Why Does `std::chrono::high_resolution_clock`'s Precision Differ from Measured Values?

Why does the accuracy of std::chrono::high_resolution_clock not match the measured value?

Problem:

In the following test program, the program tries to measure the execution time of std::cout, but the measurement results obtained are not consistent and are always 1 or 2 microseconds. This is because the precision of std::chrono::high_resolution_clock does not seem to be 100 nanoseconds, or there is a problem with the code that measures std::cout execution time.

#include <iostream>
#include <chrono>

using std::chrono::nanoseconds;
using std::chrono::duration_cast;

int main()
{
    std::cout << "Resolution (nano) = " << (double) std::chrono::high_resolution_clock::period::num / 
                                          std::chrono::high_resolution_clock::period::den * 
                                          1000 * 1000 * 1000 << std::endl;

    auto t1 = std::chrono::high_resolution_clock::now();
    std::cout << "How many nanoseconds does std::cout take?" << std::endl;
    auto t2 = std::chrono::high_resolution_clock::now();

    auto diff = t2 - t1;
    nanoseconds ns = duration_cast<nanoseconds>(diff);

    std::cout << "std::cout takes " << ns.count() << " nanoseconds" 
              << std::endl;
}

Answer:

In Visual Studio 2012, std::chrono::high_resolution_clock is defined as system_clock, which results in poor accuracy (approx. 1 ms). For greater accuracy, the following custom high-res clock can be used:

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));
}

This clock can be used instead of std::chrono::high_resolution_clock for high-precision time measurement.

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