std::atomic<T>::exchange

 

C++

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Concurrency support library

Threads thread (C++11) jthread (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) Cooperative cancellation stop_token (C++20) inplace_stop_token (C++26) never_stop_token (C++26) stop_source (C++20) inplace_stop_source (C++26)   stop_callback (C++20) inplace_stop_callback (C++26) stop_callback_for_t (C++26) stoppable_token (C++26) unstoppable_token (C++26) stoppable-source (C++26) stoppable-callback-for (C++26)   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 (C++11) lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) once_flag (C++11) call_once (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lockdefer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)(C++11)(C++11)(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) Safe Reclamation rcu_obj_base (C++26) rcu_domain (C++26) rcu_default_domain (C++26) rcu_synchronize (C++26) rcu_barrier (C++26) rcu_retire (C++26) Hazard Pointers hazard_pointer_obj_base (C++26) hazard_pointer (C++26) make_hazard_pointer (C++26)

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) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (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_fetch_maxatomic_fetch_max_explicit (C++26)(C++26) atomic_fetch_minatomic_fetch_min_explicit (C++26)(C++26) 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)

 

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

Specialized for integral, floating-point (C++20) and pointer types

atomic::fetch_add

atomic::fetch_sub

atomic::operator+=atomic::operator-=

Specialized for integral and pointer types only

atomic::fetch_max (C++26)

atomic::fetch_min (C++26)

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

Specialized for integral types only

atomic::fetch_and

atomic::fetch_or

atomic::fetch_xor

atomic::operator&=atomic::operator|=atomic::operator^=

 

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.

It is deprecated if std::atomic<T>::is_always_lock_free is false and overload (2) participates in overload resolution.

(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()
{
    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_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)