A type that provides a bidirectional iterator that can read a const element in the map.
A type const_iterator cannot be used to modify the value of an element.
The const_iterator defined by map points to elements that are objects of value_type, that is of type pair<const Key, Type>, whose first member is the key to the element and whose second member is the mapped datum held by the element.
To dereference a const_iterator cIter pointing to an element in a map, use the -> operator.
To access the value of the key for the element, use cIter -> first, which is equivalent to (*cIter).first.
To access the value of the mapped datum for the element, use cIter -> second, which is equivalent to (*cIter). second.