std::atomic_exchange, std::atomic_exchange_explicit
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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) [edit]
specializes atomic operations for std::shared_ptr
(function template)
(C++11)
(C++20)
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(C++14)
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(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
(C++11)
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(C++20)(C++20)
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(C++11)(deprecated in C++20)
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(C++11)(deprecated in C++26)
(C++11)
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(C++11)(C++11)
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atomic_exchangeatomic_exchange_explicit
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)
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(C++11)(C++11)
(C++11)(C++11)
(C++20)(C++20)
(C++20)(C++20)
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(C++20)
Defined in header
<atomic>
template< class T >
(1)
(since C++11)
T atomic_exchange( std::atomic <T>* obj,
template< class T >
(2)
(since C++11)
T atomic_exchange( volatile std::atomic <T>* obj,
template< class T >
(3)
(since C++11)
T atomic_exchange_explicit( std::atomic <T>* obj,
typename std::atomic <T>::value_type desired,
template< class T >
(4)
(since C++11)
T atomic_exchange_explicit( volatile std::atomic <T>* obj,
typename std::atomic <T>::value_type desired,
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 :
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
[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 |
[edit] See also
atomically replaces the value of the atomic object and obtains the value held previously
(public member function of
(public member function of
std::atomic<T>) [edit]
(C++11)(C++11)(C++11)(C++11)
(function template) [edit]
(deprecated in C++20)(removed in C++26)
(function template)
C documentation for atomic_exchange, atomic_exchange_explicit