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std::chrono::duration_cast(3) C++ Standard Libarystd::chrono::duration_cast(3)

NAME
       std::chrono::duration_cast - std::chrono::duration_cast

Synopsis
	  Defined in header <chrono>
	  template <class ToDuration, class Rep, class Period>
	  constexpr		  ToDuration		   duration_cast(const
       (since C++11)
	  std::chrono::duration<Rep,Period>& d);

	  Converts a std::chrono::duration to a	duration of different type To-
       Duration.

	  The function does not	participate in overload	resolution unless  To-
       Duration	is a
	  specialization of std::chrono::duration.

	  No  implicit conversions are used. Multiplications and divisions are
       avoided where
	  possible, if it is known at compile time that	one or more parameters
       are 1.
	  Computations are done	in the widest type available and converted, as
       if by
	  static_cast, to the result type only when finished.

Parameters
	  d - duration to convert

Return value
	  d converted to a duration of type ToDuration.

Notes
	  Casting between integer durations where the source period is exactly
       divisible by
	  the target period (e.g. hours	to minutes) or between	floating-point
       durations can be
	  performed  with  ordinary casts or implicitly	via std::chrono::dura-
       tion constructors,
	  no duration_cast is needed.

	  Casting from a floating-point	duration to  an	 integer  duration  is
       subject to
	  undefined  behavior  when the	floating-point value is	NaN, infinity,
       or too large to
	  be representable by the target's integer type. Otherwise, casting to
       an integer
	  duration is subject to truncation as with any	static_cast to an  in-
       teger type.

Example
	  This example measures	the execution time of a	function

       // Run this code

	#include <iostream>
	#include <chrono>
	#include <ratio>
	#include <thread>

	void f()
	{
	    std::this_thread::sleep_for(std::chrono::seconds(1));
	}

	int main()
	{
	    auto t1 = std::chrono::high_resolution_clock::now();
	    f();
	    auto t2 = std::chrono::high_resolution_clock::now();

	    // floating-point duration:	no duration_cast needed
	    std::chrono::duration<double, std::milli> fp_ms = t2 - t1;

	    // integral	duration: requires duration_cast
	    auto  int_ms  =  std::chrono::duration_cast<std::chrono::millisec-
       onds>(t2	- t1);

	    // converting integral duration to integral	 duration  of  shorter
       divisible time unit:
	    // no duration_cast	needed
	    std::chrono::duration<long,	std::micro> int_usec = int_ms;

	    std::cout << "f() took " <<	fp_ms.count() << " ms, "
		      << "or " << int_ms.count() << " whole milliseconds "
		      << "(which is " << int_usec.count() << " whole microsec-
       onds)" << std::endl;
	}

Possible output:
	f()  took  1000.23  ms,	 or  1000 whole	milliseconds (which is 1000000
       whole microseconds)

See also
	  duration		       a time interval
	  (C++11)		       (class template)
	  time_point_cast	       converts	a time point to	 another  time
       point on	the same
	  (C++11)		       clock, with a different duration
				       (function template)
	  floor(std::chrono::duration)	converts a duration to another,	round-
       ing down
	  (C++17)		       (function template)
	  ceil(std::chrono::duration)  converts	a duration to another,	round-
       ing up
	  (C++17)		       (function template)
	  round(std::chrono::duration)	converts a duration to another,	round-
       ing to nearest,
	  (C++17)		       ties to even
				       (function template)

http://cppreference.com		  2022.07.31	 std::chrono::duration_cast(3)

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