[container.node.overview]

# 23 Containers library [[containers]](./#containers)

## 23.2 Requirements [[container.requirements]](container.requirements#container.node.overview)

### 23.2.5 Node handles [[container.node]](container.node#overview)

#### 23.2.5.1 Overview [container.node.overview]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L2318)

A [*node handle*](#def:node_handle "23.2.5.1 Overview [container.node.overview]") is an object that accepts ownership of a single element
from an associative container ([[associative.reqmts]](associative.reqmts "23.2.7 Associative containers")) or an unordered
associative container ([[unord.req]](unord.req "23.2.8 Unordered associative containers"))[.](#1.sentence-1)

It may be used to transfer that
ownership to another container with compatible nodes[.](#1.sentence-2)

Containers with
compatible nodes have the same node handle type[.](#1.sentence-3)

Elements may be transferred in
either direction between container types in the same row of
Table [75](#tab:container.node.compat "Table 75: Container types with compatible nodes")[.](#1.sentence-4)

Table [75](#tab:container.node.compat) — Container types with compatible nodes [[tab:container.node.compat]](./tab:container.node.compat)  

| [🔗](#tab:container.node.compat-row-1)<br>map<K, T, C1, A> | map<K, T, C2, A> |
| --- | --- |
| [🔗](#tab:container.node.compat-row-2)<br>map<K, T, C1, A> | multimap<K, T, C2, A> |
| [🔗](#tab:container.node.compat-row-3)<br>set<K, C1, A> | set<K, C2, A> |
| [🔗](#tab:container.node.compat-row-4)<br>set<K, C1, A> | multiset<K, C2, A> |
| [🔗](#tab:container.node.compat-row-5)<br>unordered_map<K, T, H1, E1, A> | unordered_map<K, T, H2, E2, A> |
| [🔗](#tab:container.node.compat-row-6)<br>unordered_map<K, T, H1, E1, A> | unordered_multimap<K, T, H2, E2, A> |
| [🔗](#tab:container.node.compat-row-7)<br>unordered_set<K, H1, E1, A> | unordered_set<K, H2, E2, A> |
| [🔗](#tab:container.node.compat-row-8)<br>unordered_set<K, H1, E1, A> | unordered_multiset<K, H2, E2, A> |

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L2347)

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[.](#2.sentence-1)

If a node handle
is empty, it contains no allocator[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L2352)

Class *node-handle* is for exposition only[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L2355)

If a user-defined specialization of pair exists forpair<const Key, T> or pair<Key, T>, where Key is the
container's key_type and T is the container'smapped_type, the behavior of operations involving node handles is
undefined[.](#4.sentence-1)

template<*unspecified*>class *node-handle* {public:// These type declarations are described in [[associative.reqmts]](associative.reqmts "23.2.7 Associative containers") and [[unord.req]](unord.req "23.2.8 Unordered associative containers").using value_type = *see below*; // not present for map containersusing key_type = *see below*; // not present for set containersusing mapped_type = *see below*; // not present for set containersusing allocator_type = *see below*;

private:using container_node_type = *unspecified*; // *exposition only*using ator_traits = allocator_traits<allocator_type>; // *exposition only*typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_; // *exposition only* optional<allocator_type> alloc_; // *exposition only*public:// [[container.node.cons]](container.node.cons "23.2.5.2 Constructors, copy, and assignment"), constructors, copy, and assignmentconstexpr *node-handle*() noexcept : ptr_(), alloc_() {}constexpr *node-handle*(*node-handle*&&) noexcept; constexpr *node-handle*& operator=(*node-handle*&&); // [[container.node.dtor]](container.node.dtor "23.2.5.3 Destructor"), destructorconstexpr ~*node-handle*(); // [[container.node.observers]](container.node.observers "23.2.5.4 Observers"), observersconstexpr value_type& value() const; // not present for map containers key_type& key() const; // not present for set containersconstexpr mapped_type& mapped() const; // not present for set containersconstexpr allocator_type get_allocator() const; constexpr explicit operator bool() const noexcept; constexpr bool empty() const noexcept; // [[container.node.modifiers]](container.node.modifiers "23.2.5.5 Modifiers"), modifiersconstexpr void swap(*node-handle*&)noexcept(ator_traits::propagate_on_container_swap::value || ator_traits::is_always_equal::value); friend constexpr void swap(*node-handle*& x, *node-handle*& y) noexcept(noexcept(x.swap(y))) { x.swap(y); }};