26 Containers library [containers]

26.2 Container requirements [container.requirements]

26.2.4 Node handles [container.node]

26.2.4.1 node_­handle overview [container.node.overview]

A node handle is an object that accepts ownership of a single element from an associative container or an unordered associative container.
It may be used to transfer that ownership to another container with compatible nodes.
Containers with compatible nodes have the same node handle type.
Elements may be transferred in either direction between container types in the same row of Table 89.
Table 89 — Container types with compatible nodes
map<K, T, C1, A>
map<K, T, C2, A>
map<K, T, C1, A>
multimap<K, T, C2, A>
set<K, C1, A>
set<K, C2, A>
set<K, C1, A>
multiset<K, C2, A>
unordered_­map<K, T, H1, E1, A>
unordered_­map<K, T, H2, E2, A>
unordered_­map<K, T, H1, E1, A>
unordered_­multimap<K, T, H2, E2, A>
unordered_­set<K, H1, E1, A>
unordered_­set<K, H2, E2, A>
unordered_­set<K, H1, E1, A>
unordered_­multiset<K, H2, E2, A>
If a node handle is not empty, then it contains an allocator that is equal to the allocator of the container when the element was extracted.
If a node handle is empty, it contains no allocator.
Class node_­handle is for exposition only.
An implementation is permitted to provide equivalent functionality without providing a class with this name.
If a user-defined specialization of pair exists for pair<const Key, T> or pair<Key, T>, where Key is the container's key_­type and T is the container's mapped_­type, the behavior of operations involving node handles is undefined.
template<unspecified>
  class node_handle {
  public:
    // These type declarations are described in Tables 90 and 91.
    using value_type     = see below;   // not present for map containers
    using key_type       = see below;   // not present for set containers
    using mapped_type    = see below;   // not present for set containers
    using allocator_type = see below;

  private:
    using container_node_type = unspecified;
    using ator_traits = allocator_traits<allocator_type>;

    typename ator_traits::rebind_traits<container_node_type>::pointer ptr_;
    optional<allocator_type> alloc_;

  public:
    constexpr node_handle() noexcept : ptr_(), alloc_() {}
    ~node_handle();
    node_handle(node_handle&&) noexcept;
    node_handle& operator=(node_handle&&);

    value_type& value() const;          // not present for map containers
    key_type& key() const;              // not present for set containers
    mapped_type& mapped() const;        // not present for set containers

    allocator_type get_allocator() const;
    explicit operator bool() const noexcept;
    bool empty() const noexcept;

    void swap(node_handle&)
      noexcept(ator_traits::propagate_on_container_swap::value ||
               ator_traits::is_always_equal::value);

    friend void swap(node_handle& x, node_handle& y) noexcept(noexcept(x.swap(y))) {
      x.swap(y);
    }
};