C++ chrono系统时间以毫秒为单位，时间运算
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C++ chrono system time in milliseconds, time operations
提问by Dejwi
I've got a small problem caused by insufficient documentation of C++11.
由于 C++11 的文档不足，我遇到了一个小问题。
I'd like to obtain a time since epoch in milliseconds, or nanoseconds or seconds and then I will have to "cast" this value to another resolution. I can do it using gettimeofday() but it will be to easy, so I tried to achieve it using std::chrono.
我想以毫秒、纳秒或秒为单位获得自纪元以来的时间，然后我必须将此值“转换”为另一个分辨率。我可以使用 gettimeofday() 来完成它，但它会很容易，所以我尝试使用 std::chrono 来实现它。
I tried:
我试过：
std::chrono::time_point<std::chrono::system_clock> now =
std::chrono::system_clock::now();
But I have no idea what is a resolution of obtained in this way time_point, and I don't know how to get this time as a simple unsigned long long, and I haven't any conception how to cast it to another resolution.
但是我不知道以这种方式获得的 time_point 的分辨率是什么，我不知道如何将这个时间作为一个简单的 unsigned long long 获得，并且我没有任何概念如何将其转换为另一个分辨率。
回答by R. Martinho Fernandes
You can do now.time_since_epoch()to get a duration representing the time since the epoch, with the clock's resolution. To convert to milliseconds use duration_cast:
您可以now.time_since_epoch()使用时钟的分辨率来获取表示自纪元以来时间的持续时间。要转换为毫秒，请使用duration_cast：
auto duration = now.time_since_epoch();
auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();
