std::chrono is meant to take advantage of information about periodicity at compile time, but if you want to count in terms of a dynamic periodicity not known until runtime, you can still do things. E.g. use time_t and some wrapper functions, or just pick one (or several) std::chrono base durations such as standardizing on nanoseconds, and then just count floating-point ticks.
Right, that's the problem I'm describing.
> or just pick one (or several) std::chrono base durations such as standardizing on nanoseconds, and then just count floating-point ticks.
None of this really fits well -- the idea is to count in integer ticks (invariant cycles), without doing expensive division or floating point math. It's like steady clock, but in ticks instead of nanos.
> (invariant cycles)
All major CPUs have had invariant cycle counters for decades at this point. It is a runtime constant.
When discussing hardware clock ticks used for time measurement, the ticks from TSC or similar counters are meant and not ticks from some counter that counts cycles of the CPU clock, like provided by the performance counters.
The variable CPU clock frequency can be measured by computing the ratio between the ticks accumulated by the corresponding performance counter and the ticks accumulated by the TSC (or similar counter), for which the clock frequency is constant and known.
I remember not being able to represent the time in fractional units, like tertias (1/60-th of a second) but that was mostly an academic problem. More importantly, it was not possible to express duration as a compound object. I wanted to be able to represent the dates with nanosecond precision, but with more than 250 years of range. I think it is still not possible?
Likewise it's annoying to cook up your own type, but as long as you can come up with 'number of ticks' type that acts like an arithmetic type then it should work with std::chrono. E.g. if I just Boost's Multiprecision library as a quick example:
#include <chrono>
#include <iostream>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std::chrono_literals;
int main()
{
using BigInt = boost::multiprecision::uint128_t;
using BigNanos = std::chrono::duration<BigInt, std::nano>;
std::cout << "Count for 1s: " << BigNanos(1s).count() << "\n";
std::cout << "Count for 300 years: " << BigNanos(std::chrono::years(300)).count() << "\n";
}
Count for 10s: 1000000000 Count for 300 years: 9467085600000000000
If you're willing to use gcc/clang builtins for 128-bit types you won't even need Boost or your own custom type to do basic arithmetic (but serializing the result may be difficult, not sure how gcc/clang handle that for custom 128-bit types).
barishnamazov•1mo ago
I was timestamping everything using steady_clock to strictly enforce the time limits, and then just applied a calculated offset to convert that to wall time for the "Submission Date" display. It worked in testing, but during a long stress-test session, my laptop did an NTP sync. The logs then showed submissions appearing to happen before the source code file was last modified, which confused the caching logic. I was essentially betting that system_clock was stable relative to steady_clock over the process lifetime.
I eventually refactored to exactly what the article suggests: use steady_clock strictly for the runtime enforcement (is duration < 2.0s?), but capture system_clock::now() explicitly at the submission boundary for the logs, rather than trying to do math on the steady timestamp.
Also, +1 for std::chrono::round in C++20. I’ve seen code where duration_cast was used to "round" execution times to milliseconds for display, but it was silently flooring them. In a competitive programming context, reporting 1000ms when the actual time was 1000.9ms is a misleading difference because the latter gets Time Limit Exceeded error. Explicit rounding makes the intent much clearer.