condition_variable event notifyAndWait() - c++

Question

I have a loop that waits for the event.
I want to be able to add a notifyAndWait() function
Im not to sure how i can add that to my current class
class Event
{
public:
Event() : _signalled(false) {}
void notify()
{
std::unique_lock<std::mutex> lock(_mutex);
_signalled = true;
_condvar.notify_one();
}
void wait()
{
std::unique_lock<std::mutex> lock(_mutex);
_condvar.wait(lock, [&] { return _signalled; }); // also takes care about spurious wakeup
_signalled = false; // auto-reset semantics
}
private:
std::mutex _mutex;
std::condition_variable _condvar;
bool _signalled;
};

Answer 1

You could use the same condition variable to pass a signal back to the calling thread, using a different flag. Something like this:
class Event
{
public:
Event() : _signalled(false) {}
void notify()
{
std::cout << "notifying" << '\n' << std::flush;
{
std::unique_lock<std::mutex> lock(_mutex);
_completed = false;
_signalled = true;
}
_condvar.notify_one();
{
// now wait for completion
std::unique_lock<std::mutex> lock(_mutex);
_condvar.wait(lock, [&] { return _completed; });
}
std::cout << "completed" << '\n' << std::flush;
}
void wait()
{
{
std::unique_lock<std::mutex> lock(_mutex);
_condvar.wait(lock, [&] { return _signalled; });
_signalled = false;
}
// do stuff
auto n = hol::random_number(10);
std::cout << "doing " << n << " different things" << std::flush;
for(int i = 0; i < n; ++i)
{
std::cout << '.' << std::flush;
std::this_thread::sleep_for(std::chrono::milliseconds(hol::random_number(3000)));
}
std::cout << " done stuff" << '\n' << std::flush;
{
std::unique_lock<std::mutex> lock(_mutex);
_completed = true;
}
_condvar.notify_one();
}
private:
std::mutex _mutex;
std::condition_variable _condvar;
bool _signalled;
bool _completed;
};
int main()
{
Event e;
std::thread wait([&]{ e.wait(); });
e.notify();
wait.join();
}