Composition vs Aggregation - heshawacooray/OOP-Heshawa GitHub Wiki

Definition: Composition and Aggregation are both types of object relationships in object-oriented programming. They describe how objects can be related to other objects.

  • Composition: A strong "has-a" relationship where the child object cannot exist without the parent object. When the parent object is destroyed, all its child objects are also destroyed.
  • Aggregation: A weaker "has-a" relationship where the child object can exist independently of the parent object. The parent object does not own the child object, and destroying the parent does not destroy the child.
Feature Composition Aggregation
Ownership The parent object owns the child object The parent object does not own the child object
Lifetime Child object’s lifetime is tied to the parent Child object’s lifetime is independent
Destruction When the parent is destroyed, child objects are also destroyed Child objects can continue to exist even after the parent is destroyed
Relationship Type Strong relationship Weak relationship

Example code:

#include <iostream>
using namespace std;

// Composition: Car owns Engine
class Engine {
public:
    Engine() {
        cout << "Engine created" << endl;
    }
    ~Engine() {
        cout << "Engine destroyed" << endl;
    }
};

class Car {
private:
    Engine engine; // Composition: Engine is part of the Car

public:
    Car() {
        cout << "Car created" << endl;
    }
    ~Car() {
        cout << "Car destroyed" << endl;
    }

    void start() {
        cout << "Car is starting..." << endl;
    }
};

int main() {
    Car car; // When the car is created, the engine is created
    car.start();
    return 0; // When car goes out of scope, both car and engine are destroyed
}
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