std::atomic::compare_exchange_weak

Defined in header <atomic>

(1) (since C++11)

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order success, 
                            std::memory_order failure );

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order success, 
                            std::memory_order failure ) volatile;

(2) (since C++11)

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order order =
                                std::memory_order_seq_cst );

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order order =
                                std::memory_order_seq_cst ) volatile;

(3) (since C++11)

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order success, 
                              std::memory_order failure );

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order success, 
                              std::memory_order failure ) volatile;

(4) (since C++11)

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order order = 
                                  std::memory_order_seq_cst );

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order order = 
                                  std::memory_order_seq_cst ) volatile;

Atomically compares the object representation of *this with the object representation of 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 stored in *this 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 success and failure respectively. In the (2) and (4) versions order is used for both read-modify-write and load operations, except that std::memory_order_acquire and std::memory_order_relaxed are used for the load operation if order == std::memory_order_acq_rel, or order == std::memory_order_release respectively.

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. All values are permitted.
failure	-	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 `success`
order	-	the memory synchronization ordering for both operations

Return value

true if the underlying atomic value was changed, false otherwise.

Exceptions

noexcept specification:

noexcept

Notes

The weak forms (1-2) of the functions are allowed to fail spuriously, that is, act as if *this != 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<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 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();
  
    return 0;
}

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