std::atomic...dd_explicit(3)

💅 NAME

bestie, welcome to std::atomic...dd_explicit(3) from the C++ Standard Libary manual (yes, that’s the header’s spelling, not mine 🙈 — section 3, we’re keeping the lore). The stars of the show are std::atomic_fetch_add and std::atomic_fetch_add_explicit: atomic addition with and without an explicit memory order. It’s giving “fetch the old value, add the new one, no cap.” ✨

🦋 Synopsis

Defined in header <atomic>.

Overload (1) — since C++11:

template< class T >
T atomic_fetch_add( std::atomic<T>* obj,
                    typename std::atomic<T>::difference_type arg ) noexcept;

Overload (2) — since C++11:

template< class T >
T atomic_fetch_add( volatile std::atomic<T>* obj,
                    typename std::atomic<T>::difference_type arg ) noexcept;

Overload (3) — 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;

Overload (4) — 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;

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)

⚠️ warning, bestie: 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. Not a vibe. 💀

🎀 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

🌸 Return value

The value immediately preceding the effects of this function in the modification order of *obj. So you get the old value, not the new one — main character energy is all about the before pic. 🫶

🔥 Example

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

#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.

• P0558R1 — applied to C++11. Behavior as published: exact type match was required because T is only deduced from obj. Correct behavior: T was deduced from multiple arguments.

👑 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<T>).
• atomic_fetch_sub, atomic_fetch_sub_explicit (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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source http://cppreference.com, version/date 2024.06.10, page std::atomic...dd_explicit(3) — stay atomic, bestie 💖✨