WEBVTT 00:00.120 --> 00:00.690 Hello again! 00:00.960 --> 00:04.020 In this video, we are going to start looking at the chrono library. 00:04.350 --> 00:06.840 This is to do with handling times and dates. 00:09.150 --> 00:13.230 C++ already had some features for doing this, which were inherited from C. 00:13.890 --> 00:17.790 These are defined in the header. As you would expect for c, 00:17.790 --> 00:21.750 these are low-level and, to be honest, they are not very well designed. 00:22.830 --> 00:26.970 C++11 introduced the chrono library for handling times. 00:27.780 --> 00:34.830 This is much more precise than the old C stuff, but the syntax is more complicated. And now, in 00:34.830 --> 00:35.250 C++20, 00:35.610 --> 00:36.450 the chrono library 00:36.450 --> 00:38.040 can also handle dates as well. 00:38.700 --> 00:44.550 I am not going to go into that, because the compiler support for C++20 is still not yet all there. 00:46.970 --> 00:51.380 Let's remind ourselves how the traditional C++ time support works. 00:52.040 --> 00:55.730 There is a clock() function, which returns the number of clock ticks 00:56.060 --> 01:02.690 since the program started. These are stored in a variable of type clock_t, which is just a synonym 01:02.690 --> 01:03.380 for integer. 01:05.090 --> 01:07.400 These can be used for intervals of up to a few minutes. 01:07.940 --> 01:13.940 The precision depends on the implementation, but usually it is to the nearest microsecond. And you do not 01:13.940 --> 01:15.740 get the number of seconds directly. 01:15.740 --> 01:17.810 You have to divide by a constant to get that. 01:19.920 --> 01:23.790 The time() function takes a variable of type time_t, by address. 01:24.450 --> 01:26.940 Again, that is a synonym for an integer. 01:27.720 --> 01:32.640 It will set the argument to the number of seconds since midnight on the 1st of January 1970. 01:33.150 --> 01:34.740 It was obviously a very good new year for somebody! 01:36.000 --> 01:37.660 This can be used for longer intervals 01:37.660 --> 01:39.930 than clock(), and you can go up to several decades. 01:40.320 --> 01:43.170 But the precision is only to the nearest second. 01:46.400 --> 01:50.050 C++11 provides a new library for managing times. 01:50.570 --> 01:56.630 This is defined in the header, in the standard chrono namespace. So this actually has its own namespace, 01:57.080 --> 01:58.880 underneath the standard namespace. 01:59.630 --> 02:01.400 And there are three main concepts. 02:04.850 --> 02:06.860 The first concept is of a clock. 02:07.250 --> 02:11.990 This is something which has a start date, known as the "epoch", and a "tick rate". 02:12.590 --> 02:17.330 So in this terminology, C has a clock which started on the 1st of January 1970. 02:17.750 --> 02:19.610 So that is the epoch of that clock. 02:20.270 --> 02:22.220 And the tick rate is once per second. 02:23.090 --> 02:28.970 We have a time point, which is the number of clock ticks since the start of the epoch, at a given point 02:28.970 --> 02:29.450 in time. 02:29.930 --> 02:35.900 So that corresponds to the return value from calling the time() function in C. And we have a duration, 02:36.230 --> 02:39.890 which is the interval between two time points, measured in clock ticks. 02:40.400 --> 02:45.380 So that is equivalent to calling the C function time() twice and subtracting the return values. 02:46.550 --> 02:47.930 Okay, so that is it for this video. 02:48.260 --> 02:49.070 I will see you next time. 02:49.070 --> 02:51.080 But until then, keep coding!