# std::atomic<T>::fetch_add

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< cpp‎ | atomic‎ | atomic

member only of `atomic<Integral>`(C++11) and `atomic<Floating>`(C++20) template specializations |  |
| (1) |
T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; |  |
T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; |  |
member only of `atomic<T*>` template specialization |  |
| (2) |
T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; |  |
T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; |  |

Atomically replaces the current value with the result of arithmetic addition of the value and `arg`. The operation is read-modify-write operation. Memory is affected according to the value of `order`.

For signed `Integral` types, arithmetic is defined to use two’s complement representation. There are no undefined results.

For floating-point types, the floating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not be conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior.

(since C++20)

For `T*` types, the result may be an undefined address, but the operation otherwise has no undefined behavior. The program is ill-formed if `T` is not an object type.

### Parameters

arg | - | the other argument of arithmetic addition
order | - | memory order constraints to enforce

### Return value

The value immediately preceding the effects of this function in the modification order of `*this`.

### Example

Run this code

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

    std::atomic<long long> data;
    void do_work()
    {
        data.fetch_add(1, std::memory_order_relaxed);
    }

    int main()
    {
        std::thread th1(do_work);
        std::thread th2(do_work);
        std::thread th3(do_work);
        std::thread th4(do_work);
        std::thread th5(do_work);

        th1.join();
        th2.join();
        th3.join();
        th4.join();
        th5.join();

        std::cout << "Result:" << data << '\n';
    }

Output:

    Result:5

### 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 | arithmetic permitted on pointers to cv void or function | made ill-formed

### See also

atomic_fetch_add atomic_fetch_add_explicit (C++11)(C++11) | adds a non-atomic value to an atomic object and obtains the previous value of the atomic (function template)

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