std::atomic_exchange, std::atomic_exchange_explicit

 

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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)

 

Defined in header <atomic>
template< class T > T atomic_exchange( std::atomic<T>* obj,                    typename std::atomic<T>::value_type desired ) noexcept;	(1)	(since C++11)
template< class T > T atomic_exchange( volatile std::atomic<T>* obj,                    typename std::atomic<T>::value_type desired ) noexcept;	(2)	(since C++11)
template< class T > T atomic_exchange_explicit( std::atomic<T>* obj,                             typename std::atomic<T>::value_type desired,                             std::memory_order order ) noexcept;	(3)	(since C++11)
template< class T > T atomic_exchange_explicit( volatile std::atomic<T>* obj,                             typename std::atomic<T>::value_type desired,                             std::memory_order order ) noexcept;	(4)	(since C++11)

1,2) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired).

3,4) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired, order).

Parameters

obj	-	pointer to the atomic object to modify
desired	-	the value to store in the atomic object
order	-	the memory synchronization ordering

Return value

The value held previously by the atomic object pointed to by obj.

Example

A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:

Run this code

#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic<bool> lock(false); // holds true when locked
                               // holds false when unlocked
 
int new_line{1}; // the access is synchronized via atomic lock variable
 
void f(int n)
{
    for (int cnt = 0; cnt < 100; ++cnt)
    {
        while (std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire))
            ; // spin until acquired
        std::cout << n << (new_line++ % 80 ? "" : "\n");
        std::atomic_store_explicit(&lock, false, std::memory_order_release);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 8; ++n)
        v.emplace_back(f, n);
    for (auto& t : v)
        t.join();
}

Possible output:

02222222222222222222222002222222222222222222222222222222222222222222222222222222
22222222200022222222202222211111111111110000011111111100000000000000110001111111
00011111000001111110000011111100000111000000001111111111111110000010000001001111
11011111111011111011000000000000111100000000000001111000011133333333333333333333
33333333333333333333333333333333333333333333333333333333333333333333333333333333
44444444444444444444444444444444444444444444444444444444444444444444444444444444
44444444444444444444555555555555555555555555555555555555555555555555555555555555
55555555555555555555555555555555555555556666666666666666666666666666666666666666
66666666666666666666666666666666666666666666666666666666666677777777777777777777
77777777777777777777777777777777777777777777777777777777777777777777777777777777

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P0558R1	C++11	exact type match was required because T was deduced from multiple arguments	T is only deduced from obj

See also

exchange	atomically replaces the value of the atomic object and obtains the value held previously (public member function of std::atomic<T>)
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)	atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template)

std::atomic_exchange(std::shared_ptr) std::atomic_exchange_explicit(std::shared_ptr) (deprecated in C++20)(removed in C++26)	specializes atomic operations for std::shared_ptr (function template)
C documentation for atomic_exchange, atomic_exchange_explicit