op_minus.md

operator- (非メンバ関数)

• iterator[meta header]
• std[meta namespace]
• function template[meta id-type]

namespace std {
  template<class Iterator1, class Iterator2>
  typename reverse_iterator<Iterator>::difference_type
  operator-(const reverse_iterator<Iterator1>& x,
            const reverse_iterator<Iterator2>& y);               // C++03

  template <class Iterator1, class Iterator2>
  auto operator-(const reverse_iterator<Iterator1>& x,
                 const reverse_iterator<Iterator2>& y)
    -> decltype(y.current - x.current);                          // C++11

  template <class Iterator1, class Iterator2>
  constexpr auto operator-(const reverse_iterator<Iterator1>& x,
                           const reverse_iterator<Iterator2>& y)
    -> decltype(y.current - x.current);                          // C++17
}

概要

2つのreverse_iteratorの差を求める

戻り値

y.current - x.current

例

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

int main()
{
  std::vector<int> v = {1, 2, 3};

  std::reverse_iterator<decltype(v)::iterator> it1(v.end());
  std::reverse_iterator<decltype(v)::iterator> it2(v.begin());

  std::ptrdiff_t result = it2 - it1;
  std::cout << result << std::endl;
}

• it2 - it1[color ff0000]
• std::ptrdiff_t[link /reference/cstddef/ptrdiff_t.md]

出力

3

参照

• LWG Issue 685. reverse_iterator/move_iterator difference has invalid signatures
• P0031R0 A Proposal to Add Constexpr Modifiers to reverse_iterator, move_iterator, array and Range Access