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std::unorde...ap::extract(3) C++ Standard Libary std::unorde...ap::extract(3) NAME std::unordered_multimap::extract - std::unordered_multimap::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 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 with- out 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 in- serted 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 re- allocation: 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 merge splices nodes from another container (C++17) (public member function) inserts elements insert or nodes (C++11) (since C++17) (public member function) erase erases elements (C++11) (public member function) http://cppreference.com 2022.07.31 std::unorde...ap::extract(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Complexity | Notes | Example | Possible output: | See also
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