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std::atomic::exchange(3) C++ Standard Libary std::atomic::exchange(3) NAME std::atomic::exchange - std::atomic::exchange Synopsis T exchange( T desired, std::memory_order order = (1) (since C++11) std::memory_order_seq_cst ) noexcept; T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) volatile (2) (since C++11) noexcept; Atomically replaces the underlying value with desired (a read-mod- ify-write operation). Memory is affected according to the value of order. It is deprecated if std::atomic<T>::is_always_lock_free is false and (since C++20) overload (2) participates in overload resolution. Parameters desired - value to assign order - memory order constraints to enforce Return value The value of the atomic variable before the call. 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 atomically replaces the value of the atomic object with atomic_exchange_explicit non-atomic argument and returns the old value of the atomic (C++11) (function template) (C++11) exchange replaces the argument with a new value and returns its (C++14) previous value (function template) http://cppreference.com 2024.06.10 std::atomic::exchange(3)
NAME | Synopsis | Parameters | Return value | Example | Possible output: | See also
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