vc 程式效率測試 與 如何擷取系統精確時間

                GetTickCount返回（retrieve）從作業系統啟動到現在所經過（elapsed）的毫秒數，它的傳回值是DWORD。

如： DWORD time；

       time = GetTickCount();

但是

GetTickcount函數：它返回從作業系統啟動到當前所經過的毫秒數，常常用來判斷某個方法執行的時間，其函數原型是DWORD GetTickCount(void)，傳回值以32位的雙字類型DWORD儲存，因此可以儲存的最大值是2^32 ms約為49.71天，因此若系統已耗用時間超過49.71天時，這個數就會歸0，MSDN中也明確的提到了:"Retrieves the number of milliseconds that have elapsed since the system was started, up to 49.7 days."。因此，如果是編寫伺服器端程式，此處一定要萬分注意，避免引起意外的狀況。特別注意：這個函數並非即時發送，而是由系統每18ms發送一次，因此其最小精度為18ms。當需要有小於18ms的精度計算時，應使用StopWatch方法進行。也就是說使用這個函數，精度不高。為了擷取精確的時間，從而獲得精確的效率測量可以使用QueryPerformanceCounter(&ticks)函數樣本程式可以這樣用：

   ////////////variables definition//////////// DWORD time1,time2,time3;//計算時間用  LARGE_INTEGER  frequency;//高效能計數器的頻率 LARGE_INTEGER start,end; start.QuadPart=0; end.QuadPart=0; frequency.QuadPart=0; long ticks=0; QueryPerformanceFrequency(&frequency);

///////////////////processing ///////////#ifdef DEBUGTIME time1 = GetTickCount(); QueryPerformanceCounter(&start);    #endif

#ifdef DEBUGTIME time2 = GetTickCount(); QueryPerformanceCounter(&end);     ticks = (long)(end.QuadPart-start.QuadPart); double millsecond=(double)ticks*1000.0/(double)frequency.QuadPart; printf("noninterpolating wasted time is %ld  number %ld  ticks %lf\n",time2-time1,arrayindex,millsecond);#endif

這是一位名叫softman11，csdn論壇裡這位作者提供的c++ 類實現

#pragma once#include<Windows.h> ///本類採用高解析度高效能計數器實現///在我的Intel T7500機器上，解析度大約是279毫微秒class StopWatch{public:    StopWatch(void);    ~StopWatch(void);private:    LARGE_INTEGER beginticks;    LARGE_INTEGER endticks ;    LARGE_INTEGER  frequency;//高效能計數器的頻率：每秒357,9545個tick  我的INTEL T7500public:    void Start();    void Stop();    double  GetCostMillisecond();    unsigned long long  GetFrequency();}; #include "StdAfx.h"#include "StopWatch.h"#include <Windows.h>#include<iostream>using namespace std; StopWatch::StopWatch(void){    beginticks.QuadPart=0;    endticks.QuadPart=0;    frequency.QuadPart=0;    QueryPerformanceFrequency(&frequency);}  StopWatch::~StopWatch(void){} void StopWatch::Start(){    //beginticks=GetTickCount();    QueryPerformanceCounter(&beginticks);     }void StopWatch::Stop(){    QueryPerformanceCounter(&endticks);     }double  StopWatch::GetCostMillisecond(){    unsigned long long cost=(unsigned long long)(endticks.QuadPart-beginticks.QuadPart);    double millsecond=(double)cost*1000.0/(double)frequency.QuadPart;        return millsecond;}unsigned long long StopWatch::GetFrequency(){    return (unsigned long long)frequency.QuadPart;}

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