C++ chrono - get duration as float or long long

Question

I have a duration

typedef std::chrono::high_resolution_clock Clock;
Clock::time_point       beginTime;
Clock::time_point       endTime;
auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(endTime - beginTime);

And I get duration in std::chrono::milliseconds. But I need duration as float or long long. How to do that?

Answer 1

From the documentation

template<
    class Rep, 
    class Period = std::ratio<1> 
> class duration;

Class template std::chrono::duration represents a time interval. It consists of a count of ticks of type Rep and a tick period, where the tick period is a compile-time rational constant representing the number of seconds from one tick to the next.

And:

count returns the count of ticks

So a duration stores a number of ticks of a specified period of time, and count will return that number using the underlying representation type. So if the duration's representation is long long, and the period is std::milli, then .count() will return a long long equal to the number of milliseconds represented by the duration.

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In general you should avoid using weak types like float or long long to represent a duration. Instead you should stick with 'rich' types, such as std::chrono::milliseconds or an appropriate specialization of std::chrono::duration. These types aid correct usage and readability, and help prevent mistakes via type checking.

• Underspecified / overly general:
  – void increase_speed(double);
  – Object obj; … obj.draw();
  – Rectangle(int,int,int,int);

• Better: – void increase_speed(Speed);
  – Shape& s; … s.draw();
  – Rectangle(Point top_left, Point bottom_right);
  – Rectangle(Point top_left, Box_hw b);

— slide 18 from Bjarne's talk

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std::chrono is "a consistent subset of a physical quantities library that handles only units of time and only those units of time with exponents equal to 0 and 1."

If you need to work with quantities of time you should take advantage of this library, or one that provides more complete unit systems, such as boost::units.

There are rare occasions where quantities must be degraded to weakly typed values. For example, when one must use an API that requires such types. Otherwise it should be avoided.