C++統計精確時間

QueryPerformanceFrequency用法

精確獲取時間：

QueryPerformanceFrequency() - 基本介紹

類型：Win32API

原型：BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);

作用：返回硬件支持的高精度計數器的頻率。

返回值：非零，硬件支持高精度計數器；零，硬件不支持，讀取失敗。

QueryPerformanceFrequency() - 技術特點

供WIN9X使用的高精度定時器：QueryPerformanceFrequency()和QueryPerformanceCounter()，要求計算機從硬件上支持高精度定時器。需包含windows.h頭文件。

函數的原形是：

BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);

BOOL QueryPerformanceCounter (LARGE_INTEGER *lpCount);

數據類型LARGEINTEGER既可以是一個作為8字節長的整數，也可以是作為兩個4字節長的整數的聯合結構，其具體用法根據編譯器是否支持64位而定。該類型的定義如下：

typeef union _ LARGE_INTEGER

{

struct

{

DWORD LowPart;

LONG HighPart;

};

LONGLONG QuadPart;

} LARGE_INTEGER;

在定時前應該先調用QueryPerformanceFrequency()函數獲得機器內部計時器的時鐘頻率。接著在需要嚴格計時的事件發生前和發生之後分別調用QueryPerformanceCounter()，利用兩次獲得的計數之差和時鐘頻率，就可以計算出事件經歷的精確時間。

測試Sleep的精確時間：

#include <stdio.h>

#include <windows.h>

void main()

{

LARGE_INTEGER nFreq;

LARGE_INTEGER nBeginTime;

LARGE_INTEGER nEndTime;

double time;



QueryPerformanceFrequency(&nFreq);

QueryPerformanceCounter(&nBeginTime);



Sleep(1000);



QueryPerformanceCounter(&nEndTime);

time=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;



printf("%f\n",time);

Sleep(1000);

system("Pause");

}

再給出我寫的一個Fibonacci的例子：

#include <iostream>
#include <cmath>
#include <ctime>
#include <windows.h>
#include<iomanip>
using namespace std;

unsigned long Fib1(unsigned long n)
{
return (n == 1 || n == 2) ? 1 : Fib1(n - 1) + Fib1(n - 2);
}

unsigned long Fib2(unsigned long n)
{
if (n == 1 || n == 2)
{
return 1;
}
unsigned long m1 = 1, m2 = 1;
for (unsigned long i = 3; i <= n; i++)
{
m2 = m1 + m2;
m1 = m2 - m1;
}

return m2;
}

int main()
{
LARGE_INTEGER nFreq;

LARGE_INTEGER nBeginTime;

LARGE_INTEGER nEndTime;

double time1 = 0, time2 = 0;
unsigned long result1 = 0;
unsigned long result2 = 0;
unsigned long fib_num = 45;
cout.precision(10);
cout.setf(cout.showpoint); //設置為始終輸出小數點後的數字，就是說 a = 3，它也輸出 3.00000 這
QueryPerformanceFrequency(&nFreq);

QueryPerformanceCounter(&nBeginTime);

result1 = Fib1(fib_num);

QueryPerformanceCounter(&nEndTime);

time1=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

cout<<"Fib1: "<< fixed<<setprecision(8)<<setiosflags(ios::showpoint)<<time1<<endl;

QueryPerformanceFrequency(&nFreq);

QueryPerformanceCounter(&nBeginTime);

result2 = Fib2(fib_num);

QueryPerformanceCounter(&nEndTime);

time2=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

cout<<"Fib2: "<< fixed<<setprecision(20)<<setiosflags(ios::showpoint)<<time2<<endl;

cout<<"Fib2 cost "<<time2*(unsigned long)100/time1<<"% time of Fib1."<<endl;
//cout << "Hello world!" << endl;
return 0;
}

C++ Primer Plus 第6版 中文版 清晰有書簽PDF+源代碼 http://www.linuxidc.com/Linux/2014-05/101227.htm

讀C++ Primer 之構造函數陷阱 http://www.linuxidc.com/Linux/2011-08/40176.htm

讀C++ Primer 之智能指針 http://www.linuxidc.com/Linux/2011-08/40177.htm

讀C++ Primer 之句柄類 http://www.linuxidc.com/Linux/2011-08/40175.htm