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).
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) [edit] 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 [edit] Return valuetrue if the underlying atomic value was successfully changed, false otherwise.
[edit] NotesThe 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) [edit] ExampleCompare-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.
#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[edit] See also
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