WEBVTT 00:00.090 --> 00:00.630 Hello again! 00:00.870 --> 00:03.750 In this video, we are going to look at clocks and time points. 00:05.780 --> 00:08.120 The chrono library provides three clocks. 00:08.630 --> 00:13.610 The system clock is the one that is provided by the system hardware, through the operating system. 00:14.120 --> 00:20.330 This measures the so-called "wall time". So that expression goes back to the days when every office, 00:20.330 --> 00:23.390 factory, laboratory, and so on, had a clock on the wall. 00:25.090 --> 00:27.940 There is a steady clock which is provided in software. 00:28.330 --> 00:32.530 This is an ideal clock which can only go forwards, one tick at a time. 00:33.190 --> 00:37.960 You might think that is how a system clock is supposed to work. But, in fact, it is not quite as simple 00:37.960 --> 00:38.380 as that. 00:41.380 --> 00:43.750 There is also a high resolution clock. 00:44.170 --> 00:48.520 This is guaranteed to support the shortest possible tick period on that system. 00:49.330 --> 00:55.720 How it does that, is implementation is defined. And usually library will make life easy for themselves. 00:56.350 --> 00:59.950 So they implement one of these two, using the highest possible precision. 01:00.430 --> 01:02.770 And then the high-resolution clock is just an alias. 01:03.520 --> 01:07.180 So in fact, it is not very useful, because you do not know which one you are going to get. 01:11.460 --> 01:15.180 The system clock is similar to the one which comes with the C library. 01:15.630 --> 01:19.140 This will measure the "wall time", using the operating system clock. 01:19.980 --> 01:26.460 So if the computer is correctly synchronized, this will be the same time as measured by calendars, 01:26.460 --> 01:28.110 watches, phones and so on. 01:29.190 --> 01:35.130 So this makes it the best clock for interactive use, because it corresponds to what the user thinks the 01:35.130 --> 01:35.610 time is. 01:38.180 --> 01:42.470 However, it's not good for measuring time intervals, because the clock can be changed. 01:43.160 --> 01:49.730 For example, if the clock runs fast or is running slow, then that could be corrected. Either by the computer 01:49.730 --> 01:52.490 operator, or by time synchronization software. 01:53.390 --> 01:58.070 Every now and again, the time changes, because of daylight savings or leap seconds. 01:58.790 --> 02:03.680 So it is possible that if you are in the middle of measuring an interval, the clock could jump forwards. 02:03.920 --> 02:04.730 Or even backwards! 02:05.450 --> 02:07.490 So you get completely the wrong result. 02:10.370 --> 02:15.650 So the best clock for measuring time intervals is the steady clock, because this can only go forwards, 02:16.010 --> 02:18.560 and it can only go up by one tick at a time. 02:18.950 --> 02:24.020 So this is a bit like the program having its own internal stopwatch, and this will tick away constantly, 02:24.320 --> 02:27.860 regardless of what the time is doing in the outside world. 02:30.730 --> 02:31.990 To use these clocks, 02:31.990 --> 02:38.740 we have a static function called now(). And this will return the current timepoint, as seen by that clock. 02:39.280 --> 02:43.150 So system_clock::now() for the hardware clock, and steady_clock:: 02:43.150 --> 02:44.950 now() for the steady clock. 02:48.090 --> 02:51.060 The time_point class represents a point in time. 02:51.600 --> 02:58.410 It has a member, which is a duration, and that duration corresponds to the time interval since the 02:58.410 --> 03:01.350 clock's epoch. Since the clock started ticking. 03:01.830 --> 03:04.650 So for the C library clock, that would be 1970. 03:06.380 --> 03:08.950 A time_point does not exist in isolation. 03:08.960 --> 03:11.000 It is associated with a particular clock. 03:11.750 --> 03:16.310 And to get a time_point, we call the now() function for that clock. 