WEBVTT 00:00.120 --> 00:07.020 Hello again! In this video, we are going to look at member and non-member operators. When we are writing 00:07.020 --> 00:08.250 an overloaded operator. 00:08.490 --> 00:13.290 We can usually implement it either way: as a member function or a non-member function. 00:14.910 --> 00:17.490 If you can, you should implement it as a member function. 00:18.000 --> 00:23.460 This will provide direct access to the private data. It is a member function, so we do not need any friends 00:23.460 --> 00:26.160 or delegate member functions. 00:26.760 --> 00:31.860 And also, it keeps all the code that is related to the class together, in the class definition. 00:34.680 --> 00:40.980 There are some operators which work better as non-member functions. And there are a few which cannot be implemented 00:40.980 --> 00:42.180 as member functions at all. 00:46.540 --> 00:52.060 The problem with having operators as member functions arises with binary operators. If we 00:52.060 --> 00:56.050 want a type conversion - actually, let's look at this in the IDE. 00:58.090 --> 00:59.860 So here is our String class again, 00:59.860 --> 01:06.460 with a capital 'S'. We have briefly gone back to using the library string as the member, to keep things simple. 01:07.420 --> 01:12.220 I have added a constructor which takes a C-style string, because we are going to use one of those. 01:13.780 --> 01:16.960 We have a plus operator as a member function. 01:17.380 --> 01:24.320 This will take another object of the same class. And it will add the library string member to our own library string 01:24.340 --> 01:26.080 member, and return the result. 01:27.100 --> 01:30.760 We have the member function for printing out the member. 01:32.610 --> 01:39.540 And in our main() function, we create a couple of objects of this class. And then we try out our new operator. 01:40.440 --> 01:43.830 So if both the arguments are objects of our class, it works fine. 01:44.610 --> 01:51.950 This is actually called as a member function call. So this will call the member function on "w", operator plus, with 01:51.950 --> 01:53.040 "bang" as the argument. 01:54.570 --> 01:59.070 So this will add the library string inside "w" to the library string inside "bang". 02:01.330 --> 02:07.570 And that all works. If we have a string object with data "world" and one with an exclamation mark, then we get 02:08.200 --> 02:09.430 "world", exclamation mark. 02:14.300 --> 02:21.170 The problem arises is if we want to use a string conversion. We have a constructor which can convert 02:21.620 --> 02:24.800 C-style string into one of our Strings with a capital 'S'. 02:26.750 --> 02:32.930 The problem is, we only have a member function. And you cannot call a member function on a C-style string. 02:33.890 --> 02:35.920 So this is going to give a compiler error. 02:38.810 --> 02:46.190 There we are. No global operator found. So the compiler could not work out how to call this member function 02:46.190 --> 02:52.390 from the C-style string, and it looked for a non-member operator plus. But, because we implemented 02:52.400 --> 02:56.900 the operator as a member function, there is no suitable non-member function. 02:57.950 --> 02:59.330 So that is the problem that can arise. 03:04.000 --> 03:10.000 If we use a non-member function, so the operator plus is defined outside the class, then it all works. 03:11.440 --> 03:17.470 The call to the plus operator is the call to a non-member function, with arguments "w" and "bang", and the 03:17.470 --> 03:20.920 compiler will convert function arguments if it can. 03:21.370 --> 03:27.790 So in this case, it is going to convert the C-style string "Hello" to a String with a capital 'S', by 03:27.790 --> 03:31.720 calling that constructor, and then "w" will be the second argument. 03:34.470 --> 03:38.730 So that does all work. And here is that code. 03:39.060 --> 03:42.020 So now we have the operator plus outside the class. 03:43.390 --> 03:46.440 I have made the operator a friend, here. 03:47.970 --> 03:52.260 If you like, you can try making it a member function and having this one delegating to it. 03:54.160 --> 03:56.800 And then the main() function is the same as before. 03:57.310 --> 03:58.390 So let's see if this works. 04:00.190 --> 04:00.700 And there we are. 04:01.060 --> 04:04.510 So now we can add "hello" and "world" to get "hello world". 04:07.020 --> 04:13.440 So when should you use member and non-member operators? If you have an operator which changes the state 04:13.440 --> 04:16.650 of the object, that should be implemented as a member function. 04:17.190 --> 04:23.490 So things like plus equals, compound assignments. And also increment and decrement operators. 04:25.440 --> 04:31.170 If you have an operator which is closely bound to the type of a member. So something like the dereference 04:31.200 --> 04:35.580 operator, which will return an object of the same type as the member. 04:39.760 --> 04:43.160 There are some operators which can only be defined as member functions. 04:43.180 --> 04:46.870 So that is the assignment operator. The subscript operator. 04:47.350 --> 04:54.130 So this will take an argument which is an integer, and it will return some element within your object. 04:55.210 --> 05:02.320 The mysterious function call operator again. And the arrow operator, crow's foot, whatever you want to call it. 05:03.160 --> 05:04.600 These have to be member functions. 05:07.850 --> 05:13.280 If you have a binary operator, which might require a type conversion, then you should implement that 05:13.280 --> 05:14.540 as a non-member function. 05:14.900 --> 05:16.700 So we saw that with the plus operator. 05:17.120 --> 05:19.130 And that applies to all arithmetic operators. 05:20.000 --> 05:25.640 Also the equality and relational operators. And the bitwise operators. 05:27.470 --> 05:31.630 And that includes the bitwise operators which we use for input and output. 05:31.640 --> 05:33.260 So, that is, left shift and right shift. 05:34.190 --> 05:39.110 These should be non-member functions, so they are compatible with all the built-in types and the library 05:39.110 --> 05:39.530 types. 05:41.300 --> 05:43.130 Okay, so that is it for this video. 05:43.580 --> 05:46.670 I will see you next time, but until then, keep coding!