std::flat_m...emplace_hint(3)

💅 std::flat_map::emplace_hint 💅

okay so this is the std::flat_m...emplace_hint(3) page from the C++ Standard Libary, and it's giving insert but make it hinted. slay. ✨ iykyk.

🌸 NAME 🌸

std::flat_map::emplace_hint — yeah, that's the whole name. iconic, no notes.

✨ Synopsis ✨

template< class... Args >                                      (since C++23)
iterator emplace_hint( const_iterator hint, Args&&... args );

So this one drops a new element into the container as close as possible to the position just before hint. 🫶

The constructor of the element type (value_type, that is, std::pair<const Key, T>) gets called with exactly the same arguments you passed to the function, forwarded with std::forward<Args>(args).... no extra seasoning. 💅

🧚 information on iterator invalidation is copied from here — that's literally what the page says, bestie.

🎀 Parameters 🎀

• hint — iterator to the position before which the new element will be inserted.
• args — arguments to forward to the constructor of the element.

💖 Return value 💖

An iterator to the inserted element, or to the element that prevented the insertion. yes, it might hand you the blocker instead. 😭

🔥 Exceptions 🔥

If an exception is thrown for any reason, this function has no effect — that's the strong exception safety guarantee. so if it throws, it's a clean no-op. periodt.

🌀 Complexity 🌀

This section is incomplete. (the page said it, not me 💀)

🍓 Example 🍓

// Run this code

#include <chrono>
#include <cstddef>
#include <functional>
#include <iomanip>
#include <iostream>
#include <flat_map>

const int n_operations = 100'500'0;

std::size_t map_emplace()
{
    std::flat_map<int, char> map;
    for (int i = 0; i < n_operations; ++i)
        map.emplace(i, 'a');
    return map.size();
}

std::size_t map_emplace_hint()
{
    std::flat_map<int, char> map;
    auto it = map.begin();
    for (int i = 0; i < n_operations; ++i)
    {
        map.emplace_hint(it, i, 'b');
        it = map.end();
    }
    return map.size();
}

std::size_t map_emplace_hint_wrong()
{
    std::flat_map<int, char> map;
    auto it = map.begin();
    for (int i = n_operations; i > 0; --i)
    {
        map.emplace_hint(it, i, 'c');
        it = map.end();
    }
    return map.size();
}

std::size_t map_emplace_hint_corrected()
{
    std::flat_map<int, char> map;
    auto it = map.begin();
    for (int i = n_operations; i > 0; --i)
    {
        map.emplace_hint(it, i, 'd');
        it = map.begin();
    }
    return map.size();
}

std::size_t map_emplace_hint_closest()
{
    std::flat_map<int, char> map;
    auto it = map.begin();
    for (int i = 0; i < n_operations; ++i)
        it = map.emplace_hint(it, i, 'e');
    return map.size();
}

double time_it(std::function<std::size_t()> map_test,
               std::string what = "", double ratio = 0.0)
{
    const auto start = std::chrono::system_clock::now();
    const std::size_t map_size = map_test();
    const auto stop = std::chrono::system_clock::now();
    std::chrono::duration<double, std::milli> time = stop - start;
    if (what.size() && map_size)
        std::cout << std::setw(8) << time << " for " << what << " (ratio: "
                  << (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n";
    return time.count();
}

int main()
{
    std::cout << std::fixed << std::setprecision(2);
    time_it(map_emplace); // cache warmup
    const auto x = time_it(map_emplace, "plain emplace");
    time_it(map_emplace_hint, "emplace with correct hint", x);
    time_it(map_emplace_hint_wrong, "emplace with wrong hint", x);
    time_it(map_emplace_hint_corrected, "corrected emplace", x);
    time_it(map_emplace_hint_closest, "emplace using returned iterator", x);
}

🌫️ Possible output 🌫️

...TODO...

👀 See also 👀

• emplace — constructs element in-place (public member function).
• insert — inserts elements (public member function).

🗂️ Category 🗂️

• * Todo without reason

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source/version: http://cppreference.com · date: 2024.06.10 · page: std::flat_m...emplace_hint(3) — byeee bestie, go emplace something cute 💖🦋🎀