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In this video, we'll learn about how to implement member methods in our c++ classes.

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You'll be glad to hear that class methods are implemented

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in a manner very similar to the way we implemented functions.

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But I'd say that they're generally less complex.

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This is because member methods have access to the classes member attributes,

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so we don't have to pass so many arguments around.

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C++ gives you many options regarding where to write code for member methods.

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You can define the member methods right inside the class declaration.

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If you do this, then the methods become implicitly in line.

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While this is okay for small and relatively simple methods,

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we tend to implement larger, more complex methods outside of the class declaration.

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In order to do so, we need to tell the compiler that we're implementing a method

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that belongs to a specific class.

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So we use the class name

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followed by the scope resolution operator and then the method name.

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This unambiguously tells the compiler that you're implementing a method

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for a specific class.

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In c++, it's also very common to separate

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a class's specification from its implementation.

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This makes the class much easier to manage.

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We usually create a header or include file

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that has a .h or .hpp extension

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for the class specification and then a separate .cpp file

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for the class implementation.

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First, let's look at implementing methods inside the class declaration.

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In this example, we're declaring an account class.

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Notice that the account class has a private double named balance,

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and two public methods: set balance and get balance.

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Set balance sets the balance to the value passed in by the user,

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and get balance returns a copy of the account balance.

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These public methods are necessary

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to access the balance since the balance is private.

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Notice the syntax is exactly as it was for defining regular c++ functions.

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Also notice that the public methods

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have direct access to the private balance because they're members of the same class.

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Now let's see how we would implement these methods outside the class declaration.

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In this example, we're defining the set balance and get balance methods

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outside the class declaration.

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Inside the class declaration, we simply provide the method prototype or declaration.

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This tells the compiler the name of the methods,the parameters they expect and the return type.

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The compiler can then type check the method implementation when it sees them.

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In order for the compiler to know that these methods belong to the account class,

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we have to prefix the method names

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with account followed by the scope resolution operator.

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Then we implement the method as we normally would.

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In this case, the compiler knows that the two method implementations

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are for the method declarations in the account class we provided.

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As our programs get larger, we really want to separate class specification

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and class implementation into different files. Let's see an example of how we can do that.

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In this example, we create a file named account.h.

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This is an includer header file

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that will pound include in our .cpp files

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whenever we need to use the account class.

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Notice that in this file we provide the specification for the account class.

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Before we can use or pound include this account.h header file,

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we need to take care of a potential problem.

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If this file is included by more than one cpp file,

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then the compiler will see the declaration for the account class more than once

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and give us an error about duplicate declarations.

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In order to solve this, we can use what's called an include

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guard that ensures that the compiler will process this file only once,

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no matter how many times it's included.

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An include guard is just a series of pre-processor directives that guarantees the file will only be

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that guarantees the file will only be included once.

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The way it works is we wrap up our entire class declaration in this include guard.

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The first line is pound if not defined

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underscore account underscore h underscore, all in uppercase.

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This checks to see if the preprocessor has a symbol named

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underscore account underscore h underscore.

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This simple name can be anything you want,

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but best practice is to use some sort of combination of the file name

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in uppercase with preceding and ending underscores.

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Any name is fine as long as it's unique in the program.

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If the preprocessor has that name defined,

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then it skips the entire file and goes to the pound end if at the end.

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In other words, it doesn't process the file since it's already seen it

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because the symbol has been defined.

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If the symbol has not been defined, then the symbol is set

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and the file is processed as usual.

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As you can see, it's pretty simple

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but not very elegant compared to more modern programming languages.

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So now this is what the account.h file typically contains.

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You can see the include guard wrapping the class declaration.

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Now we can provide the implementation for the account class

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in a separate file named account.cpp.

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Many compilers also support the pound pragma once compiler directive.

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The effect of this directive is the same as the include guard,

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while many compounders support this directive, some don't.

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So check your compiler documentation to be sure.

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Now we can provide the implementation for the account class in a separate file

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named account.cpp. Let's take a look at that.

