# std::atomic<T>::compare_exchange_weak, std::atomic<T>::compare_exchange_strong

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< cpp‎ | atomic‎ | atomic

| (1) | (since C++11)
bool compare_exchange_weak( T& expected, T desired, std::memory_order success, std::memory_order failure ) noexcept; |  |
bool compare_exchange_weak( T& expected, T desired, std::memory_order success, std::memory_order failure ) volatile noexcept; |  |
| (2) | (since C++11)
bool compare_exchange_weak( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept; |  |
bool compare_exchange_weak( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; |  |
| (3) | (since C++11)
bool compare_exchange_strong( T& expected, T desired, std::memory_order success, std::memory_order failure ) noexcept; |  |
bool compare_exchange_strong( T& expected, T desired, std::memory_order success, std::memory_order failure ) volatile noexcept; |  |
| (4) | (since C++11)
bool compare_exchange_strong( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept; |  |
bool compare_exchange_strong( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; |  |

Atomically compares the object representation (until C++20)value representation (since C++20) of `*this` with that of `expected`, 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).

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` (until C++17)
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.

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, (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

Run this code

    #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.

| This section is incomplete Reason: more practical use of the strong CAS would be nice, such as where Concurrency in Action uses it

Run this code

    #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

atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) | 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)

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