23 Containers library [containers]

23.2 Container requirements [container.requirements]

23.2.4 Node handles [container.node]

23.2.4.1 node_handle overview [container.node.overview]

1

A node handle is an object that accepts ownership of a single element from an associative container ([associative.reqmts]) or an unordered associative container ([unord.req]). 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 [tab:containers.node.compat].

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>
2

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.

3

Class node_handle is for exposition only. An implementation is permitted to provide equivalent functionality without providing a class with this name.

4

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 [tab:containers.associative.requirements] and [tab:HashRequirements]
  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); }
};