std::atomic...or_explicit(3)

✨ std::atomic_fetch_or / std::atomic_fetch_or_explicit ✨

okay so bestie, welcome to the C++ Standard Library corner of the (3) manual pages 🎀 — today's main character is a little pair of function templates called std::atomic_fetch_or and std::atomic_fetch_or_explicit. They're lowkey iconic, fr fr.

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🌸 SYNOPSIS 🌸

bestie, these live in <atomic>, and here's the whole cast, exactly as written:

template< class T >

T atomic_fetch_or( std::atomic<T>* obj,
(1) (since C++11)

                   typename std::atomic<T>::value_type arg )
noexcept;

template< class T >

T atomic_fetch_or( volatile std::atomic<T>* obj,
(2) (since C++11)

                   typename std::atomic<T>::value_type arg )
noexcept;

template< class T >

T atomic_fetch_or_explicit( std::atomic<T>* obj,
                            typename std::atomic<T>::value_type
(3) (since C++11)
arg,

                            std::memory_order order ) noexcept;

template< class T >

T atomic_fetch_or_explicit( volatile std::atomic<T>* obj,
                            typename std::atomic<T>::value_type
(4) (since C++11)
arg,

                            std::memory_order order ) noexcept;

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💜 DESCRIPTION 💜

So what does it actually do? It atomically replaces the value pointed to by obj with the result of the bitwise OR between the old value of obj and arg. Then it hands you back the value obj held previously. ✨

The operation is performed as if the following is executed:

1. obj->fetch_or(arg) — that's for overloads 1, 2
2. obj->fetch_or(arg, order) — that's for overloads 3, 4

🫶 heads up: if std::atomic<T> has no fetch_or member — and that member is only provided for integral types except bool — then the program is ill-formed. no cap, not a vibe, it just won't compile 😭

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🦋 PARAMETERS 🦋

• obj — pointer to the atomic object you wanna modify
• arg — the value to bitwise OR with the value stored in the atomic object
• order — the memory synchronization ordering

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👑 RETURN VALUE 👑

The value immediately preceding the effects of this function in the modification order of *obj. literally the old value, the one from before you did the thing 💅

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🌈 EXAMPLE 🌈

// Run this code

#include <atomic>
#include <chrono>
#include <functional>
#include <iostream>
#include <thread>

// Binary semaphore for demonstrative purposes only.
// This is a simple yet meaningful example: atomic operations
// are unnecessary without threads.
class Semaphore
{
    std::atomic_char m_signaled;
public:
    Semaphore(bool initial = false)
    {
        m_signaled = initial;
    }
    // Block until semaphore is signaled
    void take()
    {
        while (!std::atomic_fetch_and(&m_signaled, false))
        {
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }

    void put()
    {
        std::atomic_fetch_or(&m_signaled, true);
    }
};

class ThreadedCounter
{
    static const int N = 100;
    static const int REPORT_INTERVAL = 10;
    int m_count;
    bool m_done;
    Semaphore m_count_sem;
    Semaphore m_print_sem;

    void count_up()
    {
        for (m_count = 1; m_count <= N; ++m_count)
            if (m_count % REPORT_INTERVAL == 0)
            {
                if (m_count == N)
                    m_done = true;
                m_print_sem.put(); // signal printing to occur
                m_count_sem.take(); // wait until printing is complete proceeding
            }
        std::cout << "count_up() done\n";
        m_done = true;
        m_print_sem.put();
    }

    void print_count()
    {
        do
        {
            m_print_sem.take();
            std::cout << m_count << '\n';
            m_count_sem.put();
        }
        while (!m_done);
        std::cout << "print_count() done\n";
    }

public:
    ThreadedCounter() : m_done(false) {}
    void run()
    {
        auto print_thread = std::thread(&ThreadedCounter::print_count, this);
        auto count_thread = std::thread(&ThreadedCounter::count_up, this);
        print_thread.join();
        count_thread.join();
    }
};

int main()
{
    ThreadedCounter m_counter;
    m_counter.run();
}

🌸 Output 🌸

10
20
30
40
50
60
70
80
90
100
print_count() done
count_up() done

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🧚 DEFECT REPORTS 🧚

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. retroactive glow-up, we love to see it 🔥

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

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⭐ SEE ALSO ⭐

• fetch_or — atomically performs bitwise OR between the argument and the value of the atomic object and obtains the value held previously (public member function of std::atomic<T>)
• atomic_fetch_and, atomic_fetch_and_explicit (C++11), (C++11) — replaces the atomic object with the result of bitwise AND with a non-atomic argument and obtains the previous value of the atomic (function template)
• atomic_fetch_xor, atomic_fetch_xor_explicit (C++11), (C++11) — replaces the atomic object with the result of bitwise XOR with a non-atomic argument and obtains the previous value of the atomic (function template)
• C documentation for atomic_fetch_or, atomic_fetch_or_explicit

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and that's the whole page, bestie 🫶✨ this one's from cppreference.com, dated 2024.06.10 — go forth and OR some bits atomically, periodt. 💅🎀