Why Does `std::chrono::high_resolution_clock` Show Discrepancies Despite Nanosecond Resolution?

std::chrono::high_resolution_clock Resolution Discrepancy

Despite specifying a nanosecond resolution for the std::chrono::high_resolution_clock, discrepancies arise when measuring time intervals. This can be seen in the provided test program where the reported time taken by std::cout is not precisely within the expected resolution range.

Resolving the Discrepancy

The cause of this discrepancy lies in the implementation of std::chrono::high_resolution_clock on Visual Studio 2012. Under this environment, high_resolution_clock is defined as an alias for system_clock, which has a resolution around 1 millisecond.

Fix for Visual Studio 2012

To obtain a truly high-resolution timer on Visual Studio 2012, a custom high-resolution clock implementation is recommended:

HighResClock.h

struct HighResClock
{
    typedef long long                              rep;
    typedef std::nano                              period;
    typedef std::chrono::duration<rep, period>     duration;
    typedef std::chrono::time_point<HighResClock>  time_point;
    static const bool is_steady = true;

    static time_point now();
};

HighResClock.cpp

namespace
{
    const long long g_Frequency = []() -> long long
    {
        LARGE_INTEGER frequency;
        QueryPerformanceFrequency(&frequency);
        return frequency.QuadPart;
    }();
}

HighResClock::time_point HighResClock::now()
{
    LARGE_INTEGER count;
    QueryPerformanceCounter(&count);
    return time_point(duration(count.QuadPart * static_cast<rep>(period::den) / g_Frequency));
}

Usage of Custom Clock

The custom HighResClock can be used in place of std::chrono::high_resolution_clock with the same functionality. By utilizing QueryPerformanceCounter directly, it provides accurate high-resolution timing.

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