时间chrono

template <intmax_t N, intmax_t D = 1> class ratio;

std::ratio用于实例化由分子和分母表示的有理数，第一个是分子，第二个是分母，这是两个非类型模板参数

template <class Rep, class Period = ratio<1> >
class duration;

std::chrono::duration第一个参数是个数，第二个参数是以秒为单位的比率类型，乘起来就是最后总共的秒数

seconds是duration的一个全特化的类

typedef duration<long long,ratio<1>> seconds;

type	Representation	Period
hours	signed integral type of at least 23 bits	ratio<3600,1>
minutes	signed integral type of at least 29 bits	ratio<60,1>
seconds	signed integral type of at least 35 bits	ratio<1,1>
milliseconds	signed integral type of at least 45 bits	ratio<1,1000>
microseconds	signed integral type of at least 55 bits	ratio<1,1000000>
nanoseconds	signed integral type of at least 64 bits	ratio<1,1000000000>