WEBVTT 00:00.120 --> 00:00.690 Hello again! 00:01.080 --> 00:04.050 In this video, we are going to look at constexpr functions. 00:06.060 --> 00:09.790 In C++11, we can say that a function is constexpr. 00:10.470 --> 00:16.290 This means that the arguments are constant expressions, and the return value is also a constant expression. 00:17.430 --> 00:21.420 These constexpr functions are executed at compile time. 00:21.930 --> 00:24.960 This uses an interpreter which runs inside the compiler. 00:25.290 --> 00:29.010 So this is something which understands most of the C++ language. 00:29.820 --> 00:35.220 So you can write this code in normal C++, and you get normal C++ error messages. 00:37.410 --> 00:41.650 To do this, we put the constexpr key word before the function declaration. 00:42.300 --> 00:52.030 So this function miles_to_km() is constexpr. A constexpr function must be what is known as "pure". 00:52.770 --> 00:59.430 So this means it does not modify the arguments. And also, it does not modify any global or static variables. 01:00.090 --> 01:04.200 So a pure function is not allowed to change anything outside the function body. 01:06.290 --> 01:12.110 constexpr functions are in line, by definition. Because the function call is omitted. 01:12.620 --> 01:18.890 And this also means we can have multiple definitions, so we can include a constexpr function definition 01:19.250 --> 01:24.440 in a header. And we can include that header in as many source files as we like. And we are not going 01:24.440 --> 01:25.010 to get any 01:25.010 --> 01:27.590 errors about violating the "one definition rule". 01:28.580 --> 01:33.890 So this means you can use constexpr functions in header-only libraries. And this is actually a very 01:33.890 --> 01:38.480 common practice now, with C++, to distribute libraries as headers only. 01:39.080 --> 01:44.390 So you can just include these into your program, and you do not need to worry about compiling source files 01:44.720 --> 01:46.400 and linking them into your binary. 01:49.400 --> 01:55.550 It is also possible to call a constexpr function with arguments which are not constant expressions. And 01:55.550 --> 01:58.860 that means that the return value will not be a constant expression. 01:59.900 --> 02:05.390 If you are using this in a situation where the return value is supposed to be a constant expression, 02:06.050 --> 02:07.340 then you get a compiler ever. 02:08.090 --> 02:11.780 But if it is not, then the function will be evaluated at runtime. 02:12.050 --> 02:14.840 So that will work just like any other function in C++. 02:15.830 --> 02:20.270 And the reason for allowing this, is so you can have a function which does the same thing for constant 02:20.270 --> 02:25.400 expressions and variables, without having to write two almost identical versions. 02:28.150 --> 02:31.630 So here is our constexpr function, miles_to_km(). 02:32.410 --> 02:34.990 So we can call this with a constant expression as argument. 02:35.680 --> 02:38.140 The return value will be a constant expression. 02:39.400 --> 02:43.930 We can call it with an argument, which is a variable. So not a constant expression, because it 02:43.930 --> 02:48.880 can be modified. And the return value will not be a constant expression. 02:49.720 --> 02:52.180 And this is going to be evaluated at runtime. 02:54.290 --> 02:59.150 I have put a dummy main() function in here, just so we do not get any annoying errors about not having a 02:59.150 --> 02:59.810 main() function. 03:01.310 --> 03:03.080 So there we are. That compiles. 03:04.130 --> 03:10.700 However, if we have a... If we call this function with an argument which is not a constant expression, 03:11.630 --> 03:15.080 and the return value is supposed to be a constant expression. 03:15.470 --> 03:21.500 So we are using this to initialize a constexpr variable. Which has to be initialized from a constant expression 03:22.370 --> 03:24.080 Then this is going to give a compiler error. 03:27.890 --> 03:29.640 So let's look at that. "Expression 03:29.640 --> 03:31.640 "did not evaluate to a constant. 03:32.240 --> 03:37.610 "Failure was caused by non-constant argument... See usage of 'arg'". 03:38.120 --> 03:40.010 So that is an extremely clear error message. 03:40.250 --> 03:46.010 It tells you what the problem is, where it is, and why there is an error. So, ten out of ten to Microsoft 03:46.010 --> 03:50.630 for that! Just imagine what that would have been, if this was a template function! 03:53.980 --> 03:58.120 constexpr functions are useful for performing calculations at compile time. 03:58.780 --> 04:04.210 So this means the calculation is only done once. Or, at least, only once for every time you compile the 04:04.210 --> 04:10.660 program. As opposed to every time that the program is run, by every user, on every site where it is installed. 04:12.310 --> 04:13.810 There is no runtime overhead. 04:13.810 --> 04:19.030 It takes no time at all to do this calculation, because it has been done before the program starts. And this 04:19.030 --> 04:21.410 means your program will use less electricity. 04:22.780 --> 04:26.020 In C++11, the rules were very limiting. 04:26.320 --> 04:30.490 You could only have a single return statement in the body of a constexpr function. 04:30.910 --> 04:36.970 So that is similar to the C++11 rules for lambda expressions. In C++14, 04:37.330 --> 04:43.120 these were relaxed quite a bit. As they were for lambda expressions. And now you can have multiple statements 04:43.120 --> 04:45.340 in the body of a constexpr function. 04:46.120 --> 04:51.940 You can also declare local variables in the function body, provided they are initialized, and not static. 04:52.810 --> 04:54.370 And you can use flow control. 04:54.700 --> 04:59.800 You can have if statements, switch statements, and loops. Including rang-for loops. 05:02.880 --> 05:05.100 There are still restrictions on side-effects. 05:05.370 --> 05:11.820 The body of a constexpr function may not contain any code which could cause an action at run-time. 05:12.510 --> 05:14.160 Which includes memory management. 05:14.400 --> 05:20.460 So that means you cannot use things like string and vector; calling virtual functions, and throwing 05:20.460 --> 05:21.120 exceptions. 05:24.920 --> 05:27.140 You can also have member functions which are constexpr. 05:27.320 --> 05:31.850 So these take constant expressions as arguments, and return a constant expression. 05:32.900 --> 05:37.190 This is different from having a member function, which is const. A const member function 05:37.190 --> 05:39.980 means you cannot modify the object it was called on. 05:40.370 --> 05:40.880 So the 05:40.880 --> 05:43.120 "this" pointer is a pointer to const. 05:44.800 --> 05:46.000 In C++11. 05:46.000 --> 05:50.470 If you have a member function which is constexpr, then it was also const. 05:50.800 --> 05:54.250 So these functions could not modify the objects they were called on. 05:55.210 --> 05:58.290 In C++14, they are no longer const. 05:58.300 --> 05:59.950 So you can modify the object. 06:00.370 --> 06:06.460 If you want to prevent that, then you can declare them as const additionally. So you can have a member function which 06:06.460 --> 06:08.290 is constexpr and const. 06:11.210 --> 06:15.440 And finally, we can also have members of classes and structs which are constexpr. 06:15.950 --> 06:20.330 So these are initialized from a constant expression, and they cannot be modified. 06:21.620 --> 06:27.020 This is only possible if you declare the member as static. Which means it is not associated with any particular 06:27.020 --> 06:30.050 object. Which means it is available at compile-time. 06:31.070 --> 06:35.570 If you have a member which is not static, then it has to belong to a particular object. 06:36.230 --> 06:39.560 And you can only have objects at run-time, because you need to call the constructor. 06:41.050 --> 06:43.450 So that is why you have to declare them as static. 06:44.290 --> 06:45.760 Okay, so that is it for this video. 06:46.150 --> 06:47.020 I will see you next time. 06:47.020 --> 06:49.210 But until then, keep coding!