std::unorde...t_or_assign(3)

💖 std::unorde...t_or_assign(3) 💖

okay so bestie, this is std::unorde...t_or_assign(3) from the C++ Standard Libary manual, and it's literally the insert-or-assign glow-up for std::unordered_map 🫶. It’s giving “put it in if it’s not there, overwrite it if it is,” and I’m obsessed. No cap.

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✨ NAME ✨

• std::unordered_map::insert_or_assign — std::unordered_map::insert_or_assign

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🦋 SYNOPSIS 🦋

(1) (since C++17)

template< class M >
std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj );

(2) (since C++17)

template< class M >
std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj );

(3) (since C++26)

template< class K, class M >
std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj );

(4) (since C++17)

template< class M >
iterator insert_or_assign( const_iterator hint, const Key& k, M&& obj );

(5) (since C++17)

template< class M >
iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj );

(6) (since C++26)

template< class K, class M >
iterator insert_or_assign( const_iterator hint, K&& k, M&& obj );

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🌸 DESCRIPTION 🌸

• 1,4) If a key equivalent to k already exists in the container, assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, inserts the new value as if by insert, constructing it from value_type(k, std::forward<M>(obj)).
• 2,5) Same as (1,4), except the mapped value is constructed from value_type(std::move(k), std::forward<M>(obj)).
• 3,6) If a key equivalent to k already exists in the container, assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, constructs an object u of value_type with std::forward<K>(k), std::forward<M>(obj)), then inserts u into *this. If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the behavior is undefined. The value_type must be EmplaceConstructible into unordered_map from std::forward<K>(k), std::forward<M>(obj). This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. 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.

🚨 Important behavior check, bestie: The behavior is undefined (until C++20) / The program is ill-formed (since C++20) if std::is_assignable_v<mapped_type&, M&&> is false. Do not skip this 😭.

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place. If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.

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🎀 PARAMETERS 🎀

• k — the key used both to look up and to insert if not found
• hint — iterator to the position before which the new element will be inserted
• obj — the value to insert or assign

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💅 RETURN VALUE 💅

• 1-3) The bool component is true if the insertion took place and false if the assignment took place. The iterator component is pointing at the element that was inserted or updated.
• 4-6) Iterator pointing at the element that was inserted or updated.

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🔥 COMPLEXITY 🔥

• 1-3) Same as for emplace.
• 4-6) Same as for emplace_hint.

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👑 NOTES 👑

insert_or_assign returns more information than operator[] and does not require default-constructibility of the mapped type. Slay.

Feature-test macros, because we love receipts:

• Feature-test macro: __cpp_lib_unordered_map_try_emplace — Value: 201411L — Std: (C++17) — covers std::unordered_map::try_emplace, std::unordered_map::insert_or_assign.
• Feature-test macro: __cpp_lib_associative_heterogeneous_insertion — Value: 202311L — Std: (C++26) — Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6).

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🌈 EXAMPLE 🌈

run this code, it’s literally right here:

#include <iostream>
#include <string>
#include <unordered_map>

void print_node(const auto& node)
{
    std::cout << '[' << node.first << "] = " << node.second << '\n';
}

void print_result(auto const& pair)
{
    std::cout << (pair.second ? "inserted: " : "assigned: ");
    print_node(*pair.first);
}

int main()
{
    std::unordered_map<std::string, std::string> myMap;

    print_result(myMap.insert_or_assign("a", "apple"));
    print_result(myMap.insert_or_assign("b", "banana"));
    print_result(myMap.insert_or_assign("c", "cherry"));
    print_result(myMap.insert_or_assign("c", "clementine"));

    for (const auto& node : myMap)
        print_node(node);
}

🍓 POSSIBLE OUTPUT 🍓

inserted: [a] = apple
inserted: [b] = banana
inserted: [c] = cherry
assigned: [c] = clementine
[c] = clementine
[a] = apple
[b] = banana

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🦋 SEE ALSO 🦋

• operator[] — access or insert specified element (public member function)
• at — access specified element with bounds checking (public member function)
• insert — inserts elements or nodes (since C++17) (public member function)
• emplace — constructs element in-place (public member function)

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💖 sourced from http://cppreference.com · dated 2024.06.10 · byeee bestie, go insert-or-assign your dreams 🫶