oops · high

Composition vs Inheritance (IS-A vs HAS-A)

The most common OOP design question: when to inherit and when to compose. IS-A vs HAS-A, the fragile-base-class coupling of inheritance, why 'prefer composition over inheritance' is the default advice, and the Liskov rule for when inheritance is actually correct.

The code

// IS-A  (inheritance): a Car IS-A Vehicle
struct Vehicle {
    void start();
};
struct Car : public Vehicle {}; // inherits start()

// HAS-A (composition): a Car HAS-A Engine
struct Engine {
    void run();
};
struct Car2 {
    Engine engine; // owns an Engine
    void start() {
        engine.run();
    } // delegates to it
}; // swap Engine freely

What this lesson walks through

1. 01Two ways to reuse: IS-A vs HAS-A
2. 02Inheritance couples tight; composition stays loose
3. 03Prefer composition — inherit only for a true IS-A

Inheritance says IS-A: a Car IS-A Vehicle, inheriting its interface. Composition says HAS-A: a Car HAS-A Engine and delegates to it. Both reuse code — but they couple very differently.