std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::extract

 

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std::unordered_multimap

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unordered_multimap::unordered_multimap unordered_multimap::~unordered_multimap    unordered_multimap::operator= unordered_multimap::get_allocator Iterators unordered_multimap::beginunordered_multimap::cbegin unordered_multimap::endunordered_multimap::cend Capacity unordered_multimap::size unordered_multimap::max_size unordered_multimap::empty Modifiers unordered_multimap::clear unordered_multimap::insert unordered_multimap::insert_range (C++23) unordered_multimap::emplace unordered_multimap::emplace_hint unordered_multimap::erase unordered_multimap::swap unordered_multimap::extract (C++17) unordered_multimap::merge (C++17)	Lookup unordered_multimap::count unordered_multimap::find unordered_multimap::contains (C++20) unordered_multimap::equal_range Bucket interface unordered_multimap::begin(size_type)unordered_multimap::cbegin(size_type) unordered_multimap::end(size_type)unordered_multimap::cend(size_type) unordered_multimap::bucket_count unordered_multimap::max_bucket_count unordered_multimap::bucket_size unordered_multimap::bucket Hash policy unordered_multimap::load_factor unordered_multimap::max_load_factor unordered_multimap::rehash unordered_multimap::reserve Observers unordered_multimap::hash_function unordered_multimap::key_eq
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std::swap(std::unordered_multimap) erase_if(std::unordered_multimap) (C++20)	operator==operator!= (until C++20)
Deduction guides (C++17)

 

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 the first such 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 change a key of a map element without reallocation:

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

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 <unordered_map>
 
void print(std::string_view comment, const auto& data)
{
    std::cout << comment;
    for (auto [k, v] : data)
        std::cout << ' ' << k << '(' << v << ')';
 
    std::cout << '\n';
}
 
int main()
{
    std::unordered_multimap<int, char> cont{{1, 'a'}, {2, 'b'}, {3, 'c'}};
 
    print("Start:", cont);
 
    // Extract node handle and change key
    auto nh = cont.extract(1);
    nh.key() = 4;
 
    print("After extract and before insert:", cont);
 
    // Insert node handle back
    cont.insert(std::move(nh));
 
    print("End:", cont);
}

Possible output:

Start: 1(a) 2(b) 3(c)
After extract and before insert: 2(b) 3(c)
End: 2(b) 3(c) 4(a)

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)