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As you saw in the previous lecture, the compiler will compile your source file as long as individual

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items called translation units.

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So the compiler will determine the objects and variables you declare and the types and functions you

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define.

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So and once declared, you can use any of these in the subsequent code with the scope of the declaration.

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So at its very broad set, you can declare an item at the global scope by declaring it in a header file.

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So that will be used by all of the source files in your project.

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If you do not use a namespace, it is often wise when you use such global variables to name them as

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being part of the global namespace.

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So now I want to create a new header file here in C plus plus which I will explain later what this header

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file is.

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So version here, let's create a version header file.

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And as you can see here, the Sea Lion compiler automatically created the version that Cplusplus file

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as well.

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So let's change my username to our online computer science academy.

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Oxley Name.

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And here we have a main dot C plus plus file and version file.

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And also we have a header file of this version cpp file.

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So in version in version header file, let's add the extern integer version here.

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Here and I will explain all of this code later as I do and version in versions dot cpp file.

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Include the version file.

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As you can see here, it's actually included by automatically.

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So we don't need to write here and version here.

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Equals 17.

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As you can see, we got an problem here because we have the insert class here.

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So, um.

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Oh integer version of.

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Integer version.

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Return zero.

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Here UPS Actually, we need to main here and then we will assign version as version.

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As seven.

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17 year.

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So.

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Now let's use the main main.cpp here as well.

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Here we're going to add the print where we're going to add print function in this main dot CPP file.

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So we have to include the firstly, we need to include the version here.

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Version dot header file.

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As you can see, it comes automatically.

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And then we let's define the print print function here.

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Void print version.

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Here.

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And let's use the braces here.

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STD.

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Or actually, let's use the print function here instead of STD.

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Printf.

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Printf.

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Okay, so version equals.

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Is equals to.

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Uh, the decimal, because we will get decimal here.

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And then here version.

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As you can see, we use this double dots here.

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As you can see here, because.

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We included the version that H mentioned, that header file and in version that header file we have

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an external integer version variable which I will explain what these are.

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So the code here as as you see here, has two source files, the version CPP and main dot CPP here.

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And we have also we also we have a header file as you know, also we have a header file included by

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both source files here.

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As you can see here in version of CPP, it's included version header file and in also in Maine dot CPP,

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we have the included version dot h file here.

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So.

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Um.

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So here the header file.

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A header file declares the global variable version, which can be used by both source files so it declares

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the variable but does not define it here.

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As you can see, we defined it here.

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We defined version as 17, so the actual variable is defined and initialized as you can see here here.

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So it is here that the compiler will allocate memory for the variable.

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So the extend keyword here used on the declaration in the header in the case to the compiler that the

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version has external linkage.

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So let's let me write it.

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So when you download this code you will see and read this and remind us the extern here means external

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external linkage.

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Linkage here, and that is the name is visible in files other than where the variables is defined.

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So the version variable is used in the main.cpp source file.

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So in this file, the scope resolution operator here, this is the scope resolution operator.

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As you can see, this is the two double dots here is used without a namespace name.

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As you can see in previous lecture, we actually used some namespace.

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For example, we did, for example, Revolver, and then we use this scope resolution operator here.

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So actually let me write the scope resolution.

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Operator.

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This means the scope resolution.

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Operator And I want to also note this extern terminology.

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So actually this terminology is matters because you will use them a lot if you do the recording programming

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or just any C plus plus programming, you will use this external linkage and scope resolution.

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Operator So of course there is a programs, the simple programs that don't use these operators here

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or keywords.

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So you can also declare items that will only be used within the current translation unit by declaring

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them within the source file before they are used.

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So usually here top on the file.

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So this produce a level of modularity and allows you to hide implementation details details from code

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in other source files.

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So for example, let's create an, let's write our um, header file here.

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Actually, let's create a.

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Um.

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Main here.

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So.

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Oxley and lets in main dot header file.

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Here we will declare the void usage.

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Here usage function just a function.

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We will not write a code in it yet.

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And in print.

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In here.

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Main.cpp.

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We're going to include the version age as we did earlier, and then we will use this standard.

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We will use string from the standard library.

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String.

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Application name here.

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For example, application name, for example.

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Port scanner.

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And here we have an app name variable string called app name here, which is this is a string.

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And inside this string we have a port scanner string here.

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So as you know, in C plus plus string in actually in a lot of language strings is just of arrays of

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arrays of characters here, which you will learn about these strings later also.

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So then let's use our as you can see, we have a print function here.

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Uh, and.

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We will use we will define usage function here also.

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Actually, let's include our main that include our include main header file here and define usage.

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Avoid usage.

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Here and.

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STD.

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See out.

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Application name.

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Here.

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Actually, we have to define out of this main function here, application name and then here.

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And then let's we will call the print function here.

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Print function.

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Print version.

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Here.

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And.

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And so the print that the main dot header file here contains the interface for the code in the main

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dot CPP file.

