Welcome to the lesson on class composition in C++. You have now learned the basics of classes, methods, constructors, and encapsulation. Today, we are going to take a step further into object-oriented programming by exploring class composition. The goal of this lesson is to understand how to construct complex classes by combining simpler, reusable components.

Class composition is an essential design principle that enables you to create complex objects. We will learn how to build classes containing other objects and see practical applications of this approach. Let's get started!

What is Composition? A real-life analogy would be a car. A car comprises various parts, such as an engine, wheels, and a transmission. These parts work together to make the car function. Similarly, in programming, composition allows you to build a Car class that includes an Engine class, a Wheel class, and so on.

Let's start with a simple example to understand class composition:

C++
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1#include <iostream>
2
3class Engine {
4public:
5    void start() {
6        std::cout << "Engine started";
7    }
8};
9
10class Car {
11private:
12    Engine engine;  // Car has an Engine
13public:
14    void start() {
15        engine.start();               // Start the engine first
16        std::cout << " Car started";
17    }
18};
19
20int main() {
21    Car myCar;
22    myCar.start();  // Outputs: Engine started Car started
23    return 0;
24}

In this example, the Car class contains an Engine object. Here's what's happening:

1. The Engine class has a method called start(), which prints "Engine started".
2. The Car class has a private data member engine of type Engine.
3. The Car class also has a start() method that calls the Engine's start() method before printing "Car started".

When you create a Car object and call its start() method, it first starts the engine and then starts the car, showing how the Car uses functionality provided by the Engine.

Now let's look at a more advanced example, where a Car is composed of both Engine and Wheel objects:

C++
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1#include <iostream>
2
3class Engine {
4private:
5    int horsepower;   // Engine has a horsepower attribute
6public:
7    Engine(int hp) : horsepower(hp) {}  // Constructor initializes horsepower
8    int getHorsepower() const {
9        return horsepower;
10    }
11};
12
13class Wheel {
14private:
15    int size;  // Wheel has a size attribute
16public:
17    Wheel(int s) : size(s) {}  // Constructor initializes size
18    int getSize() const {
19        return size;
20    }
21};

In this example, the Car class will have both Engine and Wheel objects. Here are some key points:

1. The Engine class has a private attribute horsepower and a method getHorsepower() to retrieve it.
2. The Wheel class has a private attribute size and a method getSize() to retrieve it.

Continuing from the previous snippet, we now construct the Car class:

C++
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1class Car {
2private:
3    Engine engine;
4    Wheel wheels[4];  // Composition with multiple instances of a component
5public:
6    // Constructor initializes Engine and Wheel objects
7    Car(int hp, int ws) : engine(hp), wheels{Wheel(ws), Wheel(ws), Wheel(ws), Wheel(ws)} {}
8
9    void displaySpecifications() const {
10        std::cout << "Engine: " << engine.getHorsepower() << " HP\n";
11        std::cout << "Wheel size: " << wheels[0].getSize() << " inches\n";
12    }
13};
14
15int main() {
16    Car myCar(300, 18);
17    myCar.displaySpecifications();  // Outputs: Engine: 300 HP \n Wheel size: 18 inches
18    return 0;
19}

In this example:

1. The Car class contains an Engine object and an array of four Wheel objects.
2. The constructor of the Car class initializes the Engine and Wheel objects.
3. The displaySpecifications() method in the Car class prints the engine horsepower and wheel size.

This approach shows how you can aggregate multiple simpler objects to build a complex object like a Car that consists of an Engine and multiple Wheels.

There are several advantages to using class composition:

1. Reusability: You can reuse simple, well-defined classes to build larger, more complex classes without duplicating code.
2. Maintainability: Since each class is focused on a single responsibility, it is easier to understand, test, and maintain.
3. Flexibility: You can easily change the behavior of a complex object by modifying its components or substituting different components.

Using our car example, suppose you want to upgrade the car's engine or change the wheel size. With composition, you can make these changes without affecting the other parts of the car.

In this lesson, we explored the concept of class composition in C++. We learned how to combine simple classes to create complex ones, as demonstrated with our Car, Engine, and Wheel examples. Composition helps in building maintainable, flexible, and reusable code by allowing the construction of complex objects from simpler components.

Next, you will move to the practice section where you will have the opportunity to apply what you've learned by creating your own classes and composing them. This hands-on experience will solidify your understanding and help you become more proficient in using composition in your programming projects. Happy coding!