Class counting_iterator

extlibs/fmt/include/fmt/format.h:366–397

An output iterator that counts the number of objects written to it and discards them.

// An output iterator that counts the number of objects written to it and
// discards them.
class counting_iterator {
 private:
  std::size_t count_;

 public:
  using iterator_category = std::output_iterator_tag;
  using difference_type = std::ptrdiff_t;
  using pointer = void;
  using reference = void;
  using _Unchecked_type = counting_iterator;  // Mark iterator as checked.

  struct value_type {
    template <typename T> void operator=(const T&) {}
  };

  counting_iterator() : count_(0) {}

  std::size_t count() const { return count_; }

  counting_iterator& operator++() {
    ++count_;
    return *this;
  }

  counting_iterator operator++(int) {
    auto it = *this;
    ++*this;
    return it;
  }

  value_type operator*() const { return {}; }
};

template <typename OutputIt> class truncating_iterator_base {
 protected: