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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, 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:

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.

Exceptions

noexcept specification:

noexcept

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, 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);
}