std::set<Key,Compare,Allocator>::extract

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

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 the qualified-id Compare::is_transparent is valid and denotes a type, and neither iterator nor const_iterator is implicitly convertible from K. It 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 (rebalancing may occur, as with erase()).

Extracting a node invalidates only the iterators to the extracted element. 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 Compare object.

Complexity

1) Amortized constant.

2,3) log(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
`__cpp_lib_associative_heterogeneous_erasure`	202110L	(C++23)	Heterogeneous erasure in associative containers and unordered associative containers, (3)

Example

Run this code

#include <algorithm>
#include <iostream>
#include <string_view>
#include <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::set<int> cont{1, 2, 3};
 
    print("Start:", cont);
 
    // Extract node handle and change key
    auto nh = cont.extract(1);
    nh.value() = 4;
 
    print("After extract and before insert:", cont);
 
    // Insert node handle back
    cont.insert(std::move(nh));
 
    print("End:", cont);
}

Output:

Start: 1 2 3
After extract and before insert: 2 3
End: 2 3 4

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
vector<bool>
inplace_vector (C++26)
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Tables
Iterator invalidation
Member function table
Non-member function table

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)