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In this video, we'll learn about using access modifiers

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to determine what will be hidden in the class and what parts of the program have access to this information.

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C++ has three basic class member access modifiers:

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these are public, private and protected.

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Class members that are public are accessible everywhere.

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Private class members are only accessible by other members of the same class

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or by friends of the class.

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We'll talk about friends later in this section. Finally,

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protected members are used with inheritance, which we'll talk about later in the course.

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Using the class member access modifiers is super simple.

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In the body of the class, you specify the axis modifier followed by colon.

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Then everything you declare from that point forward

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will have that access modifier applied to it.

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In this example, we're using the public access modifier.

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And anything we declare after this will be public until we see a different access modifier.

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In this example, we're using the private modifier.

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By default, everything in the class will be private if no access modifier is specified.

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Protected works exactly the same way as private.

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For now you can think of protected and private as being pretty much the same.

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The different comes in when we use inheritance.

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Of course, we commonly use public, private and protected all at the same time

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to tell the compiler what class members should not be available outside the class,

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and what class members should be part of the public interface of the class.

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This is how c++ implements information hiding.

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In this example the player's name, health and xp are all private.

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This means that they're only accessible from the other members of this class

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and from friends of the class.

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The talk and is dead methods are publicly accessible.

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So what happens if we try to access a private class member.

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Simple, you get a compiler error. Here are some examples.

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First, we create a player object named frank.

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If we try to access the player object's name instance variable,

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then we get a compiler error because name is private,

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and it's not accessible from outside the class. The same is true for health.

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Why would we want to make some class members private and others public.

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The idea is that you want to control access to certain data and methods

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by making them private.

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For example, you want to prevent users from directly setting frank's health to

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a 1000 as in this example.

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Maybe the maximum health for the player is a 100.

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In order to control this, you would create a public method in the class

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that sets this health property.

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And in the code for that method, it makes sure to max out the health at a 100.

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Since that method is a class member, it would have access to the private health property.

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In the second example we create a pointer named enemy to a player object.

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If we try to directly access the xp property, we get a compiler error because it's private.

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However, we can call the talk method with no error since it's public.

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Let's see another example.

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In this example, we have an account class

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and the name of the account and the balance of the account are private.

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These properties cannot be accessed directly from outside the class.

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We also provide public withdraw and deposit methods

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that can be called from outside the class.

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Since these methods are member methods, they have access to the balance

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and can update its value based on whatever business logic we have for the class.

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Here are a few examples that use the account object.

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Frank account is an account object.

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If we try to access the object's balance property directly,

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we'll get a compiler error because balance is private.

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However, we can use the deposit method and provide the amount to deposit

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since deposit is public.

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The idea is that the deposit method would somehow verify

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that the 10 million dollars that I want to deposit are indeed available before incrementing balance.

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If I try to directly access the name of the account,

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I also get a compiler error for the same reason, the account name is private.

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The same behavior applies whether we access an object using the dot operator

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or via a pointer using the arrow operator.

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In the case of Mary's account trying to access the balance

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or name directly results in a compiler error,

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but using the deposit method is fine since it's public.

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You can see how powerful this is.

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For example, if we have an object whose balance is 20 million dollars

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and we know that that must have been an error,

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then the only place where that error could have happened in this example is in the deposit method.

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There's no way that any other part of the program could have changed that value

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since the value is private. This makes testing and debugging code much, much easier.

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

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in the AccessModifiers project.

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And in this project, we've got our player class that we've been using.

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And you'll notice i made a couple of changes to it. I made

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the attributes all private, so the name the health and the xp are all private.

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And I made the two methods talking is dead public. Okay.

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Okay. So I want to show you the compiler errors that you'll get when you try to access those

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private data members.

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So in this case, I want to create a player object and I'll call it frank.

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And if i try to set frank's name,

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so I can say frank.name

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equals anything, in this case, I'll just say frank.

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And we're trying to compile, I'm going to build and run with control F5.

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You see the error I'm getting.

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And you can see it here in the box as well, it says player's name is private within this context,

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same thing we're seeing here. So you're trying to access

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a private attribute which is name from outside of the class.

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This is exactly the same area you'll get if you try to use health or xp.

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And it's not just a matter of trying to set that variable,

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it's trying to use that variable as well.

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Okay. So if i try to do something like cout frank.health,

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and I'll do an endline here,

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and we'll compile this as well.

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You'll see I get two errors now,

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same idea. I don't have access to health.

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It's not just a matter of not allowing you to set it, it's not allowing you to access it.

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Okay. Now these -- I'm going to comment those two out for a moment.

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And let's initialize the name here just to player,

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just so we have something to see when we call the talk method.

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Okay. So now what I can do is I can say frank

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.talk,

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and I can pass in the string hello there.

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Now if I try to build and run,

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I says player says hello there. Okay.

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So we know it works in this case because we're accessing

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a publicly accessible method right here talk.

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Same thing with is dead. We can't call is dead because we haven't implemented it yet,

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but we did implement talk right here.

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Okay. So that's really all there is to it.

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If you get these errors that are saying you don't have access to something, that's exactly why.

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Now why do we do this?

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It's really good programming practice to hide certain parts of your classes declarations.

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The idea is you want to control that variable

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via some sort of public method, so they've got to go through the public method

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and then the public method

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makes sure that the health is within some reasonable range.

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And as I mentioned in the slides, if we're doing this

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and we output some value of health that's 20000 or something like that,

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that's really really out of whack. If health is public,

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then anything could have changed that property.

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But if health is private, then I know that the changes that were wrong

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happen somewhere in here, right, somewhere in the public interface. My methods

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made that mistake, and I can find and fix that error much, much easier.