03:18.830 --> 03:22.730 We can subtract 2 time_points, and that will give us a duration. 03:22.820 --> 03:29.260 So that will be the interval between those two time points. And we can also add a time_point and a duration 03:29.270 --> 03:31.010 together, or subtract them. 03:31.250 --> 03:33.290 So that will give us a new time_point. 03:37.040 --> 03:42.710 As an example, I am going to measure how long it takes to calculate a number in the Fibonacci sequence. 03:43.610 --> 03:48.350 I am going to call the now() function just before the calculation, and just afterwards. 03:48.830 --> 03:53.330 So that will give me the starting time_point and the finishing time_point for the calculation. 03:54.720 --> 03:59.430 I'm using the steady_clock, because that is the best one to use for measuring intervals. 04:00.510 --> 04:03.240 The Fibonacci calculation, I am sure you're all familiar with. 04:04.830 --> 04:05.850 The number to use. 04:05.850 --> 04:07.410 You may need to tune this a bit. 04:07.890 --> 04:14.820 If I am running on the command line in release mode, then 45 is about right. For Visual Studio in debug mode, 04:15.540 --> 04:19.920 that takes a lot longer. Because, of course, it has to support all the debugging functionality. 04:20.220 --> 04:22.170 So the number 40 works better for me. 04:23.730 --> 04:27.210 And then, I subtract the start time_point from the finish. 04:27.630 --> 04:31.170 So that will give me the duration, how long it took for the calculation. 04:32.610 --> 04:35.210 Then I want to get the result in milliseconds. 04:35.220 --> 04:41.220 So I do a duration_cast with milliseconds as the parameter, and this duration as the argument. 04:41.850 --> 04:46.710 And then I call the count() member function of the duration, to get the number of milliseconds. 04:47.370 --> 04:48.540 So let's see what we get. 04:51.860 --> 04:54.970 This should take about 5 seconds or maybe six. 04:54.980 --> 04:56.780 So about now, I think. 04:57.530 --> 04:58.010 And there we go. 04:58.490 --> 05:00.140 So that is the Fibonacci number. 05:00.710 --> 05:04.850 And the time taken was 5463 milliseconds. 05:07.140 --> 05:09.390 And, finally, the sleep_for() function. 05:09.840 --> 05:13.560 I have mentioned this a couple of times, so perhaps we should cover it, just to make sure. 05:14.250 --> 05:21.360 So, in C++11, it is now possible to make a program, or a thread, sleep for a specified duration. 05:24.640 --> 05:26.620 This came in with the thread library. 05:27.550 --> 05:29.500 There is a static function. 05:29.950 --> 05:34.690 this_thread::sleep_for(). And this will apply to the currently executing thread. 05:35.290 --> 05:39.670 And this thread will pause, or "sleep", for the duration of its argument. 05:41.350 --> 05:46.120 If you are using a single-threaded program, this will make the main thread sleep. 05:47.920 --> 05:49.630 The duration is only a minimum. 05:49.780 --> 05:53.350 What happens, is that this program, or the thread, will stop executing. 05:53.740 --> 05:59.620 So the operating system scheduler or the thread scheduler will start doing other things. And it will 05:59.620 --> 06:02.680 not come back to this, until all those other things are finished. 06:02.950 --> 06:05.110 And the scheduler thinks that you were due for another turn. 06:05.620 --> 06:08.470 So it is possible that the delay may be more 2 seconds. 06:10.990 --> 06:14.260 So let's quickly try that out. "Waiter?" 06:15.270 --> 06:15.830 "You called, sir?" 06:16.380 --> 06:16.710 "Yes! 06:16.710 --> 06:17.700 There is a fly in my soup!" 06:18.600 --> 06:19.350 "Do not say that, sir." 06:19.380 --> 06:19.890 "They will all want 06:19.890 --> 06:20.130 one!" 06:21.180 --> 06:21.480 Okay. 06:21.480 --> 06:22.560 So that is it for this video! 06:22.980 --> 06:23.820 I will see you next time. 06:23.820 --> 06:25.980 But until then, keep coding!