C++ High Resolution Clock Precision – How to Get It

c++c++-chronoc++11clockduration

C++11 defines high_resolution_clock and it has the member types period and rep. But I can not figure out how I can get the precision of that clock.

Or, if I may not get to the precision, can I somehow at least get a count in nanoseconds of the minimum representable time duration between ticks? probably using period?

#include <iostream>
#include <chrono>
void printPrec() {
    std::chrono::high_resolution_clock::rep x = 1;
    // this is not the correct way to initialize 'period':
    //high_resolution_clock::period y = 1;

    std::cout << "The smallest period is "
              << /* what to do with 'x' or 'y' here? */
              << " nanos\n";
}

Best Answer

The minimum representable duration is high_resolution_clock::period::num / high_resolution_clock::period::den seconds. You can print it like this:

std::cout << (double) std::chrono::high_resolution_clock::period::num
             / std::chrono::high_resolution_clock::period::den;

Why is this? A clock's ::period member is defined as "The tick period of the clock in seconds." It is a specialization of std::ratio which is a template to represent ratios at compile-time. It provides two integral constants: num and den, the numerator and denominator of a fraction, respectively.

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C++ – Fix std::chrono::high_resolution_clock Resolution Issue

I'm going to guess you are using Visual Studio 2012. If not, disregard this answer. Visual Studio 2012 typedef's high_resolution_clock to system_clock. Sadly, this means it has crappy precision (around 1 ms). I wrote a better high-resolution clock which uses QueryPerformanceCounter for use in Visual Studio 2012...

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));
}

(I left out an assert and #ifs to see if it's being compiled on Visual Studio 2012 from the above code.)

You can use this clock anywhere and in the same way as standard clocks.

C++11 High Resolution Clock Time_Point – How to Print

There is no truly graceful way to do this. high_resolution_clock is not known to be related to UTC, or any other calendar. One thing you can portably do is output its current duration from its unspecified epoch, along with the units of that duration:

#include <chrono>
#include <iostream>

int
main()
{
    using Clock = std::chrono::high_resolution_clock;
    constexpr auto num = Clock::period::num;
    constexpr auto den = Clock::period::den;
    std::cout << Clock::now().time_since_epoch().count()
              << " [" << num << '/' << den << "] units since epoch\n";
}

Which for me outputs:

516583779589531 [1/1000000000] units since epoch

which means it is 516583779589531 nanoseconds (or 516,583.779589531 seconds) since the epoch of this clock on my machine. On my machine this translates to: my machine has been booted up for nearly 6 days. But that translation is not portable.

Ah! And I note from your error message that you are using libc++. If you are also on OS X, then we have the same definition of high_resolution_clock: It counts nanoseconds since your computer was booted.

Best Answer

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C++ – Fix std::chrono::high_resolution_clock Resolution Issue

C++11 High Resolution Clock Time_Point – How to Print

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