set_difference.hpp

#include <iostream>
#include <iterator>
#include <vector>

template<typename InputIterator1, typename InputIterator2, typename OutputIterator>
OutputIterator my_set_difference(InputIterator1 first1, InputIterator1 last1,
                                 InputIterator2 first2, InputIterator2 last2,
                                 OutputIterator out)
{
    auto current1 = first1;
    auto current2 = first2;
  
    while (current1 != last1)
    {
        if (current2 == last2) return std::copy(current1, last1, out);
        
        if (*current1 < *current2)
        {
            *out = *current1;
            ++out;
            ++current1;
        }
        else
        {
            if (!(*current2 < *current1))
            {
                ++current1;
            }
            ++current2;
        }
    }
    return out;
}
int main()
{
    std::vector<int> A = {1, 2, 3, 6, 7, 7, 10, 11, 12};
    std::vector<int> B = {4, 6, 7, 8};
    
    my_set_difference(begin(A), end(A), begin(B), end(B), std::ostream_iterator<int>(std::cout, " "));
}