std::atomic<T>::compare_exchange_weak, std::atomic<T>::compare_exchange_strong
| bool compare_exchange_weak( T& expected, T desired, std::memory_order success, |
(1) | (since C++11) |
| bool compare_exchange_weak( T& expected, T desired, std::memory_order success, |
(2) | (since C++11) |
| bool compare_exchange_weak( T& expected, T desired, std::memory_order order = |
(3) | (since C++11) |
| bool compare_exchange_weak( T& expected, T desired, std::memory_order order = |
(4) | (since C++11) |
| bool compare_exchange_strong( T& expected, T desired, std::memory_order success, |
(5) | (since C++11) |
| bool compare_exchange_strong( T& expected, T desired, std::memory_order success, |
(6) | (since C++11) |
| bool compare_exchange_strong( T& expected, T desired, std::memory_order order = |
(7) | (since C++11) |
| bool compare_exchange_strong ( T& expected, T desired, |
(8) | (since C++11) |
Atomically compares the object representation(until C++20)value representation(since C++20) of *this with that of expected. If those are bitwise-equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value stored in *this into expected (performs load operation).
| Overloads | Memory model for | |
|---|---|---|
| read‑modify‑write operation | load operation | |
| (1,2,5,6) | success | failure |
| (3,4,7,8) | order |
|
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.
|
It is deprecated if std::atomic<T>::is_always_lock_free is false and any volatile overload participates in overload resolution. |
(since C++20) |
Parameters
| expected | - | reference 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 |
| success | - | the memory synchronization ordering for the read-modify-write operation if the comparison succeeds |
| failure | - | the memory synchronization ordering for the load operation if the comparison fails |
| order | - | the memory synchronization ordering for both operations |
Return value
true if the underlying atomic value was successfully changed, false otherwise.
Notes
The comparison and copying are bitwise (similar to std::memcmp and std::memcpy); no constructor, assignment operator, or comparison operator are used.
compare_exchange_weak is allowed to fail spuriously, that is, acts as if *this != expected even if they are equal. When a compare-and-exchange is in a loop, compare_exchange_weak will yield better performance on some platforms.
When compare_exchange_weak would require a loop and compare_exchange_strong would not, compare_exchange_strong 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, compare_exchange_weak 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) |
Example
Compare-and-exchange operations are often used as basic building blocks of lock-free data structures.
#include <atomic> template<typename T> struct node { T data; node* next; node(const T& data) : data(data), next(nullptr) {} }; template<typename 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 (!head.compare_exchange_weak(new_node->next, new_node, std::memory_order_release, std::memory_order_relaxed)) ; // the body of the loop is empty // Note: the above use 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). The following is a workaround: // node<T>* old_head = head.load(std::memory_order_relaxed); // do // { // new_node->next = old_head; // } // while (!head.compare_exchange_weak(old_head, new_node, // std::memory_order_release, // std::memory_order_relaxed)); } }; int main() { stack<int> s; s.push(1); s.push(2); s.push(3); }
Demonstrates how std::compare_exchange_strong either changes the value of the atomic variable or the variable used for comparison.
| This section is incomplete Reason: more practical use of the strong CAS would be nice, such as where Concurrency in Action uses it |
#include <atomic> #include <iostream> std::atomic<int> ai; int tst_val = 4; int new_val = 5; bool exchanged = false; void valsout() { std::cout << "ai = " << ai << " tst_val = " << tst_val << " new_val = " << new_val << " exchanged = " << std::boolalpha << exchanged << '\n'; } int main() { ai = 3; valsout(); // tst_val != ai ==> tst_val is modified exchanged = ai.compare_exchange_strong(tst_val, new_val); valsout(); // tst_val == ai ==> ai is modified exchanged = ai.compare_exchange_strong(tst_val, new_val); valsout(); }
Output:
ai = 3 tst_val = 4 new_val = 5 exchanged = false ai = 3 tst_val = 3 new_val = 5 exchanged = false ai = 5 tst_val = 3 new_val = 5 exchanged = true
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 (function template) |