c++ - std::condition_variable在阻塞之前是否真的解锁了给定的unique_lock对象？

Question

正如引用所言:1) Atomically unlocks lock, blocks the current executing thread, and...
我有以下代码:

#include <iostream>
#include <thread>
#include <condition_variable>

std::mutex mutex_;
std::condition_variable condVar;
std::unique_lock<std::mutex> lck(mutex_); //purposely global to check return of owns_lock() in main

void waitingForWork()
{
    std::cout << "Before wait, lck.owns_lock() = " << lck.owns_lock() << '\n';

    condVar.wait(lck);

    std::cout << "After wait, lck.owns_lock() = " << lck.owns_lock() << '\n';
}

int main()
{
  std::thread t1(waitingForWork);

  std::this_thread::sleep_for(std::chrono::seconds(10));

  std::cout << "In main, lck.owns_lock() = " << lck.owns_lock() << '\n';
  condVar.notify_one();

  t1.join();
  return 0;
}

编译使用:g++ with c++17 on ubuntu输出:

Before wait, lck.owns_lock() = 1
In main, lck.owns_lock() = 1
After wait, lck.owns_lock() = 1

但是根据引用，我希望互斥锁会在等待时被解锁，即:

In main, lck.owns_lock() = 0

有人可以告诉我为什么吗？

Answer 1

您必须进一步阅读:

When unblocked, regardless of the reason, lock is reacquired and wait exits. If this function exits via exception, lock is also reacquired.

因此，始终保证在退出等待时重新获得该锁。