# 标准库头文件 <unordered_map>

< cpp‎ | header

此头文件是容器库的一部分。

### 包含
<compare> (C++20) | 三路比较运算符支持
<initializer_list> (C++11) | std::initializer_list 类模板
### 类
unordered_map (C++11 起) | 键值对的集合，按照键生成散列，键是唯一的 (类模板)
unordered_multimap (C++11 起) | 键值对的集合，按照键生成散列 (类模板)
### 函数
operator==operator!= (C++20 中移除) | 比较 unordered_map 中的值 (函数模板)
std::swap(std::unordered_map) (C++11) | 特化 std::swap 算法 (函数模板)
erase_if(std::unordered_map) (C++20) | 擦除所有满足特定判别标准的元素 (函数模板)
operator==operator!= (C++20 中移除) | 比较 unordered_multimap 中的值 (函数模板)
std::swap(std::unordered_multimap) (C++11) | 特化 std::swap 算法 (函数模板)
erase_if(std::unordered_multimap) (C++20) | 擦除所有满足特定判别标准的元素 (函数模板)
##### 范围访问
begin cbegin (C++11)(C++14) | 返回指向容器或数组起始的迭代器 (函数模板)
end cend (C++11)(C++14) | 返回指向容器或数组结尾的迭代器 (函数模板)
rbegin crbegin (C++14) | 返回指向一个容器或数组的逆向迭代器 (函数模板)
rend crend (C++14) | 返回容器或数组的逆向尾迭代器 (函数模板)
size ssize (C++17)(C++20) | 返回容器或数组的大小 (函数模板)
empty (C++17) | 检查容器是否为空 (函数模板)
data (C++17) | 获得指向底层数组的指针 (函数模板)

### 概要

    #include <compare>
    #include <initializer_list>

    namespace std {
      // 类模板 unordered_map
      template<class Key,
               class T,
               class Hash = hash<Key>,
               class Pred = equal_to<Key>,
               class Alloc = allocator<pair<const Key, T>>>
        class unordered_map;

      // 类模板 unordered_multimap
      template<class Key,
               class T,
               class Hash = hash<Key>,
               class Pred = equal_to<Key>,
               class Alloc = allocator<pair<const Key, T>>>
        class unordered_multimap;

      template<class Key, class T, class Hash, class Pred, class Alloc>
        bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                        const unordered_map<Key, T, Hash, Pred, Alloc>& b);

      template<class Key, class T, class Hash, class Pred, class Alloc>
        bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                        const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);

      template<class Key, class T, class Hash, class Pred, class Alloc>
        void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
                  unordered_map<Key, T, Hash, Pred, Alloc>& y)
          noexcept(noexcept(x.swap(y)));

      template<class Key, class T, class Hash, class Pred, class Alloc>
        void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
          noexcept(noexcept(x.swap(y)));

      template<class K, class T, class H, class P, class A, class Predicate>
        typename unordered_map<K, T, H, P, A>::size_type
          erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

      template<class K, class T, class H, class P, class A, class Predicate>
        typename unordered_multimap<K, T, H, P, A>::size_type
          erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

      namespace pmr {
        template<class Key,
                 class T,
                 class Hash = hash<Key>,
                 class Pred = equal_to<Key>>
          using unordered_map =
            std::unordered_map<Key, T, Hash, Pred,
                               polymorphic_allocator<pair<const Key, T>>>;
        template<class Key,
                 class T,
                 class Hash = hash<Key>,
                 class Pred = equal_to<Key>>
          using unordered_multimap =
            std::unordered_multimap<Key, T, Hash, Pred,
                                    polymorphic_allocator<pair<const Key, T>>>;

      }
    }

#### 类模板 std::unordered_map

    namespace std {
      template<class Key,
               class T,
               class Hash = hash<Key>,
               class Pred = equal_to<Key>,
               class Allocator = allocator<pair<const Key, T>>>
      class unordered_map {
      public:
        // 类型
        using key_type             = Key;
        using mapped_type          = T;
        using value_type           = pair<const Key, T>;
        using hasher               = Hash;
        using key_equal            = Pred;
        using allocator_type       = Allocator;
        using pointer              = typename allocator_traits<Allocator>::pointer;
        using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
        using reference            = value_type&;
        using const_reference      = const value_type&;
        using size_type            = /* 由实现定义 */;
        using difference_type      = /* 由实现定义 */;

        using iterator             = /* 由实现定义 */;
        using const_iterator       = /* 由实现定义 */;
        using local_iterator       = /* 由实现定义 */;
        using const_local_iterator = /* 由实现定义 */;
        using node_type            = /* 未指明 */;
        using insert_return_type   = /*insert-return-type*/<iterator, node_type>;

