op_compare_3way.md

operator<=>

• memory[meta header]
• std[meta namespace]
• function template[meta id-type]
• cpp20[meta cpp]

namespace std {
  template <class T1, class D1, class T2, class D2>
    requires three_way_comparable_with<
               typename unique_ptr<T1, D1>::pointer,
               typename unique_ptr<T2, D2>::pointer>
  compare_three_way_result_t<
    typename unique_ptr<T1, D1>::pointer,
    typename unique_ptr<T2, D2>::pointer>
  operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // (1) C++20

  template <class T, class D>
    requires three_way_comparable_with<typename unique_ptr<T, D>::pointer>
  compare_three_way_result_t<typename unique_ptr<T, D>::pointer>
  operator<=>(const unique_ptr<T, D>& x, nullptr_t);                     // (2) C++20

  template <class T, class D>
    requires three_way_comparable_with<typename unique_ptr<T, D>::pointer>
  constexpr compare_three_way_result_t<typename unique_ptr<T, D>::pointer>
  operator<=>(const unique_ptr<T, D>& x, nullptr_t);                     // (2) C++23
}

• nullptr_t[link /reference/cstddef/nullptr_t.md]

概要

unique_ptrオブジェクトの三方比較を行う。

テンプレートパラメータ制約

• (1) : 型unique_ptr<T1, D1>::pointerと型unique_ptr<T2, D2>::pointerが三方比較可能であること
• (2) : 型unique_ptr<T, D>::pointer同士が三方比較可能であること

戻り値

• (1) :

  return compare_three_way()(x.get(), y.get());
  • compare_three_way[link /reference/compare/compare_three_way.md]
  • get()[link get.md]

• (2) :

  return compare_three_way()(x.get(), static_cast<typename unique_ptr<T, D>::pointer>(nullptr));
  • compare_three_way[link /reference/compare/compare_three_way.md]
  • get()[link get.md]

例

#include <iostream>
#include <memory>

int main()
{
  std::unique_ptr<int> p1(new int(3));
  if ((p1 <=> p1) == 0) {
    std::cout << "equal" << std::endl;
  }

  std::unique_ptr<int> p2;
  if ((p2 <=> nullptr) == 0) {
    std::cout << "p2 is nullptr" << std::endl;
  }

  if ((nullptr <=> p2) == 0) {
    std::cout << "p2 is nullptr" << std::endl;
  }
}

出力

equal
p2 is nullptr
p2 is nullptr

バージョン

言語

• C++20

処理系

• Clang:
• GCC: 10 [mark verified]
• Visual C++: ??

参照

• P1614R2 The Mothership has Landed
  • C++20での三方比較演算子の追加と、関連する演算子の自動導出
• LWG3426 operator<=>(const unique_ptr<T, D>&, nullptr_t) can't get no satisfaction
• P2273R3 Making std::unique_ptr constexpr