Re: Multimap: how to get a key list?

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 02 Mar 2010 08:32:08 -0500

Message-ID:

<daniel_t-EAA199.08320802032010@70-3-168-216.pools.spcsdns.net>

tonydee <tony_in_da_uk@yahoo.co.uk> wrote:

Alternatively, we could shoe-horn an STL algorithm. For example (and
assuming we want each instance of duplicate keys:

    template <class Inserter>
    struct Assign_First : public Inserter
    {
        typedef typename Inserter::container_type container_type;

        Assign_First(container_type& c) : Inserter(c) { }

        Assign_First& operator*() { return *this; }

        template <typename Pair>
        Assign_First& operator=(const Pair& value)
        {
            Inserter::operator=(value.first);
            return *this;
        }
    };

    ...

    typedef std::multimap<int, std::string> MM;
    MM mm;

    typedef std::vector<MM::key_type> Keys;
    Keys keys;

    std::copy(mm.begin(), mm.end(),
              Assign_First<std::back_insert_iterator<Keys> >(keys));

I think part of the point here was to remove duplicates. Without that
requirement, simply:

   transform(mm.begin(), mm.end(), back_inserter(keys),
                           select1st<mm_t::value_type>());

(Where 'select1st' has a rather obvious implementation...)

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