# std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
| (1) |
template< class T > T atomic_fetch_sub( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg ) noexcept; |  |
template< class T > T atomic_fetch_sub( volatile std::atomic<T>* obj, typename std::atomic<T>::difference_type arg ) noexcept; |  |
| (2) |
template< class T > T atomic_fetch_sub_explicit( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg, std::memory_order order ) noexcept; |  |
template< class T > T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj, typename std::atomic<T>::difference_type arg, std::memory_order order ) noexcept; |  |

Performs atomic subtraction. Atomically subtracts `arg` from 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_sub(arg)
2) obj->fetch_sub(arg, order)

### Parameters

obj | - | pointer to the atomic object to modify
arg | - | the value to subtract from 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`.

### Possible implementation

    template< class T >
    T atomic_fetch_sub( std::atomic<T>* obj,
                        typename std::atomic<T>::difference_type arg ) noexcept
    {
        return obj->fetch_sub(arg);
    }

### Example

Multiple threads may use `fetch_sub` to concurrently process an indexed container

Run this code

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

    const int N = 50;
    std::atomic<int> cnt;
    std::vector<int> data(N);

    void reader(int id)
    {
        for (;;) {
            int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
            if (idx >= 0) {
                std::cout << "reader " << std::to_string(id) << " processed item "
                          << std::to_string(data[idx]) << '\n';
            } else {
                std::cout << "reader " << std::to_string(id) << " done\n";
                break;
            }
        }
    }

    int main()
    {
        std::iota(data.begin(), data.end(), 1);
        cnt = data.size() - 1;

        std::vector<std::thread> v;
        for (int n = 0; n < 5; ++n) {
            v.emplace_back(reader, n);
        }
        for (auto& t : v) {
            t.join();
        }
    }

Output:

    reader 2 processed item 50
    reader 1 processed item 44
    reader 4 processed item 46
    <....>
    reader 0 done
    reader 4 done
    reader 3 done

### 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

fetch_sub | atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously (public member function of `std::atomic<T>`)
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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