set_difference_size.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <list>

template <typename T>
class Counter : public std::iterator<std::output_iterator_tag, T> {
  size_t &Value;

public:
  Counter(size_t &Result) : Value(Result) {}
  const Counter &operator++() const { return *this; }
  const Counter &operator*() const { return *this; }
  void operator=(const T &Item) const { ++Value; }
};

template <typename ForwardIt1, typename ForwardIt2>
size_t set_difference_size(ForwardIt1 First1, ForwardIt1 Last1,
                           ForwardIt2 First2, ForwardIt2 Last2) {
  using OutputIt = Counter<typename ForwardIt1::value_type>;
  size_t Result = 0;
  std::set_difference(First1, Last1, First2, Last2, OutputIt(Result));
  return Result;
}

int main() {
  std::list<int64_t> FirstList = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37};
  std::cout << set_difference_size(std::istream_iterator<int>(std::cin),
                                   std::istream_iterator<int>(),
                                   begin(FirstList), end(FirstList))
            << std::endl;
}