        // 构造/复制/销毁
        unordered_map();
        explicit unordered_map(size_type n,
                               const hasher& hf = hasher(),
                               const key_equal& eql = key_equal(),
                               const allocator_type& a = allocator_type());
        template<class InputIt>
          unordered_map(InputIt f, InputIt l,
                        size_type n = /* 见描述 */,
                        const hasher& hf = hasher(),
                        const key_equal& eql = key_equal(),
                        const allocator_type& a = allocator_type());
        unordered_map(const unordered_map&);
        unordered_map(unordered_map&&);
        explicit unordered_map(const Allocator&);
        unordered_map(const unordered_map&, const Allocator&);
        unordered_map(unordered_map&&, const Allocator&);
        unordered_map(initializer_list<value_type> il,
                      size_type n = /* 见描述 */,
                      const hasher& hf = hasher(),
                      const key_equal& eql = key_equal(),
                      const allocator_type& a = allocator_type());
        unordered_map(size_type n, const allocator_type& a)
          : unordered_map(n, hasher(), key_equal(), a) { }
        unordered_map(size_type n, const hasher& hf, const allocator_type& a)
          : unordered_map(n, hf, key_equal(), a) { }
        template<class InputIt>
          unordered_map(InputIt f, InputIt l, size_type n, const allocator_type& a)
            : unordered_map(f, l, n, hasher(), key_equal(), a) { }
        template<class InputIt>
          unordered_map(InputIt f, InputIt l, size_type n, const hasher& hf,
                        const allocator_type& a)
            : unordered_map(f, l, n, hf, key_equal(), a) { }
        unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
          : unordered_map(il, n, hasher(), key_equal(), a) { }
        unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf,
                      const allocator_type& a)
          : unordered_map(il, n, hf, key_equal(), a) { }
        ~unordered_map();
        unordered_map& operator=(const unordered_map&);
        unordered_map& operator=(unordered_map&&)
          noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                   is_nothrow_move_assignable_v<Hash> &&
                   is_nothrow_move_assignable_v<Pred>);
        unordered_map& operator=(initializer_list<value_type>);
        allocator_type get_allocator() const noexcept;

        // 迭代器
        iterator       begin() noexcept;
        const_iterator begin() const noexcept;
        iterator       end() noexcept;
        const_iterator end() const noexcept;
        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;

        // 容量
        [[nodiscard]] bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // 修改器
        template<class... Args> pair<iterator, bool> emplace(Args&&... args);
        template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
        pair<iterator, bool> insert(const value_type& obj);
        pair<iterator, bool> insert(value_type&& obj);
        template<class P> pair<iterator, bool> insert(P&& obj);
        iterator       insert(const_iterator hint, const value_type& obj);
        iterator       insert(const_iterator hint, value_type&& obj);
        template<class P> iterator insert(const_iterator hint, P&& obj);
        template<class InputIt> void insert(InputIt first, InputIt last);
        void insert(initializer_list<value_type>);

        node_type extract(const_iterator position);
        node_type extract(const key_type& x);
        template<class K> node_type extract(K&& x);
        insert_return_type insert(node_type&& nh);
        iterator           insert(const_iterator hint, node_type&& nh);

        template<class... Args>
          pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
        template<class... Args>
          pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
        template<class... Args>
          iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
        template<class... Args>
          iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
        template<class M>
          pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
        template<class M>
          pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
        template<class M>
          iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
        template<class M>
          iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

        iterator  erase(iterator position);
        iterator  erase(const_iterator position);
        size_type erase(const key_type& k);
        template<class K> size_type erase(K&& x);
        iterator  erase(const_iterator first, const_iterator last);
        void      swap(unordered_map&)
          noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                   is_nothrow_swappable_v<Hash> &&
                   is_nothrow_swappable_v<Pred>);
        void      clear() noexcept;

        template<class H2, class P2>
          void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
        template<class H2, class P2>
          void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
        template<class H2, class P2>
          void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
        template<class H2, class P2>
          void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

