std::atomic_compare_exchange_weak, std::atomic_compare_exchange_strong, std::atomic_compare_exchange_weak_explicit, std::atomic_compare_exchange_strong_explicit

Defined in header `<atomic>`
	(1)	(since C++11)
template< class T > bool atomic_compare_exchange_weak( std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired ) noexcept;
template< class T > bool atomic_compare_exchange_weak( volatile std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired ) noexcept;
	(2)	(since C++11)
template< class T > bool atomic_compare_exchange_strong( std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired ) noexcept;
template< class T > bool atomic_compare_exchange_strong( volatile std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired ) noexcept;
	(3)	(since C++11)
template< class T > bool atomic_compare_exchange_weak_explicit( std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired, std::memory_order succ, std::memory_order fail ) noexcept;
template< class T > 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, std::memory_order succ, std::memory_order fail ) noexcept;
	(4)	(since C++11)
template< class T > bool atomic_compare_exchange_strong_explicit( std::atomic<T>* obj, typename std::atomic<T>::value_type* expected, typename std::atomic<T>::value_type desired, std::memory_order succ, std::memory_order fail ) noexcept;
template< class T > 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, std::memory_order succ, std::memory_order fail ) noexcept;

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). 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:

1) obj->compare_exchange_weak(*expected, desired)

2) obj->compare_exchange_strong(*expected, desired)

3) obj->compare_exchange_weak(*expected, desired, succ, fail)

4) 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` (until C++17)

Return value

The result of the comparison: true if *obj was equal to *expected, false otherwise.

Notes

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, (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)

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);
}

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 required because `T` is deduced from multiple arguments	`T` is deduced from the `atomic` argument only

compare_exchange_weakcompare_exchange_strong	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<T>`)
atomic_exchangeatomic_exchange_explicit (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)
std::atomic_compare_exchange_weak(std::shared_ptr) std::atomic_compare_exchange_strong(std::shared_ptr)	specializes atomic operations for std::shared_ptr (function template)