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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../atomic/atomic_exchange.html below:

std::atomic_exchange, std::atomic_exchange_explicit - cppreference.com

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) (3) (since C++11) (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).

[edit] Parameters obj - pointer to the atomic object to modify desired - the value to store in the atomic object order - the memory synchronization ordering [edit] Return value

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

[edit] Example

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

#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

[edit] 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 [edit] See also

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