std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::extract
From cppreference.com
< cpp | container | unordered multimap
| 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(a.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: | __cpp_lib_associative_heterogeneous_erasure (for overload (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
| (C++17) |
splices nodes from another container (public member function) |
| (C++11) |
inserts elements or nodes (since C++17) (public member function) |
| (C++11) |
erases elements (public member function) |