Re: Multimap: how to get a key list?

Michael Doubez <>
Tue, 2 Mar 2010 01:55:00 -0800 (PST)
On 2 mar, 01:35, "Daniel T." <> wrote:

In article

 Brian <> wrote:

On Feb 28, 5:09 pm, Sam <> wrote:

Rui Maciel writes:

Is there any way to get a list of keys from a multimap besides rely=

ing on

a couple of nested
loops to assemble that list?

What nested loops? Only one loop is required to iterate over the mult=


There is no single function that gives you a set of all keys stored i=

n the

multimap, but a single loop is all that's needed to retrieve all the =


It's fairly easy to define a template function that gives them to you=


something like this:

template<typename multimap_t>
void keys(const multimap_t &m,
          std::set<typename multimap_t::key_type> &k)
    for (typename multimap_t::const_iterator b(m.begin()), e(m.en=


         b != e; ++b)


The std::set automatically takes care of deduping the multimap's keys=


I think that works fine. This might speed it up a little:

  template<typename multimap_t>
  void keys(const multimap_t &m,
            std::set<typename multimap_t::key_type> &k)
      typename std::set<typename multimap_t::key_type>::iterator
keys_end = k.end();
      for (typename multimap_t::const_iterator b(m.begin()),
           b != e; ++b)
           k.insert(keys_end, b->first);

That would work best if k is being passed in
as an empty container as seems likely.

Now that I understand the question I can give a better answer than I did
the first time around...

Using the code below, you don't have to insert into a set, you can
insert into any output iterator (of course all of these examples work
with both multimaps and regular maps:

template < typename map_t, typename OutIt>
void keys(const map_t& aMap, OutIt out)
   for (typename map_t::const_iterator it = aMap.begin();
         it != aMap.end();
         it = aMap.upper_bound(it->first))
      out++ = it->first;



Another solution is to use decoration:
template<typename iterator_type>
class key_iterator:
   std::iterator< typename iterator_type::iterator_category
                // return type of iterator is key type
                , typename iterator_type::value_type::first_type
                , typename iterator_type::difference_type
    // put here the typedefs iterator_category, ...

    // build from
    key_iterator(const iterator_type& i):it(i){}

    // usual operation on iterator
    key_iterator& operator++()
    { ++it;return *this;}

    view_iterator operator++(int)
    { return key_iterator(it++);}

    bool operator == (const key_iterator& rhs) const
    {return it ==;}

    // and so on for other iterator operations

    // return key
    reference operator*() const
    {return it->first; }
    pointer operator->() const
    {return &it->first; }

     iterator_type it;

And then you can use
template<class T>
key_iterator<typename T::iterator_type> key_begin(T& container)
 return container.begin();

template<class T>
key_iterator<typename T::iterator_type> key_end(T& container)
 return container.end();

You can do whatever you want on the keys of an associative container.

std::accumulate(key_begin(aMap),key_end(aMap), 0 );


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