time since epoch

C++11 提供了新的获取系统时间的库函数，在获取时间的时候一般常用的是获取time since epoch，下面来看一下如何获取这个时间。

 #include <iostream>
 #include <chrono>
 #include <ctime>
 #include <sys/time.h>
 #include <cstdint>
 constexpr int microSecPerSec = ;

 int main()
 {
     std::chrono::time_point<std::chrono::system_clock> p1, p2, p3;
     struct timeval tv;
     int64_t mSecond;

     p2 = std::chrono::system_clock::now();
     p3 = p2 - std::chrono::hours();
     gettimeofday(&tv,NULL);
     mSecond = tv.tv_sec ;

     std::time_t epoch_time = std::chrono::system_clock::to_time_t(p1); // seconds
     std::cout << "epoch: " << std::ctime(&epoch_time);
     std::time_t today_time = std::chrono::system_clock::to_time_t(p2);
     std::cout << "today: " << std::ctime(&today_time);

     std::cout << "hours since epoch: "
                 << std::chrono::duration_cast<std::chrono::hours>(   //hours
                    p2.time_since_epoch()).count()
                 << std::endl;

     std::cout << "yesterday, hours since epoch: "
                 << std::chrono::duration_cast<std::chrono::hours>(
                    p3.time_since_epoch()).count()
                 << std::endl;

     std::cout << "Microseconds since epoch: (chrono) : "   //microseconds
                 << std::chrono::duration_cast<std::chrono::microseconds>(p2.time_since_epoch()).count() // p2 - p1 ==
                 <<std::endl;

     std::cout << "Microseconds since epoch: (gettimeofday) :" << mSecond * microSecPerSec + tv.tv_usec
                 <<std::endl;

     return ;
 }

结果：

C:\Users\zhuyp\Desktop\tmp>cpp_time
epoch: Thu Jan  ::
today: Fri Jan  ::
hours since epoch:
yesterday, hours since epoch:
Microseconds since epoch: (chrono) :
Microseconds since epoch: (gettimeofday) :