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Node handle (C++17)

Since C++17
template</* unspecified */>
class /* node-handle */;

Associative containers std::set, std::map, std::multiset, std::multimap, std::unordered_set, std::unordered_map, std::unordered_multiset, std::unordered_multimap, are node-based data structures, and their nodes can be extracted as an object of unspecified type known as node handle.

Node handle is a move-only type that owns and provides access to the element (the value_type) stored in the node, and provides non-const access to the key part of the element (the key_type) and the mapped part of the element (the mapped_type). If the node handle destructs while holding the node, the node is properly destructed using the appropriate allocator for the container. The node handle contains a copy of the container’s allocator. This is necessary so that the node handle can outlive the container.

The exact type of node handle (shown here as /* node-handle */) is unspecified, but each container exposes its node handle type as the member node_type.

Node handles can be used to transfer ownership of an element between two associative containers with the same key, value, and allocator type (ignoring comparison or hash/equality), without invoking any copy/move operations on the container element (this kind of operation is known as "splicing"). Transfer between unique and non-unique containers is also permitted: a node handle from a std::map can be inserted into an std::multimap, but not into std::unordered_map or std::set.

A node handle may be empty, in which case it holds no element and no allocator. The default-constructed and moved-from node handle is empty. In addition, an empty node handle can be produced by a failed call to container member function extract.

Pointers and references to an element that are obtained while it is owned by a node handle are invalidated if the element is successfully inserted into a container.

For all map containers (std::map, std::multimap, std::unordered_map, and std::unordered_multimap) whose key_type is K and mapped_type is T, the behavior of operations involving node handles is undefined if a user-defined specialization of std::pair exists for std::pair<K, T> or std::pair<const K, T>.

Member types​

pubMember typeDefinition
pubkey_type
(map containers only)		the key stored in the node
pubmapped_type
(map containers only)		the mapped part of the element stored in the node
pubvalue_type
(set containers only)		the element stored in the node
puballocator_typethe allocator to be used when destroying the element

Member functions​

constructors​

// 1)
constexpr /* node-handle */() noexcept;
// 2)
/* node-handle */ (/* node-handle */&& nh) noexcept;
  1. The default constructor initializes the node handle to the empty state.
  2. The move constructor takes ownership of the container element from nh, move-constructs the member allocator, and leaves nh in the empty state.

Parameters​

nh - a node handle with the same type (not necessarily the same container)

Notes​

Node handles are move-only, the copy constructor is not defined.

operator=​

/* node-handle */& operator=(/* node-handle */&& nh);
  • If the node handle is not empty,
    • destroys the value_type subobject in the container element object managed by this node handle by calling std::allocator_traits<allocator_type>::destroy;
    • deallocates the container element by calling std::allocator_traits<allocator_type>::rebind_traits</* container-node-type */>::deallocate;
  • Acquires ownership of the container element from nh;
  • If node handle was empty (and so did not contain an allocator) or if std::allocator_traits<allocator_type>::propagate_on_container_move_assignment is true, move-assigns the allocator from nh;
  • sets nh to the empty state.

The behavior is undefined if the node is not empty and std::allocator_traits<allocator_type>::propagate_on_container_move_assignment is false and the allocators do not compare equal.

Parameters​

nh - node handle with the same type (not necessarily the same container)

Return​

*this

Exceptions​

Throws nothing.

Notes​

Node handles are move-only, the copy assignment is not defined.

destructor​

~/* node-handle */();
  • If the node handle is not empty,
    • destroys the value_type subobject in the container element object managed by this node handle by calling std::allocator_traits<allocator_type>::destroy;
    • deallocates the container element by calling std::allocator_traits<allocator_type>::rebind_traits</* container-node-type */>::deallocate.

empty​

Until C++20
bool empty() const noexcept;
Since C++20
[[nodiscard]] bool empty() const noexcept;

Returns true if the node handle is empty, false otherwise.

operator bool​

explicit operator bool() const noexcept;

Converts to false if the node handle is empty, true otherwise.

get_allocator​

allocator_type get_allocator() const;

Returns a copy of the stored allocator (which is a copy of the allocator of the source container). The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

value​

set containers only
value_type& value() const;

Returns a reference to the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

key​

map containers only
key_type& key() const;

Returns a non-const reference to the key_type member of the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

Notes​

This function makes it possible to modify the key of a node extracted from a map, and then re-insert it into the map, without ever copying or moving the element.

mapped​

map containers only
mapped_type& mapped() const;

Returns a reference to the mapped_type member of the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

swap​

void swap(/* node-handle */& nh) noexcept(/* see below */);
  • swaps ownership of container nodes;
  • if one node is empty or if both nodes are non-empty and std::allocator_traits<allocator_type>::propagate_on_container_swap is true, swaps the allocators as well.

The behavior is undefined if both nodes are not empty and std::allocator_traits<allocator_type>::propagate_on_container_swap is false and the allocators do not compare equal.

Exceptions​

noexcept specification:

noexcept(std::allocator_traits<allocator_type>::propagate_on_container_swap::value ||
         std::allocator_traits<allocator_type>::is_always_equal::value)

Non-member functions​

swap​

friend void swap(/* node-handle */& x, /* node-handle */& y) noexcept(noexcept(x.swap(y)));

Effectively executes x.swap(y).

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when node-handle is an associated class of the arguments.

