std::atomic_compare_exchange_weak, std::atomic_compare_exchange_strong, std::atomic_compare_exchange_weak_explicit, std::atomic_compare_exchange_strong_explicit
<atomic>
bool atomic_compare_exchange_weak
( std::atomic <T>* obj, typename std::atomic <T>::value_type* expected,
bool atomic_compare_exchange_weak
( volatile std::atomic <T>* obj,
typename std::atomic <T>::value_type* expected,
bool atomic_compare_exchange_strong
( std::atomic <T>* obj, typename std::atomic <T>::value_type* expected,
bool atomic_compare_exchange_strong
( volatile std::atomic <T>* obj,
typename std::atomic <T>::value_type* expected,
bool atomic_compare_exchange_weak_explicit
( std::atomic <T>* obj, typename std::atomic <T>::value_type* expected,
typename std::atomic <T>::value_type desired,
bool atomic_compare_exchange_weak_explicit
( volatile std::atomic <T>* obj,
typename std::atomic <T>::value_type* expected,
typename std::atomic <T>::value_type desired,
bool atomic_compare_exchange_strong_explicit
( std::atomic <T>* obj, typename std::atomic <T>::value_type* expected,
typename std::atomic <T>::value_type desired,
bool atomic_compare_exchange_strong_explicit
( volatile std::atomic <T>* obj,
typename std::atomic <T>::value_type* expected,
typename std::atomic <T>::value_type desired,
Atomically compares the object representation (until C++20)value representation (since C++20) of the object pointed to by obj with that of the object pointed to by expected, 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).
| Overloads | Memory model for | |
|---|---|---|
| read‑modify‑write operation | load operation | |
| (1-4) | std::memory_order_seq_cst | std::memory_order_seq_cst |
| (5-8) | success | failure |
These functions are defined in terms of member functions of std::atomic :
If failure is stronger than success or(until C++17) is one of std::memory_order_release and std::memory_order_acq_rel , the behavior is undefined.
[edit] Parameters
[edit] Return value
The result of the comparison: true if *obj was equal to *expected, false otherwise.
[edit] Notes
std::atomic_compare_exchange_weak and std::atomic_compare_exchange_weak_explicit (the weak versions) 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, they 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,(until C++20) 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.
For a union with bits that participate in the value representations of some members but not the others, compare-and-exchange might always fail because such padding bits have indeterminate values when they do not participate in the value representation of the active member.
Padding bits that never participate in an object's value representation are ignored.
(since C++20)[edit] 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年05月05日 (bug 18899) // MSVC prior to 2014年03月17日 (bug 819819). See member function version for workaround } }; int main() { stack<int> s; s.push(1); s.push(2); s.push(3); }
[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
(public member function of
std::atomic<T>) [edit]
(function template) [edit]
(function template)