15.3. Introduction to Iterators

Iterators have many similarities to pointers and are used to point to first-class container elements and for other purposes. Iterators hold state information sensitive to the particular containers on which they operate; thus, iterators are implemented for each type of container. Certain iterator operations are uniform across containers. For example, the dereferencing operator (*) dereferences an iterator so that you can use the element to which it points. The ++ operation on an iterator moves it to the container’s next element (much as incrementing a pointer into a built-in array aims the pointer at the next array element).

First-class containers provide member functions begin and end. Function begin returns an iterator pointing to the first element of the container. Function end returns an iterator pointing to the first element past the end of the container (one past the end)—a non-existent element that’s frequently used to determine when the end of a container is reached. If iterator i points to a particular element, then ++i points to the “next” element and *i refers to the element pointed to by i. The iterator resulting from end is typically used in an equality or inequality comparison to determine whether the “moving iterator” (i in this case) has reached the end of the container.

An object of a container’s iterator type refers to a container element that can be modified. An object of a container’s const_iterator type refers to a container element that cannot be modified.