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std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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

Performs atomic subtraction.

1-2) 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)

3-4) Atomically decrements 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_sub(arg)

4) 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.

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_sub( std::atomic<T>* obj, T arg ); { return obj->fetch_sub(arg); }
Second version
template< class T > T* atomic_fetch_sub( std::atomic<T> obj, std::ptrdiff_t arg) { return obj->fetch_sub(arg); }

First version

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

Second version

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

Example

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

#include <string>
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
#include <numeric>
 
const int N = 10000;
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 < 10; ++n) {
        v.emplace_back(reader, n);
    }
    for (auto& t : v) {
        t.join();
    }
}

Output:

reader 2 processed item 10000
reader 6 processed item 9994
reader 4 processed item 9996
reader 6 processed item 9992
<....>
reader 0 done
reader 5 done
reader 3 done
reader 9 done