std::unordered_map extract() method

od C++23

// (1) Non const version only
node_type extract( const_iterator position );

// (2) Non const version only
node_type extract( const Key& k );

// (3) Non const version only
template< class K >
node_type extract( K&& x );

od C++17

// (1) Non const version only
node_type extract( const_iterator position );

// (2) Non const version only
node_type extract( const Key& k );

• (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.

Invalidation

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).

Complexity

Average case, constant - O(1).
Worst case, linear in the size of the container - O(size()).

Exceptions

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

Notes

Using 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 testing macro: __cpp_lib_associative_heterogeneous_erasure (for overload (3))

Example

Main.cpp

#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_map<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)