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This is the source code for the account.cpp file.

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Notice that we include account.h and it's in double quotes.

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Up to this point, our includes have used angle brackets with no extensions.

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There's an important difference between the two forms of include.

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Include with angle brackets as in include iostream

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is used to include system header files

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and the compiler knows where these are located.

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Includes with double quotes tell the compiler to include header files

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that are local to this project, the compiler also knows where those are.

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Notice that this file implements the two methods declared in the account.h header file.

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Now our account class is complete,

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the specification is in account.h

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and the implementation is in account.cpp.

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How do we use an account in our main cpp file though.

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Let's see in the next slide.

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Here's our main.cpp file.

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Notice how we include iostream as well as account.h.

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It's important that you always include .h files, never include .cpp files.

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Then we just write our main as we normally would.

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When the program is compiled,

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both the main.cpp

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and the account.cpp files are compiled

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and then linked to produce the executable.

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Let's see how all this works in the IDE.

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Okay. So I'm in the IDE, and I'm in the section 13 workspace

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in the ImplementingMethods1 project.

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What I'm going to do here is

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I'm going to implement these methods in this account class and we'll implement some of them

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inside the class declaration and some of them outside the class declaration.

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Then we'll create another project ImplementingMethods2

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where we'll separate the accept specification from the implementation

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and we'll create the account.h file and the account.cpp file.

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Okay. So let's start here.

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What we're doing here is we have our class account right here,

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and we've got two private attributes: name and balance.

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And then we've got the methods. And the methods are really what we're interested in here.

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Now in this case, I've got six methods, all public.

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We've got a set balance and a get balance.

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And you can see that these methods have been implemented in line

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right inside the class declaration.

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This is easy to do, and this is okay to do when you've got methods like

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these that are really really straightforward methods.

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They're done. They're implemented, and we're all set.

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Now we can also implement methods outside the class declaration.

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Notice these last four methods.

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I've got set name, get name, deposit and withdraw.

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And what you see right here are basically the function prototypes or the method prototypes.

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They're not defined here. These are only declarations.

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What we want to do is we want to define them outside the class,

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and you'll notice the class begins here on line 8, and it ends right here on line 26.

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So what we want to do is define them outside the class. And let me show you how to do that.

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It's really straightforward.

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What we've got here is -- so let me show you how easy it is.

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Here are the methods that have been implemented outside the class.

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We're starting on line 28, right here. The method is set name, which is this method right there.

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It's called set name.

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Now it's not just good enough to say void set name, that would just

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define any method.

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We want to define the set name for the account class.

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So we need to prefix the name of the method set name

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with the name of the class followed by the scope resolution operator.

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Now when the compiler sees this, it knows okay,

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you're defining the set name method that you declared in the account class.

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So it's going to come up here, and I'll be able to type check

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all of these things together and make sure that there's no mistakes.

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Okay. So you can see what's going on here.

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In this example, it's really straightforward.

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A string is being passed in,

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and the name is set to whatever string was passed in, which is n. Remember, name is

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one of the attributes of the class.

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It was private, and that's okay. This is a member method. So it has

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access to private information.

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Get name simply returns a copy of the string.

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Okay. And in this case, we've got our deposit or withdraw.

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So you're going to pass in an amount to deposit.

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This method would somehow verify that that amount is indeed there,

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so that you can deposit it.

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And all it does is it increments balance by whatever the amount is.

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And it returns true. Obviously, you would return false if it didn't verify.

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Okay. And then the last method that we'll implement is

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this account withdraw method.

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Again, it's the same idea. We prefixed the name of the method

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with the name of the class because it's a class method.

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And here's the amount. And what do we do here?

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Well, we look at the balance and we subtract the amount from it.

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If that's greater than or equal to 0, then there is sufficient funds.

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We'll just decrement the balance by the amount,

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and we'll return true. Okay. Simple as that.

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If the funds aren't there, then we'll just return false.

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So let's see how we can use this in the main,

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and it's really, really straightforward. I've got some break points in here that we'll follow in a second.

