std::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 ToDuration.
   The function does not participate in overload resolution unless ToDuration 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::duration 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 integer type.

Example
   This example measures the execution time of a function. // Run this code
 #include <chrono>
 #include <iostream>
 #include <ratio>
 #include <thread>
 void f()
 {
     std::this_thread::sleep_for(std::chrono::seconds(1));
 }
 int main()
 {
     const auto t1 = std::chrono::high_resolution_clock::now();
     f();
     const auto t2 = std::chrono::high_resolution_clock::now();
     // floating-point duration: no duration_cast needed
     const std::chrono::duration<double, std::milli> fp_ms = t2 - t1;
     // integral duration: requires duration_cast
     const auto int_ms = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
     // converting integral duration to integral duration of
     // shorter divisible time unit: no duration_cast needed
     const std::chrono::duration<long, std::micro> int_usec = int_ms;
     std::cout << "f() took " << fp_ms << ", or "
               << int_ms << " (whole milliseconds), or "
               << int_usec << " (whole microseconds)\n";
 }

Possible output:
 f() took 1000.14ms, or 1000ms (whole milliseconds), or 1000000us (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, rounding down
   (C++17) (function template)
   ceil(std::chrono::duration) converts a duration to another, rounding up
   (C++17) (function template)
   round(std::chrono::duration) converts a duration to another, rounding to nearest,
   (C++17) ties to even
                        (function template)