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As you can see, we use this usage.

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So only those functions declared in the header file will be callable by the other source file.

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So the color does not need to know about the implementation of the usage function.

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So and as you can see here, it's implemented using a call to a function called print version.

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So that is only available to main dot cplusplus file.

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So the variable application name is declared at a file scope, so it will only accessible to code in

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main.cpp.

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So if another source file declares a variable at file scope that is called the application name and

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is also a standard string here.

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So this is the string here.

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As you can see here.

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Um, the file will compile, but the linker will complain when it tries to link the object file.

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So the reason is that linker will see the same variable defined in two places and it will not know which

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one you to use.

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So a function also defines a scope.

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Variables defined within a function can only be accessed through the name, so the parameters of the

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function are also included as a variable within the function.

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So when you use when you declare the variables, you have to use different names.

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So if a parameter is not marked as a constant here const keyword as you remember, if your parameter

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is not marked as constant, then you can alter the value of of the parameter in your function.

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So you can declare the variables anywhere within a function as long as you declare them before you use

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them.

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Here, curly braces are used to define the code blocks and they also define local scopes.

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So if you declare a variable within the code block, then you can only use it there.

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So this means that you can declare variables within the same name outside of the code blocks, and the

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compiler will use the variable clauses to the scope it is accessed.

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So before finishing this lecture, it's important to mention one aspect of the C plus plus storage class

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here.

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A variable declared in function means that the compiler will allocate memory for the variable on the

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stack frame created for the function.

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So when the function finishes, the stack frame is torn down and memory is recycled here.

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So this means that after a function returns, the values in any local variables are lost completely.

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So when the function is called again, the variable is created a new and initialized again.

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Also in this lecture I want to talk about actually, yeah, in this lecture I want to talk about the

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C plus plus static keyword.

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So but in C plus plus here, static keyword change this behavior.

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So the static keyword means that the variables is allocated when the program starts, just like variables

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declared at a global scope.

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So applying static to a variable declared in a function means that the variable has internal linkage.

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So that is, the compiler restricts access to the variable to that function.

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So now I want to create an example code here, for example.

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Um.

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Know before this main function integer increment here in increment and.

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And then let's create a static static integer value.

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Value increment by one.

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Increment by one

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by one here and return a value.

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Here.

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Let's use the now let's print our function here.

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Increment here and.

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So increment here we will use.

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As you can see, this is the increment.

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So we're going to pass one number as a parameter.

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For example, five here, or let's create an this function type lots of types, for example.

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Five nine.

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19.

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999.

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So.

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Uh, let's compile this application.

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Actually let's create this application here project as a new project.

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Because I don't want to with these codes, because I want to share it with you as a file.

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Here and we're going to write the increment function here.

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Integer.

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Increment integer.

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Here.

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And static integer value.

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Value plus equals.

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E here and we will return the value.

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Here.

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So why we have a yeah, we have to add here.

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So then let's compile our code.

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Actually, let's add a new lines here.

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Because the cars here new lines so.

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By default.

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Here the compiler will initialize a static variable.

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Two zero.

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So but you can provide an initialization value and this will be used when the variable at first allocated.

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So when this program starts, the value variable will be initialized to zero before the main function

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called.

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So the first time the increment function is called the value variable incremented to ten, you know,

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to five here, which is returned by the function and printed to the console.

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And when the increment function returns, the value value variable is retained so that when the increment

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function is called again, the value variable incremented by.

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Nine here.

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As you can see, five plus nine equals 14.

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Actually, let's create a calculator.

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So.

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Let's create an example without static.

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Here.

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This is a static and this is not static here.

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So and now I want to.

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Notepad.

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Notepad.

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Here.

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With static.

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Here.

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Or without.

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Without static.

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And with static.

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With static here.

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And let's create, let's add static keyword and start it again.

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As you can see here, we got the different result.

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So.

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Here.

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So without static, our value here.

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Ursus itself and just does nothing.

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But here, by default, the compiler will initialize the static variable to zero, but you can provide

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an initialization value here.

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So and this will be used when the variable is first located.

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So when this program starts the value.

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A variable we will initialize to zero and is incremented.

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To increment the to five which is returned by the function and printed to the console.

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Here.

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So.

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When the INC function returns the value.

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Variable is returned so that the vendor Inc function is called again.

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The value variable is incremented by five by nine and total number is going to be five plus nine equals

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four teen.

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So it might be confusing at first, but I promise you will learn this constant and static variables

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and keywords in later lectures.

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This is just the beginning of the C plus plus files, but no.

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And you can do it.

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So in next lecture we will talk about the operators and I'm waiting you in the next lecture.

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So I know this, this lecture is actually almost 20 minutes, so I hope you watched all of this and

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I'm waiting you in the next lecture.

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This is just the beginning or intermediate C plus plus programming.

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So after.

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Watching and including your my lectures in your practically you will learn about the cplusplus a lot.

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So I'm waiting you in the next lecture.