std::atomic_exchange, std::atomic_exchange_explicit

 

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

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

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

Parameters

obj	-	pointer to the atomic object to modify
desr	-	the value to store in the atomic object
order	-	the memory synchronization ordering for this operation: all values are permitted.

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 <thread>
#include <vector>
#include <iostream>
#include <atomic>
 
std::atomic<bool> lock(false); // holds true when locked
                               // holds false when unlocked
 
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 << "Output from thread " << n << '\n';
        std::atomic_store_explicit(&lock, false, std::memory_order_release);
    }
}
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n) {
        v.emplace_back(f, n);
    }
    for (auto& t : v) {
        t.join();
    }
}

Output:

Output from thread 2
Output from thread 6
Output from thread 7
...<exactly 1000 lines>...

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 required because T is deduced from multiple arguments	T is deduced from the atomic argument only

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)

specializes atomic operations for std::shared_ptr (function template)