What are the uses of std::chrono::high_resolution_

问题:

At first I thought it can be used for performance measurements. But it is said that std::chrono::high_resolution_clock may be not steady (is_steady may be false). It is also said that std::chrono::high_resolution_clock may even be an alias of std::chrono::system_clock which is generally not steady. So I can't measure time intervals with this type of clock because at any moment the clock may be adjusted and my measurements will be wrong.

At the same time I can't convert time points of std::chrono::high_resolution_clock to calendar time because it doesn't have to_time_t method. So I can't get the real time with this type of clock either.

Then what can std::chrono::high_resolution_clock be used for?

回答1:

There are none.

Sorry, my bad.

If you are tempted to use high_resolution_clock, choose steady_clock instead. On libc++ and VS high_resolution_clock is a type alias of steady_clock anyway.

On gcc high_resolution_clock is a type alias of system_clock and I've seen more than one use of high_resolution_clock::to_time_t on this platform (which is wrong).

Do use <chrono>. But there are parts of <chrono> that you should avoid.

• Don't use high_resolution_clock.
• Avoid uses of .count() and .time_since_epoch() unless there is no other way to get the job done.
• Avoid duration_cast unless the code won't compile without it, and you desire truncation-towards-zero behavior.
• Avoid explicit conversion syntax if an implicit conversion compiles.

回答2:

I would suggest that single uses of the high res clock could lie.

If your algorithm makes use of many measurements then any error should average-out and you should be able to measure very small intervals.

I've used the high resolution clock to build timelines in network events where otherwise my intervals would have all been 0. :)

Finally, consider that while your system clock might not be steady, if your high res clock is stead, that greatly simplifies some games you have to play to make sure time always flows forwards.

In summary, yeah, it can fail, but when you have it, it's very helpful. Lots of samples are your friends. :)