Sunday, January 8, 2012

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:


The expression


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


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


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


is as undefined as is


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.

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