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;
|
obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr)
obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr, order)
| 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. |
The value held previously by the atomic object pointed to by obj.
A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:
#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>...
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 |
| atomically replaces the value of the atomic object and obtains the value held previously (public member function of std::atomic<T>) |
|
|
(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) |
specializes atomic operations for std::shared_ptr (function template) |
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