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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) std::memory_order_seq_cst ) noexcept; (since C++11) T exchange( T desired, std::memory_order order = (2) std::memory_order_seq_cst ) volatile noexcept; Atomically replaces the underlying value with desired. The operation is read-modify-write operation. Memory is affected according to the value of order. The volatile-qualified version (2) is deprecated if (since C++20) std::atomic<T>::is_always_lock_free is false. 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(){ const std::size_t ThreadNumber = 5; const int Sum = 5; std::atomic<int> atom{0}; std::atomic<int> counter{0}; // lambda as thread proc auto lambda = [&](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) << '#' << 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 < ThreadNumber; ++i){ v.emplace_back(lambda, i); } for (auto& tr : v){ tr.join(); } std::cout << ThreadNumber << " threads adding 0 to " << Sum << " takes total " << counter << " times\n"; } Possible output: #1 (140552371918592) wrote 0 replacing the old value 0 #2 (140552363525888) wrote 0 replacing the old value 0 #1 (140552371918592) wrote 1 replacing the old value 0 #1 (140552371918592) wrote 2 replacing the old value 1 #2 (140552363525888) wrote 1 replacing the old value 1 #1 (140552371918592) wrote 3 replacing the old value 2 #1 (140552371918592) wrote 4 replacing the old value 2 #2 (140552363525888) wrote 2 replacing the old value 3 #2 (140552363525888) wrote 4 replacing the old value 0 #3 (140552355133184) wrote 0 replacing the old value 4 #0 (140552380311296) wrote 0 replacing the old value 0 #0 (140552380311296) wrote 1 replacing the old value 4 #4 (140552346740480) wrote 0 replacing the old value 1 #4 (140552346740480) wrote 2 replacing the old value 0 #4 (140552346740480) wrote 3 replacing the old value 2 #4 (140552346740480) wrote 4 replacing the old value 3 5 threads adding 0 to 5 takes total 16 times 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 2022.07.31 std::atomic::exchange(3)
NAME | Synopsis | Parameters | Return value | Example | Possible output: | See also
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