Question

我正在努力想出一个最初看起来并不难的两个问题的答案。 Q1：如何获取UTC.Now（）和给定日期之间经过的秒数？ A1：就像下面的代码一样！ Q2：如何确定自上一次“完整”秒后经过的小数秒数？我想打印“total_elapsed_seconds.fractional_seconds” - ＆gt; “1234124.45”。我怎么做？ A2：???

#include <iostream>
#include <boost/date_time/gregorian/gregorian.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

using namespace std;
using namespace boost::gregorian;
using namespace boost::posix_time; 

void main()
{
    ptime Jan1st1970(date(1970, 1, 1));
    for(int i = 0; i < 10; i++)
    {
        ptime Now = second_clock::universal_time();
        time_duration diff = Now - Jan1st1970;
        cout << Now << " : " << diff.total_seconds() << "." << diff.fractional_seconds() << endl;
    }
}

Answer 1

您正在使用second_clock获取当前时间。顾名思义，它仅精确到最接近的秒。由于您的参考时间没有小数秒，因此持续时间小数秒始终为0.请改为使用microsec_clock：

ptime Now = microsec_clock::universal_time();

另外，在如此紧凑的循环中，我不希望每次迭代都更新时钟，所以你可能还想从boost :: thread添加一个sleep：

boost::this_thread::sleep(boost::posix_time::milliseconds(25));

Answer 2

你没有提到你使用的操作系统，但我知道窗口的时钟不会给你提供超过15毫秒的分辨率（除非你真的玩一些游戏）。
但是，Windows具有所谓的性能计时器，可以为您提供纳秒级别的分辨率。它实际上只是CPU循环次数的计数器（您可以除以CPU的频率来获取时间），因此要将其用作时钟，您必须将该时间添加到已知时间：

ptime Start = microsec_clock::universal_time();
initMyClockToZero();  // You have to write this to use the performance timer
.... do something ....
int64 microseconds = getMyClockMicrosec(); // this too
ptime Now = Start + posix_time::microseconds(microseconds);

我还有一个秒表式的计时器，我是用自己的Windows编写的。

#ifndef STOPWATCH_HPP
#define STOPWATCH_HPP

#include <iostream>
#include <windows.h>

//! \brief Stopwatch for timing performance values
//!
//! This stopwatch class is designed for timing performance of various
//! software operations.  If the values you get back a greater than a 
//! few seconds, you should be using a different tool.
//! On a Core 2 Duo E6850 @ 3.00GHz, the start/stop sequence takes 
//! approximately 230 nano seconds in the debug configuration and 180 
//! nano seconds in the release configuration.  If you are timing on the
//! sub-microsecond scale, please take this into account and test it on
//! your machine.
class Stopwatch{
public:
    //! \param start if set to true will initialize and then start the 
    //! timer.
    Stopwatch(bool start=false){
        _start.QuadPart = 0;
        _stop.QuadPart = 0;
        if(start)
            Start();
    }

    //! Starts the stopwatch running
    void Start(){
        QueryPerformanceCounter(&_start);
    }
    //! Run this when the event being timed is complete
    void Stop(){
        QueryPerformanceCounter(&_stop);
    }
   //! Stops the timer and returns the result
   double StopResult(){
      Stop();
      return ResultNanoseconds();
   }
    //! You can get the result of the stopwatch start-stop sequence at
    //! your leisure.  
    double ResultNanoseconds(){
        LARGE_INTEGER frequency;
        QueryPerformanceFrequency(&frequency);
        double cyclesPerNanosecond = static_cast<double>(frequency.QuadPart) / 1000000000.0;

        LARGE_INTEGER elapsed;
        elapsed.QuadPart = _stop.QuadPart - _start.QuadPart;
        return elapsed.QuadPart / cyclesPerNanosecond;
    }
    void PrintResultNanoseconds(){
        std::cout << ResultNanoseconds() << "nanosec" << std::endl;
    }
    void PrintResultMicroseconds(){
        std::cout << ResultNanoseconds()/100 << "microsec" << std::endl;
    }
    void PrintResultMilliseconds(){
        std::cout << ResultNanoseconds()/100000 << "millisec" << std::endl;
    }
    void PrintResultSeconds(){
        std::cout << ResultNanoseconds()/1000000000 << "sec" << std::endl;
    }
private:
    LARGE_INTEGER _start;
    LARGE_INTEGER _stop;
};

#endif STOPWATCH_HPP

Answer 3

这听起来确实合理，但输出给了我：

2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
2009-May-28 20:14:32 : 1243541672.0
Press any key to continue . . .

事实上，我期待更多数字 - 我希望得到10 ^（ - 6）[秒]范围内的结果。

Answer 4

将此添加到您的示例中以获取“当前”小数秒：

cout << Now.time_of_day().fractional_seconds() << endl;

将时钟从second_clock更改为microsec_clock以获得非零小数秒。

ptime Now = microsec_clock::universal_time();

C ++ :: Boost :: posix_time（经过的秒数。经过的小数秒）

4 个答案: