WEBVTT 00:00.810 --> 00:05.100 Functions are the basic infrastructure of C plus plus code. 00:05.100 --> 00:08.040 So the code is contained in functions. 00:08.040 --> 00:11.400 To execute that, you have to call a function. 00:11.400 --> 00:17.070 So C plus plus is remarkably flexible in the ways that you can define and call functions here. 00:17.070 --> 00:23.570 So you can define functions with a fixed number or parameters or variable number of parameters. 00:23.580 --> 00:29.220 So you can write generic code so that the same code can be used with different types. 00:29.220 --> 00:33.810 And you can even write generic code with a variable number of types here. 00:33.810 --> 00:37.560 So defining C plus plus functions. 00:37.560 --> 00:41.700 So at the most basic level, a function has parameters here. 00:41.700 --> 00:47.730 So it has the code to manipulate the parameters and it has the return value. 00:47.730 --> 00:52.650 So C plus plus gives you several ways to determine these three aspects. 00:52.650 --> 01:02.170 In this next lecture, we will cover these parts of C plus plus function from left to the right of declaration. 01:02.170 --> 01:08.170 So functions can also be templated, but this will be left to later sections. 01:08.590 --> 01:14.380 So a function must be defined exactly once, but should overloading. 01:14.380 --> 01:20.590 You can have many functions with the same name that differentiate by their parameters. 01:20.590 --> 01:27.580 So code that uses a function has to have access to the name of the function and so it needs to have 01:27.580 --> 01:27.940 access. 01:27.940 --> 01:34.630 So either the function definition or the declaration of the function also called the function prototype. 01:34.630 --> 01:40.690 So the compiler uses the prototype to type check that the calling code is calling the function. 01:40.690 --> 01:42.430 So using the right types here. 01:42.430 --> 01:49.120 So typically libraries are implemented as separate compiled library files and the prototypes of the 01:49.120 --> 01:56.350 library functions are provided in header files so that the many source files can use the functions by 01:56.350 --> 01:57.820 including the heaters. 01:57.820 --> 02:04.690 So however, if you know the function name parameters and return type, you can type the prototype yourself 02:04.690 --> 02:06.250 in your file here. 02:06.250 --> 02:13.210 So whichever you do, you are simply providing the information for the compiler to type check the expression 02:13.210 --> 02:14.470 that calls function. 02:14.470 --> 02:21.130 So it's up to the linker to locate the function in the library and either copy the code into the executable 02:21.130 --> 02:25.630 or set up the infrastructure to use the function from a shared library. 02:25.630 --> 02:32.200 So including the header file from for a library does not mean that you will be able to use the functions 02:32.200 --> 02:39.040 from that library because in standard C plus plus the header file does not have information about the 02:39.040 --> 02:41.620 library that contains a function. 02:42.080 --> 02:45.830 So most of the C runtime library is implemented this way here. 02:45.830 --> 02:53.720 So the function is compiled in a static library or a dynamic link library, and the function prototypes 02:53.750 --> 02:56.210 are provided in a header file. 02:56.660 --> 03:03.140 So you provide a library and the linker command line, and typically you will include the header file 03:03.140 --> 03:08.840 for the library so that the function prototypes are available to the compiler. 03:08.990 --> 03:15.530 So as long as the linker knows about the library, you can type the prototype in your code here. 03:15.530 --> 03:20.940 So this can save you from including some large files into your source files. 03:20.960 --> 03:26.810 The files that will mostly have prototypes of the function that you will not use here. 03:26.810 --> 03:33.440 So far in this course, we have used one source files, so all the functions are defined in the same 03:33.440 --> 03:35.690 file as where they are used. 03:35.690 --> 03:38.480 So and we have defined the function before calling it. 03:38.480 --> 03:44.190 So that is the function is defined above the code that calls it here. 03:44.190 --> 03:44.380 So. 03:44.430 --> 03:48.570 So here you do not have to define the function before it is used. 03:48.570 --> 03:53.220 As long as the function prototype is defined before the function is called here. 03:53.220 --> 03:55.740 So in main function we are going to print something here. 03:56.010 --> 04:01.170 Moved here for example C out mult. 04:02.280 --> 04:04.500 As you can see, we have to input the. 04:07.120 --> 04:07.870 Okay. 04:08.380 --> 04:09.640 Stay out here. 04:10.510 --> 04:16.960 And as you can see, we got an error because we have to import the iostream header file. 04:17.110 --> 04:20.070 And as you can see, we can now print something on the string. 04:20.080 --> 04:22.360 So moved here instead. 04:23.410 --> 04:24.340 And. 04:25.260 --> 04:27.660 Six, six, seven. 04:29.040 --> 04:30.150 And an line. 04:33.680 --> 04:34.190 Here. 04:34.190 --> 04:38.210 And after that, we're going to create the mold function here. 04:38.360 --> 04:40.100 Mold here. 04:40.460 --> 04:43.160 I will show you what's wrong with this code here. 04:43.870 --> 04:45.290 Error is here. 04:45.290 --> 04:50.750 And return the multiply by errors here. 04:52.100 --> 04:57.080 So the mod function here defined after the main function here. 04:57.080 --> 05:00.290 But this code will here. 05:00.320 --> 05:01.910 Let's try this out. 05:02.840 --> 05:04.940 Was actually said here. 05:10.350 --> 05:10.880 Here. 05:10.890 --> 05:18.150 This code will not compile because it is given the prototype before the main function. 05:18.150 --> 05:21.810 So this is called here if we make it like that. 05:25.330 --> 05:31.660 As you can see here, the function can compile and it will multiply six by seven here. 05:31.690 --> 05:36.740 So the prototype doesn't have to be have two parameter names. 05:36.760 --> 05:42.720 So this is because the compiler only needs to know the types of the parameters so their names. 05:42.730 --> 05:48.310 So however, since parameter names should be self-documenting, it's usually a good idea to give the 05:48.310 --> 05:54.370 parameter names so that you can see the purpose of the function here. 05:55.350 --> 06:03.780 Now, here in this lecture, in this example, we the function is defined in the same source file. 06:03.780 --> 06:09.350 So the internal linkage, this means the there is an internal linkage here in this code. 06:09.360 --> 06:15.840 So if the function is defined in another file, the prototype will have external linkage. 06:15.840 --> 06:24.390 And so the prototype will have to be defined, for example, like this here external. 06:25.830 --> 06:28.340 Extend integer mode. 06:29.200 --> 06:29.640 Here. 06:29.650 --> 06:30.520 Integer. 06:30.550 --> 06:31.630 Integer here. 06:34.350 --> 06:41.430 So the extern keyword is one of many specifiers that you can add to a function declaration. 06:41.430 --> 06:45.950 And in the previous lectures we have seen others. 06:45.960 --> 06:51.780 So for example, the static specifier can be used on a prototype to indicate that the function has internal 06:51.780 --> 06:56.760 linkage and the name can only be used in the current source file. 06:57.330 --> 07:05.280 So current source file in this example here in for example, static. 07:06.710 --> 07:07.340 Integer. 07:08.140 --> 07:11.530 So you have to write static before the function return type here. 07:11.740 --> 07:13.450 So in this example. 07:14.640 --> 07:18.730 It is appropriate to mark the function as static. 07:18.750 --> 07:19.530 In this case. 07:19.530 --> 07:27.390 So you can also declare a function as external here which affects how the name of the function is stored 07:27.390 --> 07:30.420 in the object file. 07:30.420 --> 07:35.370 So this is important for libraries and will be covered shortly.