1. Entity Component System

2. Composition over inheritance
An alternative to OOP inheritance
e.g. the Component structure of Unity
Avoids Java-like inheritance trees
e.g. Doom 3 (from “One Weird Trick to write better code”
idClass
idEntity
idAnimatedEntity
idWeapon
idAFEntity_Base
idAFEntity_ClawFourFingers
idAFEntity_Vehicle
idAFEntity_VehicleFourWheels
idAFEntity_VehicleSixWheels
idAFEntity_Gibbable
idAFEntity_WithAttachedHead
idActor
idPlayer
idAI
Why is this bad (not everyone would think it’s bad…)?
See for more...

3. In ssuge We’ll have a GameObject class
Contain a transform (SceneNode)
Can be parented (again, SceneNode)
Other info (e.g. tag, resource group, etc.)
We’ll have a base Component class
An abstract class – meant only to be a base
We’ll derive multiple types of Components from it
MeshComponent
GUIElementComponent
ParticleEmitterComponent
When setting up our scene, we can “snap-in” a component to add functionality to a GameObject

4. C++ Polymorphism What is the output? class Yarg { public:
void func() { cout << “Yarg” << endl; } };
class Tango : public Yarg {
public:
void func() override { cout << “Tango” << endl; } };
optional, but nice (like annotation in Java)
Yarg * y = new Yarg();
Tango * t = new Tango();
y->func();
t->func();
delete y;
y = t; // Valid b/c of POLYMORPHISM

5. A fix… You can make a method virtual: Syntax: Now it’ll work “better”
Makes the compiler wait until run-time to determine the method to call.
Adds a virtual function table to the object (a little overhead)
Syntax:
Now it’ll work “better”
class Yarg {
public:
virtual void func() { cout << “Yarg” << endl; } };

6. Pure virtual
You can choose to not implement a method in the base class
…and force the child class to define it.
A little like Java interface classes
The base class is now abstract
We can’t make instances of it.
We can, however, use polymorphic pointers of the base type
Syntax:
class Yarg {
public:
virtual void func() { cout << “Yarg” << endl; } = 0; };

7. Why? In our ECS, I want to have a collection of Component pointers
But…they’re not actually pointing to Components
…instead they’re pointing to MeshComponents, etc.
Design Decision
What kind of collection: array (fixed size or not?), map, vector?
Duplicate component types?
A createXYZ method in GameObject vs. an addComponent(Component*) method.

8. Factories Another common design pattern
Used to create instances of another class
In C++
Could be
a separate class (e.g. MeshComponentFactory – probably a Singleton)
a static method (e.g. in the MeshComponent class)
Advantages:
Can optimize memory allocations
[side-topic]: memory fragmentation and new operator
Isolates code to create instances of that class to one spot
(for us) could update all components “in parallel”