| Defined in header <atomic> | ||
|---|---|---|
| (1) | (since C++11) | |
template< class T >
bool atomic_compare_exchange_weak( std::atomic<T>* obj,
T* expected, T desired ); | ||
template< class T >
bool atomic_compare_exchange_weak( volatile std::atomic<T>* obj,
T* expected, T desired ); | ||
| (2) | (since C++11) | |
template< class T >
bool atomic_compare_exchange_strong( std::atomic<T>* obj,
T* expected, T desired ); | ||
template< class T >
bool atomic_compare_exchange_strong( volatile std::atomic<T>* obj,
T* expected, T desired ); | ||
| (3) | (since C++11) | |
template< class T >
bool atomic_compare_exchange_weak_explicit( std::atomic<T>* obj,
T* expected, T desired,
std::memory_order succ,
std::memory_order fail ); | ||
template< class T >
bool atomic_compare_exchange_weak_explicit( volatile std::atomic<T>* obj,
T* expected, T desired,
std::memory_order succ,
std::memory_order fail ); | ||
| (4) | (since C++11) | |
template< class T >
bool atomic_compare_exchange_strong_explicit( std::atomic<T>* obj,
T* expected, T desired,
std::memory_order succ,
std::memory_order fail ); | ||
template< class T >
bool atomic_compare_exchange_strong_explicit( volatile std::atomic<T>* obj,
T* expected, T desired,
std::memory_order succ,
std::memory_order fail ); |
Atomically compares the object representation of the object pointed to by obj with the object representation of the object pointed to by expected, as if by std::memcmp, and if those are bitwise-equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value pointed to by obj into *expected (performs load operation). Copying is performed as if by std::memcpy.
The memory models for the read-modify-write and load operations are succ and fail respectively. The (1-2) versions use std::memory_order_seq_cst by default.
These functions are defined in terms of member functions of std::atomic:
obj->compare_exchange_weak(*expected, desired) obj->compare_exchange_strong(*expected, desired) obj->compare_exchange_weak(*expected, desired, succ, fail) obj->compare_exchange_strong(*expected, desired, succ, fail) Parameters
| obj | - | pointer to the atomic object to test and modify |
| expected | - | pointer to the value expected to be found in the atomic object |
| desired | - | the value to store in the atomic object if it is as expected |
| succ | - | the memory synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted. |
| fail | - | the memory synchronization ordering for the load operation if the comparison fails. Cannot be std::memory_order_release or std::memory_order_acq_rel and cannot specify stronger ordering than succ |
Return value
The result of the comparison: true if *obj was equal to *expected, false otherwise.
Exceptions
noexcept specification: noexceptNotes
The weak forms ((1) and (3)) of the functions are allowed to fail spuriously, that is, act as if *obj != *expected even if they are equal. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.
When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable unless the object representation of T may include padding bits, trap bits, or offers multiple object representations for the same value (e.g. floating-point NaN). In those cases, weak compare-and-exchange typically works because it quickly converges on some stable object representation.
Example
compare and exchange operations are often used as basic building blocks of lockfree data structures.
#include <atomic>
template<class T>
struct node
{
T data;
node* next;
node(const T& data) : data(data), next(nullptr) {}
};
template<class T>
class stack
{
std::atomic<node<T>*> head;
public:
void push(const T& data)
{
node<T>* new_node = new node<T>(data);
// put the current value of head into new_node->next
new_node->next = head.load(std::memory_order_relaxed);
// now make new_node the new head, but if the head
// is no longer what's stored in new_node->next
// (some other thread must have inserted a node just now)
// then put that new head into new_node->next and try again
while(!std::atomic_compare_exchange_weak_explicit(
&head,
&new_node->next,
new_node,
std::memory_order_release,
std::memory_order_relaxed))
; // the body of the loop is empty
// note: the above loop is not thread-safe in at least
// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
// MSVC prior to 2014-03-17 (bug 819819). See member function version for workaround
}
};
int main()
{
stack<int> s;
s.push(1);
s.push(2);
s.push(3);
}See also
| atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (public member function of std::atomic) | |
| (C++11)(C++11) | atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template) |
| specializes atomic operations for std::shared_ptr (function template) | |
C documentation for atomic_compare_exchange, atomic_compare_exchange_explicit | |
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