#include <algorithm>
  iterator set_difference( iterator start1, iterator end1, iterator start2, iterator end2, iterator result );
  iterator set_difference( iterator start1, iterator end1, iterator start2, iterator end2, iterator result, StrictWeakOrdering cmp );

The set_difference() algorithm computes the difference between two sets defined by [start1,end1) and [start2,end2) and stores the difference starting at result.

Both of the sets, given as ranges, must be sorted in ascending order.

The return value of set_difference() is an iterator to the end of the result range.

If the strict weak ordering comparison function object cmp is not specified, set_difference() will use the < operator to compare elements.