What is the difference between inheritance and composition?

The simple test: is a versus has a

Ask yourself whether the relationship is an is a or a has a. A Dog is an Animal, so inheritance fits. A Car has an Engine, so composition fits. Getting this test right is most of the answer.

Inheritance example

class Animal { public: void eat() {} };
class Dog : public Animal { };   // Dog is an Animal

class Animal { void eat() {} }
class Dog extends Animal { }   // Dog is an Animal

class Animal:
    def eat(self): ...
class Dog(Animal):   # Dog is an Animal
    pass

Composition example

class Engine { public: void start() {} };
class Car {
    Engine engine;                 // Car has an Engine
public:
    void start() { engine.start(); }
};

class Engine { void start() {} }
class Car {
    private Engine engine = new Engine();  // Car has an Engine
    void start() { engine.start(); }
}

class Engine:
    def start(self): ...
class Car:
    def __init__(self):
        self.engine = Engine()   # Car has an Engine
    def start(self):
        self.engine.start()

Why teams often prefer composition

• Flexibility: you can swap the inner object for another type at run time.
• Loose coupling: the outer class depends on behaviour, not on a rigid parent hierarchy.
• Avoids fragile base class problems where a parent change breaks many children.