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std::atomic_fetch_or, std::atomic_fetch_or_explicit

Defined in header `<atomic>`
	(1)	(since C++11)
template< class T > T atomic_fetch_or( std::atomic<T>* obj, typename std::atomic<T>::value_type arg ) noexcept;
template< class T > T atomic_fetch_or( volatile std::atomic<T>* obj, typename std::atomic<T>::value_type arg ) noexcept;
	(2)	(since C++11)
template< class T > T atomic_fetch_or_explicit( std::atomic<T>* obj, typename std::atomic<T>::value_type 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 arg, std::memory_order order) noexcept;

Atomically replaces the value pointed by obj with the result of bitwise OR between the old value of obj and arg. Returns the value obj held previously.

The operation is performed as if the following is executed:

1) obj->fetch_or(arg)

2) obj->fetch_or(arg, order)

If std::atomic<T> has no fetch_or member (this member is only provided for integral types), the program is ill-formed.

Parameters

obj	-	pointer to the atomic object to modify
arg	-	the value to bitwise OR 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_or( std::atomic<T>* obj, typename std::atomic<T>::value_type  arg )
{
    return obj->fetch_or(arg);
}

Example

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

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