std::atomic_fetch_add, std::atomic_fetch_add_explicit

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

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:

1,2) obj->fetch_add(arg)

3,4) obj->fetch_add(arg, order)

If std::atomic<T> has no fetch_add member (this member is only provided for integral, floating-point(since C++20) and pointer types except bool), the program is ill-formed.

#include <atomic>
#include <chrono>
#include <iostream>
#include <string>
#include <thread>
#include <vector>
 
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 was required because `T` was deduced from multiple arguments	`T` is only deduced from obj

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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>`) [edit]
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) [edit]
C documentation for atomic_fetch_add, atomic_fetch_add_explicit

cppreference.com

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std::atomic_fetch_add, std::atomic_fetch_add_explicit

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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