std :: condition_variable谓词是否在锁定状态下执行？

Question

As condition variable could check whether given predicate is true or not, and return only if it is true (like below snippet), is the predicate running inside the protection of lock included in the condition variable ( mutex m in below snippet)?

std::condition_variable cv;
std::mutex m;
std::unique_lock<std::mutex> lk;

cv.wait(lk, []{/*predicate*/});

Answer 1

The version of wait() that takes a predicate is equivalent to:

while (!pred())
    wait(lock);

Hence, the predicate is checked outside the wait(lock) call (and the lock is unlocked only inside that call), meaning that the lock is locked when the predicate is checked.

Answer 2

The call cv.wait(lk, p) is defined by [thread.condition.condvar] to be equivalent to:

while (!p())
    cv.wait(lk);

Moreover, the precondition for wait is that the calling thread holds the lock. Therefore, yes indeed, the predicate check happens under the lock.

Anything other than this behaviour wouldn't make a lot of sense, since otherwise checking the predicate would produce a data race unless you would provide a separate synchronisation mechanism for the predicate.