Chrono in C++ - GeeksforGeeks

29 May, 2017
Chrono library is used to deal with date and time. This library was designed to deal with the fact that timers and clocks might be different on different systems and thus to improve over time in terms of precision. The unique thing about chrono is that it provides a precision-neutral concept by separating duration and point of time (“timepoint”) from specific clocks.
chrono is the name of a header and also of a sub-namespace: All the elements in this header (except for the common_type specializations) are not defined directly under the std namespace (like most of the standard library) but under the std::chrono namespace.The elements in this header deal with time. This is done mainly by means of three concepts:
Duration
A duration object expresses a time span by means of a count like a minute, two hours, or ten milliseconds. For example, “42 seconds” could be represented by a duration consisting of 42 ticks of a 1-second time unit.
// C++ program to illustrate the utility // function duration::count#include <iostream> #include <chrono>      int main (){    using namespace std::chrono;    // std::chrono::milliseconds is an     // instantiation of std::chrono::duration:- 1 second    milliseconds mil(1000);           mil = mil*60;          std::cout << "duration (in periods): ";    std::cout << mil.count() << " milliseconds.\n";          std::cout << "duration (in seconds): ";    std::cout << (mil.count() * milliseconds::period::num /                                milliseconds::period::den);    std::cout << " seconds.\n";      return 0;}
Output:
duration (in periods): 60000 milliseconds.
duration (in seconds): 60 seconds.

Clock
A clock consists of a starting point (or epoch) and a tick rate. For example, a clock may have an epoch of February 22, 1996 and tick every second. C++ defines three clock types:
system_clock-It is the current time according to the system (regular clock which we see on the toolbar of the computer). It is written as- std::chrono::system_clock
steady_clock-It is a monotonic clock that will never be adjusted.It goes at a uniform rate. It is written as- std::chrono::steady_clock
high_resolution_clock– It provides the smallest possible tick period. It is written as-std::chrono::high_resolution_clock
Time point
A time_point object expresses a point in time relative to a clock’s epoch. Internally, the object stores an object of a duration type, and uses the Clock type as a reference for its epoch.
// C++ program to illustrate time point// and system clock functions#include <iostream>#include <chrono>#include <ctime>  // Function to calculate// Fibonacci serieslong fibonacci(unsigned n){    if (n < 2) return n;    return fibonacci(n-1) + fibonacci(n-2);}  int main(){    // Using time point and system_clock    std::chrono::time_point<std::chrono::system_clock> start, end;      start = std::chrono::system_clock::now();    std::cout << "f(42) = " << fibonacci(42) << '\n';    end = std::chrono::system_clock::now();      std::chrono::duration<double> elapsed_seconds = end - start;    std::time_t end_time = std::chrono::system_clock::to_time_t(end);      std::cout << "finished computation at " << std::ctime(&end_time)              << "elapsed time: " << elapsed_seconds.count() << "s\n";}
Output:
f(42) = 267914296
finished computation at Wed Jan  4 05:13:48 2017
elapsed time: 2.14538s

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