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 );