std::condition_variable - 性能有什么不同吗？

Question

Is there any difference in performance between those two:

First version:

std::condition_variable conditionVarible;
std::mutex mutex;
std::chrono::steady_clock::time_point timePoint;

timePoint = std::chrono::steady_clock::now() + std::chrono::seconds(60);

std::unique_lock uniqueLock(mutex);
condition_variable.wait(uniqueLock, []() { 
    return std::chrono::steady_clock::now() >= timePoint; 
});

Second version:

std::condition_variable conditionVarible;
std::mutex mutex;
std::chrono::steady_clock::time_point timePoint;

timePoint = std::chrono::steady_clock::now() + std::chrono::seconds(60);

std::unique_lock uniqueLock(mutex);
condition_variable.wait_until(uniqueLock, timePoint, [](){ return false; });

Answer 1

Those two code snippets do two completely different things.

The first one will wait until the condition variable gets signaled, then check whether the time has elapsed. If it has, it wakes up. If it hasn't, it goes back to the wait state until the next signal. Repeat until done.

The second one ignores signals. It will simply wait until until the time has elapsed. It's an over-complicated and inefficient way to write sleep_until .

In short: this first will wake up on the first signal after the time has elapsed; the second will wake up after the time has elapsed.