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:
1,2) obj->fetch_or(arg)
3,4) obj->fetch_or(arg, order)
If std::atomic<T> has no fetch_or member (this member is only provided for integral types except bool), the program is ill-formed.
The value immediately preceding the effects of this function in the modification order of *obj.
[edit] Example#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[edit] 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 becauseT was deduced from multiple arguments T is only deduced
std::atomic<T>) [edit] replaces the atomic object with the result of bitwise AND with a non-atomic argument and obtains the previous value of the atomic
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