std::atomic<T>::exchange

 

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Threads thread (C++11) jthread (C++20) stop_token (C++20) stop_source (C++20) stop_callback (C++20) hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)     this_thread namespace get_id (C++11) yield (C++11) sleep_for (C++11) sleep_until (C++11) Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20)

Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Mutual exclusion mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17) timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14) Generic lock management lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11) lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and barriers latch (C++20) barrier (C++20) Futures promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11) launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::atomic

Member functions

atomic::atomic

atomic::operator=

atomic::is_lock_free

atomic::store

atomic::load

atomic::operator T

atomic::exchange

atomic::compare_exchange_strongatomic::compare_exchange_weak

atomic::wait (C++20)

atomic::notify_one (C++20)

atomic::notify_all (C++20)

Constants

atomic::is_always_lock_free (C++17)

Specialized member functions

atomic::fetch_add

atomic::fetch_sub

atomic::fetch_and

atomic::fetch_or

atomic::fetch_xor

atomic::operator++atomic::operator++(int)atomic::operator--atomic::operator--(int)

atomic::operator+=atomic::operator-=atomic::operator&=atomic::operator|=atomic::operator^=

 

		(since C++11)
T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept;	(1)
T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;	(2)

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 std::atomic<T>::is_always_lock_free is false.

(since C++20)

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_exchangeatomic_exchange_explicit (C++11)(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)