std::unordered_map::extract

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

span (C++20)

std::unordered_map

Member functions

unordered_map::unordered_map

unordered_map::~unordered_map

unordered_map::operator=

unordered_map::get_allocator

Iterators

unordered_map::beginunordered_map::cbegin

unordered_map::endunordered_map::cend

Capacity

unordered_map::erase

unordered_map::size

unordered_map::max_size

Modifiers

unordered_map::clear

unordered_map::insert

unordered_map::insert_or_assign (C++17)

unordered_map::emplace

unordered_map::emplace_hint

unordered_map::try_emplace (C++17)

unordered_map::erase

unordered_map::swap

unordered_map::extract (C++17)

unordered_map::merge (C++17)

Lookup

unordered_map::at

unordered_map::operator[]

unordered_map::count

unordered_map::find

unordered_map::contains (C++20)

unordered_map::equal_range

Bucket interface

unordered_map::begin(size_type)unordered_map::cbegin(size_type)

unordered_map::end(size_type)unordered_map::cend(size_type)

unordered_map::bucket_count

unordered_map::max_bucket_count

unordered_map::bucket_size

unordered_map::bucket

Hash policy

unordered_map::load_factor

unordered_map::max_load_factor

unordered_map::rehash

unordered_map::reserve

Observers

unordered_map::hash_function

unordered_map::key_eq

Deduction guides(C++17)

node_type extract( const_iterator position );	(1)	(since C++17)
node_type extract( const key_type& x );	(2)	(since C++17)

1) Unlinks the node that contains the element pointed to by position and returns a node handle that owns it

2) If the container has an element with key equivalent to x, unlinks the node that contains that element from the container and returns a node handle that owns it. Otherwise, returns an empty node handle.

In either case, no elements are copied or moved, only the internal pointers of the container nodes are repointed

Extracting a node invalidates only the iterators to the extracted element, and preserves the relative order of the elements that are not erased. Pointers and references to the extracted element remain valid, but cannot be used while element is owned by a node handle: they become usable if the element is inserted into a container.

Parameters

position	-	a valid iterator into this container
x	-	a key to identify the node to be extracted

Return value

A node handle that owns the extracted element, or empty node handle in case the element is not found in overload (2)

Complexity

1,2) Average case O(1), worst case O(a.size()).

Notes

extract is the only way to change a key of a map element without reallocation:

map<int, string> m{{1,mango}, {2,papaya}, {3,guava}};
auto nh = m.extract(2);
nh.key() = 4;
m.insert(move(nh));
// m == {{1,mango}, {3,guava}, {4,papaya}}

See also

merge (C++17)	splices nodes from another container (public member function)
insert	inserts elements or nodes (since C++17) (public member function)
erase	erases elements (public member function)