cppreference.com > C++ Algorithms > set_difference set_difference Syntax:
      #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. Related topics: includes set_intersection set_symmetric_difference set_union