c++ 标准库duration

std::chrono::duration<int> twentySecends(20);
std::chrono::duration<double, std::ratio<60>> halfAMinute(0.5);//单位60s
std::chrono::duration<long, std::ratio<1, 1000>> oneMillisend(1);//单位1ms

	// duration TYPES
using nanoseconds = duration<long long, nano>;
using microseconds = duration<long long, micro>;
using milliseconds = duration<long long, milli>;
using seconds = duration<long long>;
using minutes = duration<int, ratio<60>>;
using hours = duration<int, ratio<3600>>;
//nano milli是ratio中的一些定义.