# 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`. That is, it performs atomic post-increment. The operation is a 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 `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.

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 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. The volatile-qualified versions are deprecated if std::atomic<T>::is_always_lock_free is false. | (since C++20)

### 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>
    #include <array>

    std::atomic<long long> data{10};
    std::array<long long, 5> return_values{};

    void do_work(int thread_num)
    {
        long long val = data.fetch_add(1, std::memory_order_relaxed);
        return_values[thread_num] = val;
    }

    int main()
    {
        {
            std::jthread th0{do_work, 0};
            std::jthread th1{do_work, 1};
            std::jthread th2{do_work, 2};
            std::jthread th3{do_work, 3};
            std::jthread th4{do_work, 4};
        }

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

        for (long long val : return_values) {
            std::cout << "Seen return value : " << val << std::endl;
        }
    }

Possible output:

    Result : 15
    Seen return value : 11
    Seen return value : 10
    Seen return value : 14
    Seen return value : 12
    Seen return value : 13

### 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)
operator++operator++(int)operator--operator--(int) | increments or decrements the atomic value by one (public member function)

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