        // 观察器
        hasher hash_function() const;
        key_equal key_eq() const;

        // map 操作
        iterator         find(const key_type& k);
        const_iterator   find(const key_type& k) const;
        template<class K>
          iterator       find(const K& k);
        template<class K>
          const_iterator find(const K& k) const;
        template<class K>
        size_type        count(const key_type& k) const;
        template<class K>
          size_type      count(const K& k) const;
        bool             contains(const key_type& k) const;
        template<class K>
          bool           contains(const K& k) const;
        pair<iterator, iterator>               equal_range(const key_type& k);
        pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
        template<class K>
          pair<iterator, iterator>             equal_range(const K& k);
        template<class K>
          pair<const_iterator, const_iterator> equal_range(const K& k) const;

        // 元素访问
        mapped_type& operator[](const key_type& k);
        mapped_type& operator[](key_type&& k);
        mapped_type& at(const key_type& k);
        const mapped_type& at(const key_type& k) const;

        // 桶接口
        size_type bucket_count() const noexcept;
        size_type max_bucket_count() const noexcept;
        size_type bucket_size(size_type n) const;
        size_type bucket(const key_type& k) const;
        local_iterator begin(size_type n);
        const_local_iterator begin(size_type n) const;
        local_iterator end(size_type n);
        const_local_iterator end(size_type n) const;
        const_local_iterator cbegin(size_type n) const;
        const_local_iterator cend(size_type n) const;

        // 散列策略
        float load_factor() const noexcept;
        float max_load_factor() const noexcept;
        void max_load_factor(float z);
        void rehash(size_type n);
        void reserve(size_type n);
      };

      template<class InputIt,
               class Hash = hash</*iter-key-type*/<InputIt>>,
               class Pred = equal_to</*iter-key-type*/<InputIt>>,
               class Allocator = allocator</*iter-to-alloc-type*/<InputIt>>>
        unordered_map(InputIt, InputIt,
                      typename /* 见描述 */::size_type = /* 见描述 */,
                      Hash = Hash(), Pred = Pred(), Allocator = Allocator())
          -> unordered_map</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIt>,
                           Hash, Pred, Allocator>;

      template<class Key, class T, class Hash = hash<Key>,
               class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
        unordered_map(initializer_list<pair<Key, T>>,
                      typename /* 见描述 */::size_type = /* 见描述 */,
                      Hash = Hash(), Pred = Pred(), Allocator = Allocator())
          -> unordered_map<Key, T, Hash, Pred, Allocator>;

      template<class InputIt, class Allocator>
        unordered_map(InputIt, InputIt, typename /* 见描述 */::size_type, Allocator)
          -> unordered_map</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIt>,
                           hash</*iter-key-type*/<InputIt>>,
                           equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class InputIt, class Allocator>
        unordered_map(InputIt, InputIt, Allocator)
          -> unordered_map</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIte>,
                           hash</*iter-key-type*/<InputIt>>,
                           equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class InputIt, class Hash, class Allocator>
        unordered_map(InputIt, InputIt,
                      typename /* 见描述 */::size_type, Hash, Allocator)
          -> unordered_map</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIt>, Hash,
                           equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class Key, class T, class Allocator>
        unordered_map(initializer_list<pair<Key, T>>,
                      typename /* 见描述 */::size_type, Allocator)
          -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

      template<class Key, class T, class Allocator>
        unordered_map(initializer_list<pair<Key, T>>, Allocator)
          -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

      template<class Key, class T, class Hash, class Allocator>
        unordered_map(initializer_list<pair<Key, T>>,
                      typename /* 见描述 */::size_type, Hash, Allocator)
          -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;

      // 交换
      template<class Key, class T, class Hash, class Pred, class Alloc>
        void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
                  unordered_map<Key, T, Hash, Pred, Alloc>& y)
          noexcept(noexcept(x.swap(y)));
    }

