std::set::emplace_hint(3)

💅 std::set::emplace_hint(3) 💅

okay bestie, welcome to this page from the C++ Standard Libary manual — yes, section 3, standard library group chat, main character energy only. We're talking about std::set::emplace_hint, and she's here to insert things with a hint and zero drama. ✨

🎀 NAME 🎀

std::set::emplace_hint — std::set::emplace_hint. That's the name. Twice. It's giving manpage symmetry, and I'm obsessed. 💖

📜 Synopsis 📜

here's the signature, bestie — keep the (since C++11) exactly where it is:

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

so what does it do? it inserts a new element into the container as close as possible to the position just before hint.

The constructors of the key and mapped value are called with exactly the same arguments as supplied to the function, forwarded with std::forward<Args>(args)....

no iterators or references are invalidated. not a one. 💅

🧚 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 🎁

you get back an iterator to the inserted element, or to the element that prevented the insertion. either way, she's pointing at the relevant element. 🫶

😭 Exceptions 😭

if an exception is thrown for any reason, this function has no effect — that's the strong exception safety guarantee, and she said what she said. 💀

⚡ Complexity ⚡

logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint. that's the hint slay. ✨

🌸 Example 🌸

okay this example is long but it's literally benchmarking hint placement, and the possible output is right there. run this code, bestie:

// Run this code

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

        const int n_operations = 100'500'0;

        std::size_t set_emplace()
        {
            std::set<int> set;
            for (int i = 0; i < n_operations; ++i)
                set.emplace(i);
            return set.size();
        }

        std::size_t set_emplace_hint()
        {
            std::set<int> set;
            auto it = set.begin();
            for (int i = 0; i < n_operations; ++i)
            {
                set.emplace_hint(it, i);
                it = set.end();
            }
            return set.size();
        }

        std::size_t set_emplace_hint_wrong()
        {
            std::set<int> set;
            auto it = set.begin();
            for (int i = n_operations; i > 0; --i)
            {
                set.emplace_hint(it, i);
                it = set.end();
            }
            return set.size();
        }

        std::size_t set_emplace_hint_corrected()
        {
            std::set<int> set;
            auto it = set.begin();
            for (int i = n_operations; i > 0; --i)
            {
                set.emplace_hint(it, i);
                it = set.begin();
            }
            return set.size();
        }

        std::size_t set_emplace_hint_closest()
        {
            std::set<int> set;
            auto it = set.begin();
            for (int i = 0; i < n_operations; ++i)
                it = set.emplace_hint(it, i);
            return set.size();
        }

        double time_it(std::function<std::size_t()> set_test,
                       const char* what = nullptr,
                       double ratio = 0.0)
        {
            const auto start = std::chrono::system_clock::now();
            const std::size_t setsize = set_test();
            const auto stop = std::chrono::system_clock::now();
            const std::chrono::duration<double, std::milli> time = stop -
       start;
            if (what != nullptr && setsize > 0)
                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(set_emplace); // cache warmup
            const auto x = time_it(set_emplace, "plain emplace");
            time_it(set_emplace_hint, "emplace with correct hint", x);
            time_it(set_emplace_hint_wrong, "emplace with wrong hint", x);
            time_it(set_emplace_hint_corrected, "corrected emplace", x);
            time_it(set_emplace_hint_closest, "emplace using returned
       iterator", x);
        }

Possible output:

        392.25ms for plain emplace (ratio: 1.00)
         97.15ms for emplace with correct hint (ratio: 4.04)
        387.52ms for emplace with wrong hint (ratio: 1.01)
         84.80ms for corrected emplace (ratio: 4.63)
         83.67ms for emplace using returned iterator (ratio: 4.69)

👀 See also 👀

• emplace — constructs element in-place (C++11) (public member function).
• insert — inserts elements or nodes (since C++17) (public member function).

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