NAME¶

std::atomic_fetch_or,std::atomic_fetch_or_explicit - std::atomic_fetch_or,std::atomic_fetch_or_explicit

Synopsis¶

Defined in header <atomic>
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;

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.

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

Return value¶

The value immediately preceding the effects of this function in the modification
order of *obj.

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

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 is only deduced
T was deduced from multiple arguments from obj

See also¶

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