Re: Multimap: how to get a key list?

"Daniel T." <>
Tue, 02 Mar 2010 08:32:08 -0500
tonydee <> 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)
            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),

(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;

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