计算时间，精确到纳秒

#ifndef __TIMER_H__
#define __TIMER_H__

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

class Timer
{
public:
    Timer();
    ~Timer() {};
    void start();
    void end();
    float getTime() const;
    void display() const;

private:
    __int64     frequency; // Timer Frequency
    float       resolution; // Timer Resolution
    unsigned long mm_timer_start; // Multimedia Timer Start Value
    unsigned long mm_timer_elapsed; // Multimedia Timer Elapsed Time
    bool   performance_timer; // Using The Performance Timer?
    __int64     performance_timer_start; // Performance Timer Start Value
    __int64     performance_timer_elapsed; // Performance Timer Elapsed Time
    float       startTime;
    float       endTime;
    float       passTime;
};

Timer::Timer()
{
    // Check To See If A Performance Counter Is Available
    // If One Is Available The Timer Frequency Will Be Updated
    if (!QueryPerformanceFrequency((LARGE_INTEGER *) & frequency)) {
        // No Performace Counter Available
        performance_timer = FALSE; // Set Performance Timer To FALSE
        //mm_timer_start = timeGetTime(); // Use timeGetTime() To Get Current Time
        resolution = 1.0f/1000.0f; // Set Our Timer Resolution To .001f
        frequency = 1000; // Set Our Timer Frequency To 1000
        mm_timer_elapsed = mm_timer_start; // Set The Elapsed Time To The Current Time
    } else {
        // Performance Counter Is Available, Use It Instead Of The Multimedia Timer
        // Get The Current Time And Store It In performance_timer_start
        QueryPerformanceCounter((LARGE_INTEGER *) &performance_timer_start);
        performance_timer = TRUE; // Set Performance Timer To TRUE
        // Calculate The Timer Resolution Using The Timer Frequency
        resolution = (float) (((double)1.0f)/((double)frequency));
        // Set The Elapsed Time To The Current Time
        performance_timer_elapsed = performance_timer_start;
    }
}

void Timer::start()
{
    __int64 time; // time Will Hold A 64 Bit Integer

    if (performance_timer) { // Are We Using The Performance Timer?
        QueryPerformanceCounter((LARGE_INTEGER *) &time); // Grab The Current Performance Time
        // Return The Current Time Minus The Start Time Multiplied By The Resolution And 1000 (To Get MS)
        startTime = ( (float) ( time - performance_timer_start) * resolution)*1000.0f;
    } else {
        // Return The Current Time Minus The Start Time Multiplied By The Resolution And 1000 (To Get MS)
        //startTime = ( (float) ( timeGetTime() - mm_timer_start) * resolution)*1000.0f;
    }
}

void Timer::end()
{
    __int64 time; // time Will Hold A 64 Bit Integer

    if (performance_timer) { // Are We Using The Performance Timer?
        QueryPerformanceCounter((LARGE_INTEGER *) &time); // Grab The Current Performance Time
        // Return The Current Time Minus The Start Time Multiplied By The Resolution And 1000 (To Get MS)
        endTime = ( (float) ( time - performance_timer_start) * resolution)*1000.0f;
    } else {
        // Return The Current Time Minus The Start Time Multiplied By The Resolution And 1000 (To Get MS)
        //endTime = ( (float) ( timeGetTime() - mm_timer_start) * resolution)*1000.0f;
    }

    passTime = endTime - startTime;
}

float Timer::getTime() const
{
    return passTime/1000;
}

void Timer::display() const
{
    std::cout << "It takes " << passTime/1000 << " s." << std::endl;
}

#endif