std::atomic::exchange(3)

💖 std::atomic::exchange 💖

okay so this is the std::atomic::exchange(3) page from the C++ Standard Libary, bestie — and it’s giving “atomic swap with receipts.” 💅✨

🌈 NAME 🌈

std::atomic::exchange — std::atomic::exchange

✨ Synopsis ✨

Two overloads, both (since C++11), no cap:

T exchange( T desired, std::memory_order order =
             std::memory_order_seq_cst ) noexcept;

(1) (since C++11)

T exchange( T desired, std::memory_order order =
             std::memory_order_seq_cst ) volatile noexcept;

(2) (since C++11)

Atomically replaces the underlying value with desired (a read-modify-write operation). Memory is affected according to the value of order.

since C++20, it’s deprecated if std::atomic<T>::is_always_lock_free is false and overload (2) participates in overload resolution. that’s the real caveat, bestie — don’t skip it. 💀

🎀 Parameters 🎀

• desired — value to assign
• order — memory order constraints to enforce

🫶 Return value 🫶

The value of the atomic variable before the call. that’s the old value — the receipt, the before pic, the “what was it before you slayed” moment. 🧾✨

🦋 Example 🦋

// Run this code

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <iostream>
#include <syncstream>
#include <thread>
#include <vector>

int main()
{
    constexpr int thread_count{5};
    constexpr int sum{5};

    std::atomic<int> atom{0};
    std::atomic<int> counter{0};

    auto increment_to_sum = [&](const int id)
    {
        for (int next = 0; next < sum;)
        {
            // each thread is writing a value from its own knowledge
            const int current = atom.exchange(next);
            counter++;
            // sync writing to prevent from interrupting by other
       threads
            std::osyncstream(std::cout)
                << "Thread #" << id << " (id=" <<
       std::this_thread::get_id()
                << ") wrote " << next << " replacing the old value "
                << current << ".\n";
            next = std::max(current, next) + 1;
        }
    };

    std::vector<std::thread> v;
    for (std::size_t i = 0; i < thread_count; ++i)
        v.emplace_back(increment_to_sum, i);

    for (auto& tr : v)
        tr.join();

    std::cout << thread_count << " threads take "
              << counter << " times in total to "
              << "increment 0 to " << sum << ".\n";
}

🌸 Possible output 🌸

Thread #1 (id=139722332333824) wrote 0 replacing the old value 0.
Thread #2 (id=139722323941120) wrote 0 replacing the old value 0.
Thread #1 (id=139722332333824) wrote 1 replacing the old value 0.
Thread #1 (id=139722332333824) wrote 2 replacing the old value 1.
Thread #1 (id=139722332333824) wrote 3 replacing the old value 2.
Thread #1 (id=139722332333824) wrote 4 replacing the old value 3.
Thread #0 (id=139722340726528) wrote 0 replacing the old value 0.
Thread #3 (id=139722315548416) wrote 0 replacing the old value 0.
Thread #3 (id=139722315548416) wrote 1 replacing the old value 4.
Thread #0 (id=139722340726528) wrote 1 replacing the old value 1.
Thread #4 (id=139722307155712) wrote 0 replacing the old value 1.
Thread #4 (id=139722307155712) wrote 2 replacing the old value 2.
Thread #4 (id=139722307155712) wrote 3 replacing the old value 2.
Thread #4 (id=139722307155712) wrote 4 replacing the old value 3.
Thread #2 (id=139722323941120) wrote 1 replacing the old value 0.
Thread #0 (id=139722340726528) wrote 2 replacing the old value 1.
Thread #2 (id=139722323941120) wrote 2 replacing the old value 4.
Thread #0 (id=139722340726528) wrote 3 replacing the old value 2.
Thread #0 (id=139722340726528) wrote 4 replacing the old value 3.
5 threads take 19 times in total to increment 0 to 5.

👑 See also 👑

• atomic_exchange (C++11) — atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template)
• atomic_exchange_explicit (C++11) — atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template)
• exchange (C++14) — replaces the argument with a new value and returns its previous value (function template)

mwah 💋 — source: http://cppreference.com, version/date: 2024.06.10, page: std::atomic::exchange(3). byeee 🎀