How Can QueryPerformanceCounter Improve Timer Accuracy and How Is It Used?

What is QueryPerformanceCounter and How to Use It

To improve the timing accuracy of a Timer class, QueryPerformanceCounter can be a reliable option, particularly when seeking microsecond resolution.

Implementing QueryPerformanceCounter

To utilize QueryPerformanceCounter, follow these steps:

1. Initialize PCFreq:

   • Determine the performance counter frequency using QueryPerformanceFrequency(&li).
   • Convert the result to double and divide it by 1000 to obtain the frequency in microseconds.

LARGE_INTEGER li;
if (!QueryPerformanceFrequency(&li))
    cout << "QueryPerformanceFrequency failed!\n";

PCFreq = double(li.QuadPart) / 1000.0;

2. Start the Counter:

   • Record the initial count using QueryPerformanceCounter(&li).
   • Store this count in the CounterStart variable.

QueryPerformanceCounter(&li);
CounterStart = li.QuadPart;

3. Get Time Elapsed:

   • Call QueryPerformanceCounter(&li) to obtain the current count.
   • Calculate the time elapsed by subtracting the start count from the current count and dividing the result by PCFreq.

double GetCounter()
{
    QueryPerformanceCounter(&li);
    return double(li.QuadPart - CounterStart) / PCFreq;
}

4. Example Usage:

StartCounter();
Sleep(1000);
cout << GetCounter() << "\n"; // Output: approximately 1000

By adjusting the division of PCFreq you can control the unit of time returned:

• Milliseconds: /1000.0
• Seconds: /1.0
• Microseconds: /1000000.0

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