WEBVTT 00:00.210 --> 00:04.410 Hello again! In this video, we are going to look at which operators you should overload. 00:06.080 --> 00:10.640 In principle, you can overload almost any operator that is provided by the language, but there are 00:10.640 --> 00:12.410 some which are more useful than others. 00:13.670 --> 00:20.360 The typical textbook or course or class example is to use the arithmetic operators, usually the plus 00:20.360 --> 00:25.310 operator. Which is quite a good example, but it is only really useful for mathematical classes. 00:26.330 --> 00:29.480 C++ now has complex and rational number classes. 00:29.510 --> 00:36.280 So that is maybe not something you are going to need to use, to do very often. For general purpose programming, 00:36.290 --> 00:42.230 the most useful operators are the assignment operator, which we have talked about before. The equality 00:42.230 --> 00:49.910 and inequality operators, for testing whether two objects have the same value. The less than operator 00:49.910 --> 00:51.050 for doing comparisons. 00:51.410 --> 00:56.960 You can actually do any comparison using just one operator, and the less than operator is the one that is 00:56.960 --> 00:58.100 used in C++. 00:59.000 --> 01:04.420 And then finally, this rather mysterious one, the function call operator. Which we will talk about 01:04.460 --> 01:05.570 later on in this section. 01:09.490 --> 01:17.140 There are some operators which you should not overload or cannot overload. The logical "and" and "or" operators. 01:17.500 --> 01:20.200 The built-in versions of these have special properties. 01:20.560 --> 01:26.410 So if you have something like "a" and "b" and "c", they start executing from left to right. 01:26.470 --> 01:30.850 So first it evaluates a. And they have a "short-circuit" evaluation. 01:30.850 --> 01:35.680 So if, for example, a is true and b is false, then it will stop and return false. 01:36.160 --> 01:37.270 It will not evaluate c. 01:37.270 --> 01:42.190 You can overload these operators, but they will not have those properties. 01:42.670 --> 01:45.100 So it is probably not something that is useful. 01:48.550 --> 01:55.840 You can also overload the address-of operator and the comma operator, which is used for writing several 01:55.840 --> 01:57.340 expressions in a single statement. 01:59.170 --> 02:03.580 These are already defined for classes and if you redefine them, you are just going to confuse things. 02:05.320 --> 02:12.700 And some that you are not allowed to overload: the scope operator - the double colon for namespaces or 02:13.030 --> 02:21.610 class members - the dot operator for accessing struct and class members, the dot star operator for member 02:21.620 --> 02:27.130 function pointers, and the ternary operator. And you are not allowed to overload those. 02:31.730 --> 02:37.130 In general, you should think carefully before you add operators to a class. Only provides operators 02:37.130 --> 02:39.490 which are actually needed or add value to the class. 02:39.500 --> 02:43.070 So do not go mad and you write lots of overloads if you do not actually need them! 02:43.850 --> 02:45.710 If you are writing a database connector class, 02:45.710 --> 02:47.840 does it really need to be able to do arithmetic? 02:49.400 --> 02:54.800 The return value from the operator should correspond to the equivalent in the language for built-in 02:54.800 --> 02:56.720 or library class types. 02:58.640 --> 03:02.600 If you have a logical or relational operator, it should return bool. 03:03.110 --> 03:09.380 If it is an arithmetic operator, it should return the class type by value. And if it is an operator which 03:09.380 --> 03:10.560 performs an assignment. 03:10.580 --> 03:17.270 So something like plus equals, for example. It should return the object on the left of the operation 03:17.690 --> 03:20.540 by reference, which is not a const reference. 03:21.890 --> 03:26.330 And if you do all these, your operators will be consistent with the ones in the language and they will 03:26.330 --> 03:28.340 be easier to use and less confusing. 03:29.360 --> 03:30.860 OK, so that is it for this video. 03:31.340 --> 03:34.310 I'll see you next time, but until then, keep coding!