNamesMap(boost::assign::map_list_of
    (1,string("1st"))
    (2,string("2nd"))
    (3,string("3rd")));


  // Display all keys in order
  transform(
    ordinalNamesMap.begin(),
    ordinalNamesMap.end(),
    ostream_iterator<IntStringMap::key_type>(cout,","),
    select1st<IntStringMap::value_type>());
  cout << '\n';

  // Display all values, in key order
  transform(
    ordinalNamesMap.begin(),
    ordinalNamesMap.end(),
    ostream_iterator<IntStringMap::mapped_type>(cout,","),
    select2nd<IntStringMap::value_type>());
  cout << '\n';
}

Output

1,2,3,
1st,2nd,3rd,