std::atomic_fetch_add, std::atomic_fetch_add_explicit

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

Performs atomic addition. 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)

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 synchronization 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_add( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg )
{
    return obj->fetch_add(arg);
}

Example

Single-writer/multiple-reader lock can be made with std::atomic_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>
using namespace std::chrono_literals;
 
// meaning of cnt:
// 5: readers and writer are in race. There are no active readers or writers.
// 4...0: there are 1...5 active readers, The writer is blocked.
//-1: writer won the race and readers are blocked.
 
const int N = 5; // four concurrent readers are allowed
std::atomic<int> cnt(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() == 25)
            break;
        // unlock
        std::atomic_fetch_add(&cnt, 1);
        // pause
        std::this_thread::sleep_for(1ms);
    }
}
 
void writer()
{
    for(int n = 0; n < 25; ++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(1ms);
    }
}
 
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 2 sees 0
reader 3 sees 0
reader 1 sees 0
<...>
reader 2 sees 24
reader 4 sees 24
reader 1 sees 24

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

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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<T>`)
atomic_fetch_subatomic_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)