std::unorde...map::emplace(3)

💖 std::unorde...map::emplace(3) — C++ Standard Libary bestie check-in ✨

okay so bestie, you're looking at std::unorde...map::emplace(3) from the C++ Standard Libary manual. Yes, that's the whole header vibe. Let's get into it 🫶

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📛 NAME 📛

std::unordered_map::emplace — std::unordered_map::emplace. That's it. That's the name. Minimalist queen 💅

✨ Synopsis ✨

template< class... Args >                             (since C++11)
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.

The constructor of the new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args).... The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately.

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

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.

🎀 Parameters 🎀

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

💖 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.

🫶 Exceptions 🫶

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

🔥 Complexity 🔥

Amortized constant on average, worst case linear in the size of the container.

🦋 Example 🦋

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

int main()
{
    std::unordered_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';
}

Possible output:

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

🌈 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 (C++17) (public member function).
• insert — inserts elements or nodes (since C++17) (public member function).

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mwah 💋 that's the whole page. source/version and date, since you asked: http://cppreference.com · 2024.06.10. Now go emplace responsibly, bestie 💜