### 类模板 std::unordered_multimap=

    namespace std {
      template<class Key,
               class T,
               class Hash = hash<Key>,
               class Pred = equal_to<Key>,
               class Allocator = allocator<pair<const Key, T>>>
      class unordered_multimap {
      public:
        // 类型
        using key_type             = Key;
        using mapped_type          = T;
        using value_type           = pair<const Key, T>;
        using hasher               = Hash;
        using key_equal            = Pred;
        using allocator_type       = Allocator;
        using pointer              = typename allocator_traits<Allocator>::pointer;
        using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
        using reference            = value_type&;
        using const_reference      = const value_type&;
        using size_type            = /* 由实现定义 */;
        using difference_type      = /* 由实现定义 */;

        using iterator             = /* 由实现定义 */;
        using const_iterator       = /* 由实现定义 */;
        using local_iterator       = /* 由实现定义 */;
        using const_local_iterator = /* 由实现定义 */;
        using node_type            = /* 未指明 */;

        // 构造/复制/销毁
        unordered_multimap();
        explicit unordered_multimap(size_type n,
                                    const hasher& hf = hasher(),
                                    const key_equal& eql = key_equal(),
                                    const allocator_type& a = allocator_type());
        template<class InputIt>
          unordered_multimap(InputIt f, InputIt l,
                             size_type n = /* 见描述 */,
                             const hasher& hf = hasher(),
                             const key_equal& eql = key_equal(),
                             const allocator_type& a = allocator_type());
        unordered_multimap(const unordered_multimap&);
        unordered_multimap(unordered_multimap&&);
        explicit unordered_multimap(const Allocator&);
        unordered_multimap(const unordered_multimap&, const Allocator&);
        unordered_multimap(unordered_multimap&&, const Allocator&);
        unordered_multimap(initializer_list<value_type> il,
                           size_type n = /* 见描述 */,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
        unordered_multimap(size_type n, const allocator_type& a)
          : unordered_multimap(n, hasher(), key_equal(), a) { }
        unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
          : unordered_multimap(n, hf, key_equal(), a) { }
        template<class InputIt>
          unordered_multimap(InputIt f, InputIt l, size_type n, const allocator_type& a)
            : unordered_multimap(f, l, n, hasher(), key_equal(), a) { }
        template<class InputIt>
          unordered_multimap(InputIt f, InputIt l, size_type n, const hasher& hf,
                             const allocator_type& a)
            : unordered_multimap(f, l, n, hf, key_equal(), a) { }
        unordered_multimap(initializer_list<value_type> il, size_type n,
                           const allocator_type& a)
          : unordered_multimap(il, n, hasher(), key_equal(), a) { }
        unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf,
                           const allocator_type& a)
          : unordered_multimap(il, n, hf, key_equal(), a) { }
        ~unordered_multimap();
        unordered_multimap& operator=(const unordered_multimap&);
        unordered_multimap& operator=(unordered_multimap&&)
          noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                   is_nothrow_move_assignable_v<Hash> &&
                   is_nothrow_move_assignable_v<Pred>);
        unordered_multimap& operator=(initializer_list<value_type>);
        allocator_type get_allocator() const noexcept;

        // 迭代器
        iterator       begin() noexcept;
        const_iterator begin() const noexcept;
        iterator       end() noexcept;
        const_iterator end() const noexcept;
        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;

        // 容量
        [[nodiscard]] bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // 修改器
        template<class... Args> iterator emplace(Args&&... args);
        template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
        iterator insert(const value_type& obj);
        iterator insert(value_type&& obj);
        template<class P> iterator insert(P&& obj);
        iterator insert(const_iterator hint, const value_type& obj);
        iterator insert(const_iterator hint, value_type&& obj);
        template<class P> iterator insert(const_iterator hint, P&& obj);
        template<class InputIt> void insert(InputIt first, InputIt last);
        void insert(initializer_list<value_type>);

        node_type extract(const_iterator position);
        node_type extract(const key_type& x);
        template<class K> node_type extract(K&& x);
        iterator insert(node_type&& nh);
        iterator insert(const_iterator hint, node_type&& nh);

        iterator  erase(iterator position);
        iterator  erase(const_iterator position);
        size_type erase(const key_type& k);
        template<class K> size_type erase(K&& x);
        iterator  erase(const_iterator first, const_iterator last);
        void      swap(unordered_multimap&)
          noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                   is_nothrow_swappable_v<Hash> &&
                   is_nothrow_swappable_v<Pred>);
        void      clear() noexcept;

        template<class H2, class P2>
          void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
        template<class H2, class P2>
          void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
        template<class H2, class P2>
          void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
        template<class H2, class P2>
          void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

