stl reverse iterator

Reverse Iterators

The third kind of predefined iterator adapters are reverse iterators. Reverse iterators operate in reverse. They switch the call of an increment operator internally into a call of the decrement operator, and vice versa. All containers can create reverse iterators via their member functions rbegin() and rend(). Consider the following example:

Example:

Reverse_iterator

The expression

coll.rbegin()

returns a reverse iterator for coll. This iterator may be used as the beginning of a reverse iteration over the elements of the collection. Its position is the last element of the collection. Thus,

the expression

*coll.rbegin()

returns the value of the last element. Accordingly, the expression

coll.rend()

returns a reverse iterator for coll that may be used as the end of a reverse iteration. As usual for ranges, its position is past the end of the range, but from the opposite direction; that is, it is the position before the first element in the collection.

The expression

*coll.rend()

is as undefined as is

*coll.end()

You should never use operator * (or operator ->) for a position that does not represent a valid element.

The advantage of using reverse iterators is that all algorithms are able to operate in the opposite direction without special code. A step to the next element with operator ++ is redefined into a step backward with operator --. For example, in this case, copy() iterates over the elements of coll from the last to the first element. So, the output of the program is as follows:

9 8 7 6 5 4 3 2 1

You can also switch "normal" iterators into reverse iterators, and vice versa. However, in doing so the element of an iterator changes.

See Also:

Standard Template Library Concepts and sample codes in C++