CP guidelines for condition variables

Sorry this isn't complete, I'll try to flesh it out.

Condition variables are not semaphores. Notifications will be missed if they are sent when no other thread is blocked waiting on the condition variable, so you must not just rely on notify_one() or notify_all() to signal another thread, you must always have some predicate that is tested, e.g. a boolean flag (protected by the same mutex that is used when waiting on the condition variable). With a predicate that tests some condition, even if the notification is sent before the other thread waits on the condition variable then it won't be "missed" because the predicate will be true and the wait will return immediately.

Bad:

std::condition_variable cv;
std::mutex mx;

void thread1() {
  // do some work
  // ...
  // wake other thread
  cv.notify_one();
}

void thread2() {
  std::unique_lock<std::mutex> lock(mx);
  cv.wait(lock);  // might block forever
  // do work ...
}

Good:

std::condition_variable cv;
std::mutex mx;
bool ready = false;

void thread1() {
  // do some work
  // ...
  // make condition true:
  std::lock_guard<std::mutex> lock(mx);
  ready = true;
  // wake other thread
  cv.notify_one();
}

void thread2() {
  std::unique_lock<std::mutex> lock(mx);
  cv.wait(lock, []{ return ready; });
  // do work ...
}