Node handle (C++17)

Since C++17
template</* unspecified */>
class /* node-handle */;

Associative containers std::set, std::map, std::multiset, std::multimap, std::unordered_set, std::unordered_map, std::unordered_multiset, std::unordered_multimap, are node-based data structures, and their nodes can be extracted as an object of unspecified type known as node handle.

Node handle is a move-only type that owns and provides access to the element (the value_type) stored in the node, and provides non-const access to the key part of the element (the key_type) and the mapped part of the element (the mapped_type). If the node handle destructs while holding the node, the node is properly destructed using the appropriate allocator for the container. The node handle contains a copy of the container’s allocator. This is necessary so that the node handle can outlive the container.

The exact type of node handle (shown here as /* node-handle */) is unspecified, but each container exposes its node handle type as the member node_type.

Node handles can be used to transfer ownership of an element between two associative containers with the same key, value, and allocator type (ignoring comparison or hash/equality), without invoking any copy/move operations on the container element (this kind of operation is known as "splicing"). Transfer between unique and non-unique containers is also permitted: a node handle from a std::map can be inserted into an std::multimap, but not into std::unordered_map or std::set.

A node handle may be empty, in which case it holds no element and no allocator. The default-constructed and moved-from node handle is empty. In addition, an empty node handle can be produced by a failed call to container member function extract.

Pointers and references to an element that are obtained while it is owned by a node handle are invalidated if the element is successfully inserted into a container.

For all map containers (std::map, std::multimap, std::unordered_map, and std::unordered_multimap) whose key_type is K and mapped_type is T, the behavior of operations involving node handles is undefined if a user-defined specialization of std::pair exists for std::pair<K, T> or std::pair<const K, T>.

Member types​

pubMember typeDefinition
pubkey_type
(map containers only)		the key stored in the node
pubmapped_type
(map containers only)		the mapped part of the element stored in the node
pubvalue_type
(set containers only)		the element stored in the node
puballocator_typethe allocator to be used when destroying the element

Member functions​

constructors​

// 1)
constexpr /* node-handle */() noexcept;
// 2)
/* node-handle */ (/* node-handle */&& nh) noexcept;
  1. The default constructor initializes the node handle to the empty state.
  2. The move constructor takes ownership of the container element from nh, move-constructs the member allocator, and leaves nh in the empty state.

Parameters​

nh - a node handle with the same type (not necessarily the same container)

Notes​

Node handles are move-only, the copy constructor is not defined.

operator=​

/* node-handle */& operator=(/* node-handle */&& nh);
  • If the node handle is not empty,
    • destroys the value_type subobject in the container element object managed by this node handle by calling std::allocator_traits<allocator_type>::destroy;
    • deallocates the container element by calling std::allocator_traits<allocator_type>::rebind_traits</* container-node-type */>::deallocate;
  • Acquires ownership of the container element from nh;
  • If node handle was empty (and so did not contain an allocator) or if std::allocator_traits<allocator_type>::propagate_on_container_move_assignment is true, move-assigns the allocator from nh;
  • sets nh to the empty state.

The behavior is undefined if the node is not empty and std::allocator_traits<allocator_type>::propagate_on_container_move_assignment is false and the allocators do not compare equal.

Parameters​

nh - node handle with the same type (not necessarily the same container)

Return​

*this

Exceptions​

Throws nothing.

Notes​

Node handles are move-only, the copy assignment is not defined.

destructor​

~/* node-handle */();
  • If the node handle is not empty,
    • destroys the value_type subobject in the container element object managed by this node handle by calling std::allocator_traits<allocator_type>::destroy;
    • deallocates the container element by calling std::allocator_traits<allocator_type>::rebind_traits</* container-node-type */>::deallocate.

empty​

Until C++20
bool empty() const noexcept;
Since C++20
[[nodiscard]] bool empty() const noexcept;

Returns true if the node handle is empty, false otherwise.

operator bool​

explicit operator bool() const noexcept;

Converts to false if the node handle is empty, true otherwise.

get_allocator​

allocator_type get_allocator() const;

Returns a copy of the stored allocator (which is a copy of the allocator of the source container). The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

value​

set containers only
value_type& value() const;

Returns a reference to the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

key​

map containers only
key_type& key() const;

Returns a non-const reference to the key_type member of the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

Notes​

This function makes it possible to modify the key of a node extracted from a map, and then re-insert it into the map, without ever copying or moving the element.

mapped​

map containers only
mapped_type& mapped() const;

Returns a reference to the mapped_type member of the value_type subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.

Exceptions​

Throws nothing.

swap​

void swap(/* node-handle */& nh) noexcept(/* see below */);
  • swaps ownership of container nodes;
  • if one node is empty or if both nodes are non-empty and std::allocator_traits<allocator_type>::propagate_on_container_swap is true, swaps the allocators as well.

The behavior is undefined if both nodes are not empty and std::allocator_traits<allocator_type>::propagate_on_container_swap is false and the allocators do not compare equal.

Exceptions​

noexcept specification:

noexcept(std::allocator_traits<allocator_type>::propagate_on_container_swap::value ||
         std::allocator_traits<allocator_type>::is_always_equal::value)

Non-member functions​

swap​

friend void swap(/* node-handle */& x, /* node-handle */& y) noexcept(noexcept(x.swap(y)));

Effectively executes x.swap(y).

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when node-handle is an associated class of the arguments.