Composition in C++

What is Composition in C++?

• Composition in C++ is a design principle and a relationship between classes that represents a whole-part or contains association.
• It's a more specific form of aggregation where one class (the whole) is composed of one or more objects of another class (the parts).

Key Characteristics of Composition in C++

• Composition is a strong has-a relationship where one class contains another class as a fundamental part.
• Composition allows for encapsulating the behavior and properties of the composed objects within the containing object.

Composition in C++ Example

1#include <iostream>
2using namespace std;
3
4class Engine {
5public:
6    void start() {
7        cout << "Engine started." << endl;
8    }
9};
10
11class Car {
12public:
13    Car() {
14        // Composition: Car "has-a" Engine
15        engine.start();
16    }
17
18    void drive() {
19        cout << "Car is driving." << endl;
20    }
21
22private:
23    Engine engine; // Composition: Car contains an Engine
24};
25
26
27int main() {
28    Car myCar;
29    myCar.drive();
30
31    return 0;
32}

• Car is the containing class, and Engine is the composed class.
• The Car class contains an Engine object as a part of its structure, demonstrating composition.
• When a Car object is created, it automatically starts the engine.
• The Car class encapsulates the behavior of both the car and the engine.

Types of Object Composition

Aggregation

• Aggregation is a "has-a" relationship where one class contains another class as a part.
• It represents a weaker relationship than composition, as the contained objects can exist independently of the containing object.

Composition

• Composition is a strong "has-a" relationship where one class contains another class as a fundamental part.
• Examples include a car containing an engine, or a computer containing a motherboard.

Association

• Association represents a more loosely coupled relationship between classes.
• It indicates that two classes are related but does not specify how one class is contained within the other.
• Associations can be bi-directional, where both classes reference each other, or uni-directional, where one class knows about the other but not vice versa.
• Examples include a teacher having an association with students or a library book being associated with a borrower.

Composition vs Classification

Composition

• Composition refers to the relationship between classes where one class is composed of or contains another class as a part.
• Independence: Composed objects cannot exist independently of the containing object. They are tightly coupled.
• Examples: A car containing an engine, a computer containing a motherboard, or a house containing rooms are examples of composition.

Classification

• Classification refers to grouping objects into classes based on their common characteristics and behaviors.
• It is the process of categorizing and organizing similar objects under a common class.
• Independence: In classification, objects within the same class may share common characteristics, but they are independent instances. Each instance can exist on its own.
• Examples: In a programming context, classifying animals like "mammals," "birds," or "reptiles" based on their shared characteristics is an example of classification.

Conclusion