Re: Multimap: how to get a key list?

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

{
   public:
     // 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 == rhs.it;}

     // and so on for other iterator operations

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

   private:
      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 );