Inheritance

In the previous chapter, we saw how we could create user-defined types, helping us design our code in a way that makes sense for the specific project we're working on.

For example, were we making a game where players can fight goblin enemies, we could have a Goblin class, containing all our goblin-specific code:

1class Goblin {
2public:
3  void Render(){}
4  void Move(){}
5  void Attack(){}
6  void DropLoot(){}
7  void Enrage(){}
8};

When we start implementing more complex systems using classes, we quickly encounter a problem.

Why we need Inheritance

Our game will probably need some simple objects we can scatter around the environment, like rocks.

Rocks aren't as complicated as goblins - they just need to be rendered to the screen:

1class Rock {
2public:
3  void Render(){}
4};

We have a bit of a problem here - both of our classes have a Render() method. In fact, given we're making a game, a lot of our types are going to need this method.

With what we've learned so far, there are no good ways to deal with this. Some options might be:

Option 1: Let both classes have a Render() function

Having the same code in multiple places is a pain - it makes our projects very complicated. Given we're making a game, a lot of our types are going to need this function, so the problem gets worse and worse as we add more types.

Option 2: Repurpose the Goblin class so both Rocks and Goblins can share the same code

This reduces the need to duplicate code, but it creates extremely complicated classes. It also degrades performance - class functions like Attack() need class variables like int Damage to support those actions. Having every rock in our game carry those variables, which they never need, is a waste of resources.

To solve this problem, what we need is the ability to define multiple layers of abstraction, and that is what inheritance gives us.

Designing with Inheritance

Inheritance allows us to organize our classes into hierarchies. This allows a class to inherit the functions and variables of its parent.

For example, let's imagine a class that simply grants the object the ability to exist in our world, and render itself to the screen. We'll call this the Actor class.

Next, let's create a class for our Goblins. Goblins also need the ability to be rendered, but with inheritance, we no longer need to recreate that functionality. Actor already has that ability.

Goblins are just a more specific type of Actor, so we can have our Goblin class inherit all the abilities of the Actor class.

Now, our Goblin class has the Render ability, without needing to write any additional code. It just inherits it from the parent class.

This tree structure provides our classes with a powerful ability. We get to have our dedicated, specific classes, but they don't need to duplicate the more generic, shared functions.

They can instead just inherit those functions and variables from their ancestors.

Implementing Inheritance

Let's create our Actor class, and our Goblin class that inherits from Actor:. To do this, we add some additional syntax to our class Goblin definition:

1class Actor {
2public:
3  void Render(){};
4};
5
6class Goblin : public Actor {
7public:  
8  void Move(){}
9  void Attack(){}
10  void DropLoot(){}
11  void Enrage(){}
12};

Now, when we create an object from the Goblin class, that object will be both a Goblin and an Actor, able to use the functions of both:

1Goblin Bonker;
2
3// Available because Bonker is a Goblin
4Bonker.Attack();
5
6// Available because Bonker is an Actor
7Bonker.Render();

With this class hierarchy, all Goblin objects are inherently also Actor objects. The opposite is not necessarily true - an Actor is not necessarily also a Goblin.

It could just be a plain Actor, or it could be some other subclass of actor.

1Actor Rock;
2
3// Available because Rock is an Actor
4Rock.Render();
5
6// But we can't do this, because Rock is not a Goblin
7Rock.Attack();

1class Item {};
2
3class Weapon : public Item {};
4
5int main(){
6  Weapon IronSword;
7}

1class Item {};
2
3class Weapon : public Item {};
4
5int main(){
6  Item WoodenBarrel;
7}

Multiple Layers of Inheritance

Let's add a new type of object to our game - we need dragons, so let's define a type, using the same approach:

Something should seem off here. The Dragon and Goblin classes both have the Move, Attack, and DropLoot functions. That's too much duplicate code.

Thankfully, with our new knowledge of inheritance, we might realize a way we can improve this design.

We can move that shared code into a new class. We can have both Goblin and Dragon inherit from it. Let's call this new class Character

Now, we have multiple layers of inheritance. With this setup, any Goblin or Dragon we create will be a member of 3 classes, inheriting the abilities of all of them:

1class Actor {
2public:
3  void Render(){}
4};
5
6class Character : public Actor {
7public:
8  void Move(){}
9  void Attack(){}
10  void DropLoot(){}
11};
12
13class Goblin : public Character {
14public:
15  void Enrage(){}
16};
17
18class Dragon : public Character {
19public:
20  void Fly(){}
21};
22
23int main(){
24  Dragon Dave;
25
26  // Dave is an Actor
27  Dave.Render();
28
29  // And a Character
30  Dave.Attack();
31
32  // And a Dragon
33  Dave.Fly();
34}

Preventing Inheritance with final

Sometimes, we create a class that is not designed to ever have a child class. To clarify that intent, and have the compiler enforce it, we can add the final specifier to the class heading:

1class Demon : final {};
2
3// Combining final with a base class
4class Vampire final : public Character {};

Should anyone try to inherit from a final class, the compiler will block them:

1class Warrior : public Vampire {}

1Cannot inherit from 'Vampire' as it has been declared as 'final'

Summary

In this lesson, we delved into the concept of inheritance in C++. We explored how inheritance allows us to create hierarchies of classes, enabling child classes to inherit properties and behaviors from their parent classes.

This not only helps in organizing code into a more understandable structure but also aids in reducing redundancy.  We:

• Learned the basics of inheritance and how it helps in organizing classes into hierarchies.
• Explored public and private inheritance, understanding how access levels affect the inherited members.
• Demonstrated the implementation of inheritance in C++ with practical examples, such as Goblin and Actor classes.
• Discussed the use of the final keyword to prevent further inheritance from a class.
• Examined multiple layers of inheritance through examples, showcasing how classes can inherit from other derived classes.

Next Lesson: Protected Class Members

Our next lesson will focus on protected class members. This concept helps us control access to class members in an inheritance hierarchy.

Protected members provide a level of accessibility that falls between public and private, which we'll cover in detail.

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