std::unorde...mplace_hint(3)

💅 std::unordered_map::emplace_hint

okay so bestie, welcome to the C++ Standard Library manual page for std::unordered_map::emplace_hint — yes the whole name, it's giving "my full government name" energy 🎀 this one lives in the C++ Standard Libary index and it is so much chiller than it sounds, i promise.

---

✨ NAME ✨

std::unordered_map::emplace_hint — literally the thing this whole page is about, no plot twist 🌸

---

🦋 Synopsis 🦋

template< class... Args >                                      (since C++11)
iterator emplace_hint( const_iterator hint, Args&&... args );

So what's the vibe? It inserts a new element into the container, and it uses hint as a suggestion for where the element should go 👀 — like, it's a hint, not a command, the container is the main character here.

The constructor of the element type (value_type, that is, std::pair<const Key, T>) gets called with exactly the same arguments as you supplied to the function, forwarded with std::forward<Args>(args).... periodt ✨

And here's the important glow-up detail: 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. so like... pay attention to which one you're in, bestie.

---

🌈 Parameters 🌈

• hint — an iterator, used as a suggestion as to where to insert the new element 🫶 (again: suggestion. it's a soft launch.)
• args — the arguments to forward to the constructor of the element

---

👑 Return value 👑

You get back an iterator to the inserted element, or to the element that prevented the insertion. so you always get a valid iterator back, which is honestly iconic ⭐

---

🔥 Exceptions 🔥

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee). that's the good stuff, no half-finished mess left behind 🧚

---

💜 Complexity 💜

Amortized constant on average, worst case linear in the size of the container. ngl, that's the whole story.

---

💖 Example 💖

// Run this code

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

const int n_operations = 100'500'0;

std::size_t map_emplace()
{
    std::unordered_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::unordered_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::unordered_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::unordered_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::unordered_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 🌸

143.48ms for plain emplace (ratio: 1.00)
164.78ms for emplace with correct hint (ratio: 0.87)
171.22ms for emplace with wrong hint (ratio: 0.84)
166.55ms for corrected emplace (ratio: 0.86)
167.41ms for emplace using returned iterator (ratio: 0.86)

lowkey the ratio column is the whole tea ☕ — the "wrong hint" one is out here at 0.84 and honestly? relatable.

---

🎀 See also 🎀

• emplace — constructs element in-place (public member function)
• insert — inserts elements or nodes (since C++17) (public member function)

---

🫶 friendly reminder bestie: hint is a suggestion, not a promise, and if a rehash happens your iterators are gone gone. check the max_load_factor() * bucket_count() line before you get attached 💅

okay that's the page!! ily, go read the insert page next, it's the sibling 💜

— with love, from http://cppreference.com, version 2024.06.10 🌈✨ (that's the std::unorde...mplace_hint(3) you're reading, mwah 💋)