# std::atomic_exchange, std::atomic_exchange_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
| (1) | (since C++11)
template< class T > T atomic_exchange( std::atomic <T>* obj, typename std::atomic <T>::value_type desr ) noexcept; |  |
template< class T > T atomic_exchange( volatile std::atomic <T>* obj, typename std::atomic <T>::value_type desr ) noexcept; |  |
| (2) | (since C++11)
template< class T > T atomic_exchange_explicit( std::atomic <T>* obj, typename std::atomic <T>::value_type desr, std::memory_order order ) noexcept; |  |
template< class T > T atomic_exchange_explicit( volatile std::atomic <T>* obj, typename std::atomic <T>::value_type desr, std::memory_order order ) noexcept; |  |

1) Atomically replaces the value pointed to by `obj` with the value of `desr` and returns the value `obj` held previously, as if by obj->exchange(desr)
2) Atomically replaces the value pointed to by `obj` with the value of `desr` and returns the value `obj` held previously, as if by obj->exchange(desr, order)

### Parameters

obj | - | pointer to the atomic object to modify
desr | - | the value to store in the atomic object
order | - | the memory synchronization ordering for this operation: all values are permitted.

### Return value

The value held previously by the atomic object pointed to by `obj`

### Example

A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:

Run this code

    #include <thread>
    #include <vector>
    #include <iostream>
    #include <atomic>

    std::atomic<bool> lock(false); // holds true when locked
                                   // holds false when unlocked

    void f(int n)
    {
        for (int cnt = 0; cnt < 100; ++cnt) {
            while(std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire))
                 ; // spin until acquired
            std::cout << "Output from thread " << n << '\n';
            std::atomic_store_explicit(&lock, false, std::memory_order_release);
        }
    }
    int main()
    {
        std::vector<std::thread> v;
        for (int n = 0; n < 10; ++n) {
            v.emplace_back(f, n);
        }
        for (auto& t : v) {
            t.join();
        }
    }

Output:

    Output from thread 2
    Output from thread 6
    Output from thread 7
    ...<exactly 1000 lines>...

### 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

### See also

exchange | atomically replaces the value of the atomic object and obtains the value held previously (public member function of `std::atomic<T>`)
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_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)
std::atomic_exchange(std::shared_ptr) std::atomic_exchange_explicit(std::shared_ptr) | specializes atomic operations for std::shared_ptr (function template)

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