计时器计时器不正确转换秒数 [英] Chrono Timer Not Converting Seconds Properly

问题描述

我有一个有趣的，但奇怪的问题，我的游戏计时器。它似乎毫秒工作正常。但是，当我尝试应用 std :: chrono :: seconds cast时，突然获取 0.000000 浮动。

我的计时器如下：

  #include< ; iostream> 
 #include< time.h> 
 #include< chrono> 
 
 
 class Timer 
 {
 public：
 
 typedef std :: chrono :: high_resolution_clock时间; 
 typedef std :: chrono :: milliseconds ms; //<  - 如果改变为秒，我得到0.00000 
 typedef std :: chrono :: duration< float> fsec; 
 
 std :: chrono :: high_resolution_clock :: time_point m_timestamp; 
 
 float currentElapsed; 
 
 Timer（）
 {
 m_timestamp = Time :: now（）; 
} 
 
 float getTimeElapsed（）
 {
 return currentElapsed; 
} 
 
 void Tick（）
 {
 currentElapsed = std :: chrono :: duration_cast< ms>（Time :: now（） -  m_timestamp）.count （）; 
 m_timestamp = Time :: now（）; 
} 
 
 public：
 // Singleton stuff 
 static Timer * Instance（）; 
 static void Create（）; 
}; 
  

计时器每帧一次所以，例如我通常每帧大约33ms。 33ms / 1000 = 0.033s 秒，所以应该有足够的位空间。

编辑：对不起，秒，不是毫秒

解决方案

std :: chrono :: seconds 都被指定为具有整数表示（C ++ 11§20.11.2[time.syn]）。当您将高分辨率持续时间转换为低分辨率持续时间时，您正在执行带有结果截断的整数除法，例如

 使用命名空间std :: chrono; 
 assert（duration_cast< seconds>（milliseconds {999}）== seconds {0}）; 
  

您可以通过在缩放之前切换到浮点表示而不是之后的：

  using namespace std :: chrono; 
 currentElapsed = duration_cast< duration< float，std :: milli>>（Time :: now（） -  m_timestamp）.count（）; 
  

（在coliru演示）

更好的是， store currentElapsed 作为 duration 大小：

  class Timer {
 typedef std :: chrono :: high_resolution_clock时间; 
 typedef std :: chrono :: duration< float>持续时间; 
 
 Time :: time_point m_timestamp; 
 duration currentElapsed; 
 
 public：
 Timer（）：m_timestamp（Time :: now（））{} 
 
 duration getTimeElapsed（）const {
 return currentElapsed; 
} 
 
 void Tick（）{
 auto now = Time :: now（）; 
 currentElapsed = now  -  m_timestamp; 
 m_timestamp = now; 
} 
}; 
  

I am having an interesting, yet strange issue with my game timer. It seems like the milliseconds works just fine. However, when I try to apply the std::chrono::seconds cast I suddenly get 0.000000 when casting to a float.

My timer is as follows:

 #include <iostream>
 #include <time.h>
 #include <chrono>


 class Timer
 {
 public:

     typedef std::chrono::high_resolution_clock Time;
     typedef std::chrono::milliseconds ms; //<--If changed to seconds, I get 0.00000
     typedef std::chrono::duration<float> fsec;

     std::chrono::high_resolution_clock::time_point m_timestamp;

     float currentElapsed;

     Timer()
     {
          m_timestamp = Time::now();
     }

     float getTimeElapsed()
     {
         return currentElapsed;
     }

     void Tick()
     {
         currentElapsed = std::chrono::duration_cast<ms>(Time::now() - m_timestamp).count();
         m_timestamp = Time::now();
     }

 public:
     //Singleton stuff
     static Timer* Instance();
     static void Create();
 };

The timer gets ticked once per frame. So, for instance I normally get about 33ms per frame. 33ms / 1000 = 0.033s seconds so there should be plenty of bit space to hold that.

Any ideas on what maybe going on?

Any help is greatly appreciated!

EDIT: Sorry, Seconds, not Milliseconds

解决方案

std::chrono::seconds et al. are all specified to have integral representations (C++11 §20.11.2 [time.syn]). When you convert a high-resolution duration to a low-resolution duration, you are performing integer division with resulting truncation, e.g,

using namespace std::chrono;
assert(duration_cast<seconds>(milliseconds{999}) == seconds{0});

You can avoid this truncation by switching to a floating point representation before scaling instead of after:

using namespace std::chrono;
currentElapsed = duration_cast<duration<float,std::milli>>(Time::now() - m_timestamp).count();

(Demo at coliru)

Better yet, store currentElapsed as a duration to keep the "units" associated with the magnitude:

class Timer {
    typedef std::chrono::high_resolution_clock Time;
    typedef std::chrono::duration<float> duration;

    Time::time_point m_timestamp;
    duration currentElapsed;

public:
    Timer() : m_timestamp(Time::now()) {}

    duration getTimeElapsed() const {
        return currentElapsed;
    }

    void Tick() {
        auto now = Time::now();
        currentElapsed = now - m_timestamp;
        m_timestamp = now;
    }
};

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