Time

• This is from chapter 22 of Gregorie, fifth edition.
  • And it is a summary at best.
  • And it is only the "timing" part of time, not the date and hour part.
• The auto type
  • This allows the compiler to deduce a type available at compile time.
  • It is useful in this setting where we have huge scoping names.
• Recall or learn
  • Since C++11, numbers can use ' in as commas
  • 1'000 is a legal value.
  • And it makes things much easier to read.
• All of the c++ functions are contained in the chronos library
  • So you need to include it.
• Recall (I hope) the basic time units appropriate for computing.
  • A second is a long time.
  • 60 seconds make a minute.
  • A second is divided into 1000 milliseconds (ms)
  • A millisecond is divided into 1000 microseconds (us)
  • A microsecond is divided into 1000 nanoseconds (ns)
  • A nanosecond is divided into 1000 picoseconds (ps).
• CPU Clocks run in 100's of pico seconds per cycle.
  • So it is not reasonable to measure time at that level.
  • Especially on consumer grade machines like we have.
  • So nanoseconds are probably our best hope for a clock.
  • Remember that could encompass 100's of instructions however.
• The ratio template
  • This is used to represent finite ratios.
  • These are useful when doing dimensional analysis
  • As expected this is two integer values (std::intmax_t)
  • Which can be signed.
  • The second (Denom) can not be 0
  • There are a large number of these declared
    • see the reference.
    • We will use mano, micro, milli
    • As well as ratio <1,1>
    • And ratio <60,1>
    • And perhaps ratio <3600,1>
  • They are automatically reduced to lowest terms.
  • The other components use these, so we need the idea.
    • But that is all, if you want more, read the chapter.
• A time piont is a value produced by a clock
• A duration is the difference between two time points.
  • And a ratio of which represents the time in seconds between two ticks.
  • Thus you can force a duration into any time unit you want using the ratio.
    • Give it 1:1 and it will report minutes.
    • 1:1000 or milli and it will report milliseconds.

    • // in this case diff is the difference between two time points.
      // just to be clear, declaring a duration or two.
      duration<double, ratio<60,1>>{diff}
      duration<double, milli>{diff}
                   

  • You can print a duration.
  • Durations have a method .count() which returns the integer part of a mixed number.
  • They support most math and comparison functions.
  • See This reference.
  • Probably time to start looking at timeDemo.cpp.
• Clocks
  • The standard defines several clocks
    • system_clock: Wall clock time. This time could change backward.
    • stedy_clock: Time will never decrease
    • high_resolution_clock: the shortest possible tick for the hardware
  • It is possible for these to all be the same.
  • now() is a member function that returns a time point.
  • You probably want the high_resolution_clock for this exercise.
• Finally a time point
  • reference
  • Another templated class.
  • Supports most of the math operators.
• I wanted something more easy to use.
  • Take a look at StopWatchT.h
  • Take a look at StopWatchT.cpp
  • Take a look at TimerTest.cpp
  • Note, when compiling with the -O* option, you need to make sure that results are used, or the compiler will remove the code.
    • And your program will run much faster, which is the point of the +0* optimizer.