std::map::insert_or_assign(3)

💅 std::map::insert_or_assign — from the C++ Standard Library, bestie

okay so first things first: the manual name is std::map::insert_or_assign(3), from the C++ Standard Library, and this whole page is about one little member function that just gets you. No cap.

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✨ NAME ✨

std::map::insert_or_assign — literally what it says on the tin: it inserts a thing, or it assigns a thing. Iconic behavior. We stan a function that communicates.

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🦋 Synopsis 🦋

Six whole overloads, and yes we're listing every single one, because missing an overload is not the vibe.

(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 );

Now the glow-up explanation of what each group actually does 👇

• 1 and 4 — if a key equivalent to k already exists in the container, it assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, it inserts the new value as if by insert, constructing it from value_type(k, std::forward<M>(obj)).
• 2 and 5 — same as (1,4), except the mapped value is constructed from value_type(std::move(k), std::forward<M>(obj)).
• 3 and 6 — if a key equivalent to k already exists in the container, it assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, it constructs an object u of value_type with std::forward<K>(k), std::forward<M>(obj)), then inserts u into *this. ⚠️ If equal_range(u.first) == equal_range(k) is false, the behavior is undefined. The value_type must be EmplaceConstructible into map from std::forward<K>(k), std::forward<M>(obj). This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of Key. (iykyk — that's the hetero lookups club 🫶)

🚨 important vibe check: if std::is_assignable_v<mapped_type&, M&&> is false, then the behavior is undefined (until C++20), and the program is ill-formed (since C++20). Like, don't do that. Please.

✨ And the best part: no iterators or references are invalidated. She's polite. She doesn't disturb the neighbors.

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🎀 Parameters 🎀

• k — the key used both to look up and to insert if not found. Dual-purpose queen 👑
• 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.

So basically overloads 1–3 give you the receipt and the location 📍, and 4–6 just point at the thing.

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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. That's the whole flex. It tells you what happened and it doesn't force your mapped type to be default-constructible. main character energy, honestly.

Feature-test macro tea ☕:

• __cpp_lib_map_try_emplace — value 201411L, std (C++17) — covers std::map::try_emplace, std::map::insert_or_assign, Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers.
• __cpp_lib_associative_heterogeneous_insertion — value 202311L, std (C++26) — covers 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)

#include <iostream>
#include <string>
#include <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::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);
}

Output:

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

See how "c" got inserted once and then assigned the second time? That's the whole point, bestie. Cherry ➡️ clementine. Growth. 🍓

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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 (C++11) — constructs element in-place (public member function)

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💀 one last thing: that hint overload set is not a do-nothing parameter — it's "iterator to the position before which the new element will be inserted." Read it. Respect it. 🫶

okay bye ily, go insert_or_assign something 💜

source: cppreference.com · version: 2024.06.10 — mwah, that's the footer tea, see u next manpage 🎀✨