# std::atomic_fetch_add, std::atomic_fetch_add_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
| (1) | (since C++11)
template< class Integral > Integral atomic_fetch_add( std::atomic<Integral>* obj, Integral arg ); |  |
template< class Integral > Integral atomic_fetch_add( volatile std::atomic<Integral>* obj, Integral arg ); |  |
| (2) | (since C++11)
template< class Integral > Integral atomic_fetch_add_explicit( std::atomic<Integral>* obj, Integral arg, std::memory_order order ); |  |
template< class Integral > Integral atomic_fetch_add_explicit( volatile std::atomic<Integral>* obj, Integral arg, std::memory_order order ); |  |
| (3) | (since C++11)
template< class T > T* atomic_fetch_add( std::atomic<T*>* obj, std::ptrdiff_t arg ); |  |
template< class T > T* atomic_fetch_add( volatile std::atomic<T*>* obj, std::ptrdiff_t arg ); |  |
| (4) | (since C++11)
template< class T > T* atomic_fetch_add_explicit( std::atomic<T*>* obj, std::ptrdiff_t arg, std::memory_order order ); |  |
template< class T > T* atomic_fetch_add_explicit( volatile std::atomic<T*>* obj, std::ptrdiff_t arg, std::memory_order order ); |  |

Performs atomic addition.

1-2) Atomically adds `arg` to the value pointed to by `obj` and returns the value `obj` held previously. The operation is performed as if the following was executed:
1) obj->fetch_add(arg)
2) obj->fetch_add(arg, order)
3-4) Atomically increments the pointer value, pointed to by `obj`, by `arg`, and returns the value `obj` held previously. The operation is performed as if the following was executed:
3) obj->fetch_add(arg)
4) obj->fetch_add(arg, order)

### Parameters

obj | - | pointer to the atomic object to modify
arg | - | the value to add to the value stored in the atomic object
order | - | the memory sycnhronization ordering for this operation: all values are permitted.

### Return value

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

### Exceptions

`noexcept` specification:
noexcept

### Possible implementation

First version

    template< class T >
    typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, bool>::value, T>::type
    atomic_fetch_add( std::atomic<T>* obj, T arg );
    {
        return obj->fetch_add(arg);
    }

Second version

    template< class T >
    T* atomic_fetch_add( std::atomic<T*>* obj, std::ptrdiff_t arg)
    {
        return obj->fetch_add(arg);
    }

### Example

Single-writer/multiple-reader lock can be made with fetch_add. Note that this simplistic implementation is not lockout-free

Run this code

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

    // meaning of cnt:
    // 10: there are no active readers or writers.
    // 1...9: there are 9...1 readers active, The writer is blocked
    // 0: temporary value between fetch_sub and fetch_add in reader lock
    // -1: there is a writer active. The readers are blocked.
    const int N = 10; // nine concurrent readers are allowed
    std::atomic<int> cnt = ATOMIC_VAR_INIT(N);

    std::vector<int> data;

    void reader(int id)
    {
        for(;;)
        {
            // lock
            while(std::atomic_fetch_sub(&cnt, 1) <= 0)
                std::atomic_fetch_add(&cnt, 1);
            // read
            if(!data.empty())
                std::cout << (  "reader " + std::to_string(id)
                              + " sees " + std::to_string(*data.rbegin()) + '\n');
            if(data.size() == 100)
                break;
            // unlock
            std::atomic_fetch_add(&cnt, 1);
            // pause
            std::this_thread::sleep_for(std::chrono::milliseconds(1));
        }
    }

    void writer()
    {
        for(int n = 0; n < 100; ++n)
        {
            // lock
            while(std::atomic_fetch_sub(&cnt, N+1) != N)
                std::atomic_fetch_add(&cnt, N+1);
            // write
            data.push_back(n);
            std::cout << "writer pushed back " << n << '\n';
            // unlock
            std::atomic_fetch_add(&cnt, N+1);
            // pause
            std::this_thread::sleep_for(std::chrono::milliseconds(1));
        }
    }

    int main()
    {
        std::vector<std::thread> v;
        for (int n = 0; n < N; ++n) {
            v.emplace_back(reader, n);
        }
        v.emplace_back(writer);
        for (auto& t : v) {
            t.join();
        }
    }

Output:

    writer pushed back 0
    reader 8 sees 0
    reader 3 sees 0
    reader 1 sees 0
    <...>
    reader 2 sees 99
    reader 6 sees 99
    reader 1 sees 99

### See also

fetch_add | atomically adds the argument to the value stored in the atomic object and obtains the value held previously (public member function of `std::atomic`)
atomic_fetch_sub atomic_fetch_sub_explicit (C++11)(C++11) | subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic (function template)
C documentation for atomic_fetch_add, atomic_fetch_add_explicit

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