1 00:00:01,340 --> 00:00:03,580 Unions in Cplusplus. 2 00:00:03,620 --> 00:00:11,640 So unions, as a cousin of the pod that puts all of its member in the same place. 3 00:00:11,660 --> 00:00:20,120 So you can think of unions as a different views or interpretations of a block of a memory so they can 4 00:00:20,120 --> 00:00:26,770 be useful in some low level situations, such as when marshalling structures that must be consistent 5 00:00:26,780 --> 00:00:27,560 across architectures. 6 00:00:27,560 --> 00:00:36,230 So dealing with the type checking issues related to C, Cplusplus, Interoperation and even packing 7 00:00:36,230 --> 00:00:44,210 Bitfields So let's create a union here, a union here, union variant here variant. 8 00:00:45,810 --> 00:00:49,410 As you can see here, this is this definition is same as struct. 9 00:00:49,410 --> 00:00:51,210 So in previous lecture we defined. 10 00:00:51,240 --> 00:00:53,670 Struct and create an example from it. 11 00:00:54,210 --> 00:00:57,240 And so let's create a character. 12 00:00:57,240 --> 00:01:06,150 My string here ten then integer my integer here. 13 00:01:07,640 --> 00:01:08,690 A double. 14 00:01:09,610 --> 00:01:12,310 Double my. 15 00:01:12,920 --> 00:01:15,100 My double, for example. 16 00:01:16,160 --> 00:01:27,920 So then the union here variant can be interpreted as my string ten an integer or a double. 17 00:01:28,160 --> 00:01:32,810 So it takes up only as much memory as it largest member. 18 00:01:32,810 --> 00:01:34,520 Probably a string here. 19 00:01:34,520 --> 00:01:36,590 My string in this case. 20 00:01:36,590 --> 00:01:43,400 So you can use that operator to specify a union's interpretation. 21 00:01:43,400 --> 00:01:51,170 Syntactically this looks like this looks like accessing a member of Pod, but it's completely different 22 00:01:51,170 --> 00:01:52,190 under the hood here. 23 00:01:52,190 --> 00:01:55,640 So let's create an variant here. 24 00:01:55,670 --> 00:01:57,260 Variant V. 25 00:01:59,040 --> 00:02:07,680 Here we that integer, my integer, for example, 32 and printf. 26 00:02:08,520 --> 00:02:13,130 The answer here answer. 27 00:02:14,030 --> 00:02:14,360 Here. 28 00:02:15,850 --> 00:02:18,670 We that integer. 29 00:02:18,700 --> 00:02:19,870 My integer. 30 00:02:19,870 --> 00:02:23,260 And let's define the that floating point here. 31 00:02:23,260 --> 00:02:26,320 We got my double. 32 00:02:26,350 --> 00:02:27,910 My double. 33 00:02:27,910 --> 00:02:28,600 Yes. 34 00:02:28,810 --> 00:02:39,100 And here two point or three point here this is the double and then print f. 35 00:02:42,550 --> 00:02:44,290 My double. 36 00:02:44,320 --> 00:02:46,900 Or let's make it my integer. 37 00:02:46,900 --> 00:02:50,050 My union integer. 38 00:02:51,070 --> 00:02:55,420 And my union double. 39 00:02:56,470 --> 00:02:58,540 Here or here? 40 00:02:59,560 --> 00:02:59,880 Here. 41 00:02:59,890 --> 00:03:03,670 My union double D and. 42 00:03:06,240 --> 00:03:07,450 We that. 43 00:03:10,240 --> 00:03:11,140 My double. 44 00:03:12,880 --> 00:03:14,650 And then here. 45 00:03:17,200 --> 00:03:17,830 Print test. 46 00:03:20,260 --> 00:03:23,620 My union integer. 47 00:03:25,250 --> 00:03:27,070 Uh, print here. 48 00:03:27,070 --> 00:03:29,620 My union integer two, for example. 49 00:03:30,190 --> 00:03:34,930 Here d v.my integer. 50 00:03:34,930 --> 00:03:43,120 Actually, we have we used the wrong format specifier here we have to use F okay, so. 51 00:03:45,640 --> 00:03:47,530 Let's run our program now. 52 00:03:47,860 --> 00:03:50,410 So let's run here. 53 00:03:51,810 --> 00:03:52,010 Oops. 54 00:03:52,260 --> 00:03:54,750 Actually, let's add the new lines here. 55 00:03:55,380 --> 00:03:56,110 Okay. 56 00:03:56,130 --> 00:03:57,300 New line. 57 00:03:59,500 --> 00:03:59,980 Oops. 58 00:04:04,280 --> 00:04:05,660 New line and new line. 59 00:04:06,580 --> 00:04:07,300 Here. 60 00:04:08,860 --> 00:04:13,260 As you can see, this is the weird answer we got here. 61 00:04:13,270 --> 00:04:17,230 So firstly we declared variant V. 62 00:04:17,800 --> 00:04:26,830 Next we interpret we as an integer so and set its value to 32 here. 63 00:04:27,700 --> 00:04:29,290 And then print it. 64 00:04:30,260 --> 00:04:34,460 So you hear then reinterpret the. 65 00:04:35,410 --> 00:04:40,660 As a float and reassign its value here. 66 00:04:43,540 --> 00:04:52,900 So you printed the the to the console and it appears well as you as we've righted it so far so good 67 00:04:52,900 --> 00:04:53,200 here. 68 00:04:53,200 --> 00:05:01,900 So disaster strikes only when you try to interrupt V as an integer again here. 69 00:05:02,630 --> 00:05:07,790 As you can see, our integer is 32 and this gave us some numbers. 70 00:05:08,000 --> 00:05:09,080 So. 71 00:05:10,200 --> 00:05:15,330 When were you clobbered over the original value of V 32? 72 00:05:15,360 --> 00:05:26,250 So when assigning this my double value so we can use this printf here before my double he or let's two 73 00:05:26,250 --> 00:05:28,860 before my double. 74 00:05:29,670 --> 00:05:34,230 Here and then after my double. 75 00:05:37,180 --> 00:05:42,820 As you can see here before, my double is almost everything is completely fine. 76 00:05:42,820 --> 00:05:46,840 We got the same number, we printed the same number we assign. 77 00:05:46,840 --> 00:05:51,190 And after that we just got a completely different value. 78 00:05:52,230 --> 00:05:53,610 And disaster strike. 79 00:05:53,850 --> 00:05:56,340 So that's the main problem with unions. 80 00:05:56,370 --> 00:06:02,940 It's up to you to keep track of which interpretation is appropriate so the compiler won't help you. 81 00:06:03,090 --> 00:06:10,830 You should avoid using unions in all but the rarest of cases, and you won't see them in this course 82 00:06:10,830 --> 00:06:11,400 here. 83 00:06:12,750 --> 00:06:21,510 Maybe in later lectures because unions has a very narrow use case in C plus plus just the daily programming. 84 00:06:26,070 --> 00:06:26,700 Here.