std::unordered_multiset::extract

#include <iostream>
#include <unordered_set>

class noncopyable {
protected:
  constexpr noncopyable() noexcept = default;
  ~noncopyable() = default;
  constexpr noncopyable(noncopyable const &) noexcept = delete;
  constexpr noncopyable(noncopyable &&) noexcept = default;
  noncopyable & operator=(noncopyable const &) noexcept = delete;
  noncopyable & operator=(noncopyable &&) noexcept = default;
};

struct my_struct // ムーブオンリーな型
  : private noncopyable {
  int value;
  int num = 0;
  static inline int count = 0;
  constexpr my_struct(int i) noexcept : value(i) { num = count++; };
  bool operator == (const my_struct &rhs) const noexcept {return this->value == rhs.value;}
};

// ハッシュ関数
auto my_hash = [](my_struct const& s) noexcept -> std::size_t
{
  return std::hash<int>{}(s.value);
};

int main()
{
  // ムーブオンリーな型をキーとして扱う multiset
  std::unordered_multiset<my_struct, decltype(my_hash)> s;

  // 挿入
  s.insert(my_struct{1});
  s.insert(my_struct{1});
  s.insert(my_struct{3});

  // 要素を取り出す
  my_struct m = std::move(s.extract(s.begin()).value());
}

#include <iostream>
#include <unordered_set>

int main()
{
  std::unordered_multiset<int> s1;
  std::unordered_multiset<int> s2 = { 1, 1, 3 };

  // ノードを取得
  std::unordered_multiset<int>::node_type node = s2.extract(1);

  // 再確保なしに値を書き換える
  node.value() = 15;

  // ノードを転送
  s1.insert(std::move(node));

  if (s1.size() != 0) std::cout << "s1 = { ";
  else std::cout << "s1 = {}\n";

  for(auto&& itr = s1.begin(); itr != s1.end();)
    std::cout << *itr << (++itr != s1.end() ? ", " : " }\n");

  if (s2.size() != 0) std::cout << "s2 = { ";
  else std::cout << "s2 = {}\n";

  for(auto&& itr = s2.begin(); itr != s2.end();)
    std::cout << *itr << (++itr != s2.end() ? ", " : " }\n");
}