std::atomic_exchange, std::atomic_exchange_explicit - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| 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).
[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 #include #include #include std::atomic 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 becauseT was deduced from multiple arguments | T is only deducedfrom obj |