NAME¶

std::unordered_set::extract - std::unordered_set::extract

Synopsis¶

node_type extract( const_iterator position ); (1) (since C++17)
node_type extract( const Key& k ); (2) (since C++17)
template< class K > (3) (since C++23)
node_type extract( K&& x );

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 k, unlinks the node that
contains that element from the container and returns a node handle that owns it.
Otherwise, returns an empty node handle.
3) Same as (2). This overload participates in overload resolution only if
Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type,
and neither iterator nor const_iterator is implicitly convertible from K. This
assumes that such Hash is callable with both K and Key type, and that the KeyEqual
is transparent, which, together, allows calling this function without constructing
an instance of Key.

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
k - a key to identify the node to be extracted
x - a value of any type that can be transparently compared with a key
identifying 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 (2,3).

Exceptions¶

1) Throws nothing.
2,3) Any exceptions thrown by the Hash and KeyEqual object.

Complexity¶

1,2,3) Average case O(1), worst case O(size()).

Notes¶

extract is the only way to take a move-only object out of a set:

std::set<move_only_type> s;
s.emplace(...);
move_only_type mot = std::move(s.extract(s.begin()).value());

Feature-test macro Value Std Feature
Heterogeneous erasure in
associative containers
__cpp_lib_associative_heterogeneous_erasure 202110L (C++23) and unordered
associative containers,
(3)

Example¶

// Run this code

#include <algorithm>
#include <iostream>
#include <string_view>
#include <unordered_set>

void print(std::string_view comment, const auto& data)
{
std::cout << comment;
for (auto datum : data)
std::cout << ' ' << datum;

std::cout << '\n';
}

int main()
{
std::unordered_set<int> cont{1, 2, 3};

print("Start:", cont);

// Extract node handle and change key
auto nh = cont.extract(1);
nh.value() = 4;