std::unorde...ap::extract(3)

💅 std::unordered_multimap::extract

okay bestie, so this page is from the C++ Standard Library manpages, and it's all about std::unordered_multimap::extract — literally the girlboss move of yoinking a node out of your container without touching the actual elements. no copying, no moving, just vibes and pointer repointing. slay.

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✨ 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 );

so like, there's three flavors of this icon, and here's the tea on each:

1. Unlinks the node that contains the element pointed to by position and returns a node handle that owns it. periodt.
2. If the container has an element with key equivalent to k, it unlinks the node that contains the first such element from the container and returns a node handle that owns it. Otherwise? bestie gets an empty node handle. sorry babe. 🫶
3. Same as (2) — but this overload only participates in overload resolution 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. iykyk 🌸

In either case, no elements are copied or moved — only the internal pointers of the container nodes are repointed. literally just a glow-up for your pointers, no relocation stress 💅

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 the element is owned by a node handle: they become usable again if the element is inserted into a container. like, she's on a little vacation, okay? she'll be back. ✨

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🎀 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

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💖 Return value 💖

A node handle that owns the extracted element — or an empty node handle in case the element is not found in (2, 3). no cap, sometimes she's just not there. 😭

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🔥 Exceptions 🔥

• (1) Throws nothing. queen of calm. 👑
• (2, 3) Any exceptions thrown by the Hash and KeyEqual object. so like, if those go off, that's on them, not on extract.

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⭐ Complexity ⭐

• (1, 2, 3) Average case O(1), worst case O(size()).

average case is giving effortless, worst case is giving finals week. 💀

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🌈 Notes 🌈

extract is literally 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"}}

see how we swapped papaya's key from 2 to 4 without any reallocation drama? iconic. 🍓

Feature-test macro tea ☕:

• __cpp_lib_associative_heterogeneous_erasure — value 202110L, std (C++23), feature: Heterogeneous erasure in associative containers and unordered associative containers, relates to overload (3).

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🦋 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)

look at her go — extracted, key-swapped, reinserted, all without breaking a sweat. main character energy fr fr. 🔥

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👑 See also 👑

• merge — splices nodes from another container (C++17) (public member function)
• insert — inserts elements or nodes (since C++17) (public member function)
• erase — erases elements (public member function)

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aaaaand that's the whole page bestie! 🎀 this gem lives over at http://cppreference.com, dated 2024.06.10, under the very iconic title std::unordered_multimap::extract(3). go forth and extract responsibly. mwah! 💅✨🫶