要获取以纪元为单位的当前时间(以秒为单位)作为浮点值，可以将duration_cast设置为浮点持续时间类型:

To get the current time since the epoch in seconds as a floating-point value, you can duration_cast to a floating-point duration type:

#include <chrono>

double fractional_seconds_since_epoch
    = std::chrono::duration_cast<std::chrono::duration<double>>(
        std::chrono::system_clock::now().time_since_epoch()).count();