std::flat_map::emplace(3)

💖 std::flat_map::emplace 💖

okay so this is the std::flat_map::emplace page from the C++ Standard Libary manual, bestie. this overload is marked (since C++23), and it’s giving “insert in place with receipts” energy 💅✨

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✨ NAME ✨

std::flat_map::emplace — yep, literally std::flat_map::emplace. no notes, it’s that girl 🎀

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🌸 Synopsis 🌸

here’s the signature, copied with love and zero edits:

template< class... Args >                             (since C++23)
std::pair<iterator, bool> emplace( Args&&... args );

Inserts a new element into the container constructed in-place with the given args, if there is no element with the key in the container.

Initializes an object t of type std::pair<key_type, mapped_type> with std::forward<Args>(args)...; if the map already contains an element whose key is equivalent to t.first, *this is unchanged. Otherwise, equivalent to:

auto key_it = ranges::upper_bound(c.keys, t.first, compare);
auto value_it = c.values.begin() + std::distance(c.keys.begin(),
       key_it);
c.keys.insert(key_it, std::move(t.first));
c.values.insert(value_it, std::move(t.second));

This overload participates in overload resolution only if std::is_constructible_v<std::pair<key_type, mapped_type>, Args...> is true.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations. 🔥

🔎 Information on iterator invalidation is copied from here.

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🍓 Parameters 🍓

• args — arguments to forward to the constructor of the element 🫶

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👑 Return value 👑

A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place. periodt 💅

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😭 Exceptions 😭

If an exception is thrown for any reason, this function has no effect — that’s the strong exception safety guarantee, bestie. no half-finished chaos, we love to see it.

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🌈 Complexity 🌈

If insertion occurs, it’s linear in the size of the container; otherwise it’s logarithmic in the size of the container.

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🧚 Example 🧚

// Run this code

#include <iostream>
#include <string>
#include <utility>
#include <flat_map>

int main()
{
    std::flat_map<std::string, std::string> m;

    // uses pair's move constructor
    m.emplace(std::make_pair(std::string("a"), std::string("a")));

    // uses pair's converting move constructor
    m.emplace(std::make_pair("b", "abcd"));

    // uses pair's template constructor
    m.emplace("d", "ddd");

    // emplace with duplicate key has no effect
    m.emplace("d", "DDD");

    // uses pair's piecewise constructor
    m.emplace(std::piecewise_construct,
              std::forward_as_tuple("c"),
              std::forward_as_tuple(10, 'c'));
    // an alternative is: m.try_emplace("c", 10, 'c');

    for (const auto& p : m)
        std::cout << p.first << " => " << p.second << '\n';
}

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🍓 Output 🍓

a => a
b => abcd
c => cccccccccc
d => ddd

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🦋 See also 🦋

• emplace_hint — constructs elements in-place using a hint (public member function) ✨
• try_emplace — inserts in-place if the key does not exist, does nothing if the key exists (public member function) 💜
• insert — inserts elements (public member function) 🌸

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source/site: http://cppreference.com · version/date: 2024.06.10 · page: std::flat_map::emplace(3) — byeeee bestie, go emplace responsibly 💅🫶