STL containers' difference_type typedef

Question

I noticed STL containers (including vector, list, etc.) have a difference_type typedef, which is usually an alias for std::ptrdiff_t.

What is it used for? It is used by any member functions?

Answer 1

It's used to compute difference between iterators. For example cont.end() - cont.begin(). It's not used directly by stl container, but is widely used by iterator library. Example:

template< class InputIt >
typename std::iterator_traits<InputIt>::difference_type 
    distance( InputIt first, InputIt last );