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So what I'm doing here is I'm creating an account called Frank account.

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And I'm setting the name to Frank's account.

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Now I can't say Frank account.name

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because that's private. So I've got to go through this public method called set name.

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I'm doing the same thing for balance. Frank account.set balance to a 1000.

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I can't say Frank account.balance equals a 1000 because balance is private.

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Okay. So at this point, I've got an account.

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It's got Frank's account is the name, a 1000 is the balance.

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And then I want to deposit 200 into that account.

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I'm going to call Frank account.deposit 200.

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If that returns true, then the deposit will okay, and I'm going to just display that.

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If that returns false, then the deposit was not allowed for whatever reason.

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Now I'm going to call some withdrawal methods here.

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I'm going to call the withdrawal method on the Frank account with 500.

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So I'm going to check, I should have a 1000 in there,

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right. Actually I should have 1200 in there because I just put 200 in there.

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So I'll check to see if 1200

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minus 500 is greater than or equal to zero which it is,

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and the withdrawal should be okay.

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I should have 700 left after that.

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Then I try to do another withdrawal with 1500 and that should fail.

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Okay. So let's let's run this through the debugger here.

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Right now, I'm at line 53.

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And I'm going to set a watch to Frank account.

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That way I can see that as it changes, really easily,

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and that's already set here.

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So what I'm going to do is I'm going to hit next.

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And remember, I'm right here on line 53. So at this point,

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I've just created the Frank account object.

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Now I want to set that name. Notice that the name is empty right now and the balance is garbage.

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I'm going to set the name to Frank's account. So when I select next,

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you can see that that's changed right here to Frank account.

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And now I want to set the balance to a 1000, so we should see that change here as well.

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Again, remember, these two attributes are private.

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That's why we're going through those public methods to change them.

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Now what we're going to do is we're going to deposit 200 into that account.

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So let's step through here.

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And what we're going to do now is we're going to jump to this method right here.

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I've got a breakpoint in there, that's where I'm at right now.

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Assuming there was some business logic here that would verify that that deposit amount was there,

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I'm going to increase the balance

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by 200, so I should have 1200 here when I finish and return true.

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And I'm back now. You can see I've got 1200 here.

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And I'm going to return true, and it should display deposit okay over here.

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And exactly what happens.

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Now we're going to withdraw 500. Well, Now we're going to withdraw 500. Well,

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So let's try that. And you can see I'm right in here now.

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I've jumped to this function, this method right here.

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So balance is 1200 minus 500 is greater than or equal to 0, so we should be good.

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In that case, I'm going to decrement the balance by the amount, and I'm going to return true.

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And I'm right back here now on line 63.

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Notice I've got 700 left in the account, and it should display

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withdrawal okay.

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The last example here is I'm going to withdraw 1500. I don't have 1500.

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So it should fail.

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So I'm going to go back up here if balance amount is greater than or equal to 0, it's not.

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So it's going to go straight to false, which it's going to return.

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And then when I come back to my main,

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I'm right here on line 70 and it should display not sufficient funds.

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And that's exactly what it does.

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Okay. So there you go. This is a real simple example of

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implementing these class methods both inside the

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class declaration and outside the class declaration.

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Okay. What I'm going to do next is we'll create a new project called ImplementingMethods2.

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And what we'll do is we'll create the account.h and account.cpp,

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and then we'll have a main cpp that runs the whole thing. So I'll set that up and be right back.

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Okay. So I'm back. I'm still in the section 13 workspace,

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but I've just created a new project called ImplementingMethods2.

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And all it has in it is a main cpp file that you're looking at right here.

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What we're going to do here is we're going to create our account.h file,

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our account.cpp file.

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And then we'll use this main cpp

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file as a driver so that we can create some objects and so forth.

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Okay. So what do we do? Well, we need to create an account class.

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Every IDE has its way of creating a new class.

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CodeLite is no different. If you select the source directory and right click,

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you'll have an option for new class. We'll select that.

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And then you get this pop-up dialog, and it's going to ask you, what's the class name?