        // 观察器
        hasher hash_function() const;
        key_equal key_eq() const;

        // map 操作
        iterator         find(const key_type& k);
        const_iterator   find(const key_type& k) const;
        template<class K>
          iterator       find(const K& k);
        template<class K>
          const_iterator find(const K& k) const;
        size_type        count(const key_type& k) const;
        template<class K>
          size_type      count(const K& k) const;
        bool             contains(const key_type& k) const;
        template<class K>
          bool           contains(const K& k) const;
        pair<iterator, iterator>               equal_range(const key_type& k);
        pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
        template<class K>
          pair<iterator, iterator>             equal_range(const K& k);
        template<class K>
          pair<const_iterator, const_iterator> equal_range(const K& k) const;

        // 桶接口
        size_type bucket_count() const noexcept;
        size_type max_bucket_count() const noexcept;
        size_type bucket_size(size_type n) const;
        size_type bucket(const key_type& k) const;
        local_iterator begin(size_type n);
        const_local_iterator begin(size_type n) const;
        local_iterator end(size_type n);
        const_local_iterator end(size_type n) const;
        const_local_iterator cbegin(size_type n) const;
        const_local_iterator cend(size_type n) const;

        // 散列策略
        float load_factor() const noexcept;
        float max_load_factor() const noexcept;
        void max_load_factor(float z);
        void rehash(size_type n);
        void reserve(size_type n);
      };

      template<class InputIt,
               class Hash = hash</*iter-key-type*/<InputIt>>,
               class Pred = equal_to</*iter-key-type*/<InputIt>>,
               class Allocator = allocator</*iter-to-alloc-type*/<InputIt>>>
        unordered_multimap(InputIt, InputIt,
                           typename /* 见描述 */::size_type = /* 见描述 */,
                           Hash = Hash(), Pred = Pred(), Allocator = Allocator())
          -> unordered_multimap<iter-key-type<InputIt>, iter-mapped-type<InputIt>,
                                Hash, Pred, Allocator>;

      template<class Key, class T, class Hash = hash<Key>,
               class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
        unordered_multimap(initializer_list<pair<Key, T>>,
                           typename /* 见描述 */::size_type = /* 见描述 */,
                           Hash = Hash(), Pred = Pred(), Allocator = Allocator())
          -> unordered_multimap<Key, T, Hash, Pred, Allocator>;

      template<class InputIt, class Allocator>
        unordered_multimap(InputIt, InputIt, typename /* 见描述 */::size_type,
                           Allocator)
          -> unordered_multimap</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIt>,
                                hash</*iter-key-type*/<InputIt>>,
                                equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class InputIt, class Allocator>
        unordered_multimap(InputIt, InputIt, Allocator)
          -> unordered_multimap</*iter-key-type*/<InputIt>, /*iter-mapped-type*/<InputIt>,
                                hash</*iter-key-type*/<InputIt>>,
                                equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class InputIt, class Hash, class Allocator>
        unordered_multimap(InputIt, InputIt, typename /* 见描述 */::size_type, Hash,
                           Allocator)
          -> unordered_multimap</*iter-key-type*/<InputIt>, iter-mapped-type<InputIt>, Hash,
                                equal_to</*iter-key-type*/<InputIt>>, Allocator>;

      template<class Key, class T, class Allocator>
        unordered_multimap(initializer_list<pair<Key, T>>,
                           typename /* 见描述 */::size_type, Allocator)
          -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

      template<class Key, class T, class Allocator>
        unordered_multimap(initializer_list<pair<Key, T>>, Allocator)
          -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

      template<class Key, class T, class Hash, class Allocator>
        unordered_multimap(initializer_list<pair<Key, T>>,
                           typename /* 见描述 */::size_type, Hash, Allocator)
          -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>;

      // 交换
      template<class Key, class T, class Hash, class Pred, class Alloc>
        void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
          noexcept(noexcept(x.swap(y)));
    }