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Well, the class name is account, again with a capital a.

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Don't worry about namespace. And then the block guard is what you saw with the include guard that

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i talked about.

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This is that pound define.

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So I'm just going to say underscore account

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underscore h underscore you can really do anything you like, you can even

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leave it blank and it's going to use its own way of doing it.

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Okay. So that's it. There's some more advanced options here,

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which should all be off at this point.

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And you can choose to create a .hpp file instead of a .h file,

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I'm not going to. And you can also choose to use the pragma once command.

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Again, I'm not going to. I'm just going to use the block guard.

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You select okay.

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And what happens here on the left-hand side, you'll see that

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we've got two files that were created. We've got the account.h file

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and the account.cpp file, that's exactly what we want

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Now we're going to start with the account h file, notice the wrapper.

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Okay. Now I'm going to get rid of what it generated for me.

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And what I'm going to do is I'm going to copy and paste the account that we were working on.

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And I'm just going to copy and paste the declaration of the account.

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So let me do that right now, and I'll paste it right in here.

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It should be pretty familiar.

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There's the class account. Those are the two private attributes.

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Let's see what we've got here? Now we've got to do a couple of things. We're using a string here.

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So what we want to do is here we want to pound include string,

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so our account header file is complete.

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And we really don't want to use the namespace in here. So

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I'm just going to prefix this with standard.

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And let's see I've got a string here as well.

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And I believe I've got one here as well.

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And I think that'll do it. So that's my account

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.h file. You can see it's exactly as we had before.

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There's my account declaration and it's wrapped up in that guard.

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Okay. Now let's go to the account cpp.

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Now you'll notice that it's generated the include for me.

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And also it's generated these two functions, these two member functions.

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We've got to get rid of those. This is a constructor and that's a destructor.

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We'll talk about those in a couple of videos.

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But for now, I just want to get rid of them.

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And what I want to do is I want to copy and paste

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those four methods that we wrote. Remember, the ones we wrote outside the class.

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So I'm going to copy them right in here like that. There's my account.

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There's my -- and again, I'm going to prefix this with standard,

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wherever we're using strings.

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And I think that was the -- right here's the other one.

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That's it.

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Now what we'll do is we'll go to the main.

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And what we're going to do here is we're going to include

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account.h

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because we're going to need it in our main. And if you remember the code for the main,

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again, I'll copy and paste it because it's really straightforward.

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That was the code for the main where we created the Frank account,

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we set the name for the account and so forth.

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Now what we're going to do is I'm going to save this and we'll compile the whole thing.

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We'll just basically do build and run, control F5. What's going to happen is

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the .cpp files will both be compiled and everything will be linked

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into an executable. Now we may run into some errors here,

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and we'll fix those as we go. So I'm going to hit control F5,

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no errors.

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Everything worked great. You can see I run the file and i get

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deposit okay, withdrawal okay, not sufficient funds, exactly like we had before.

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We could run this through the debugger if we like.

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But it's really as simple as that.

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Throughout the course, this is the way we're going to start writing code now. We're going to have

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the specification here

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and the implementation in the .cpp file.

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And you can see -- again, let's do this one more time,

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you can see the account.h the header or the include file

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that is our declaration for the account class.

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We have to wrap that up so that the compiler only sees it once.

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We're including string here because we've got our string

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class that we're using right there.

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And we really don't want to use name spaces inside these header files

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because everything that includes them is going to get that all that name space stuff,

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which is really not necessary.

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So what we've got there is that we've got our private stuff, we've got our public stuff.

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We've defined these two methods right inside the class declaration.

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And then these other methods we've defined outside the class declaration

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in account cpp.

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Now in order for the compiler to understand what you're talking about here,

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you have to include account.h.

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Once you include account.h, the compiler now knows what this is.

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Okay. Once that's done, you go to your main and you can see the main is now

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pretty self-contained. You're just including account.h,

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it's really really simple to do. The idea is just

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you're going to have a lot of header files and a lot of cpp files

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in your application, and everything gets compiled and linked into a single executable.