Has-A vs Is-A

The first design decision is whether a relationship is is-a or has-a.

Is-A Relationship

Use inheritance when one class is a specialized type of another class.

class Animal { };

class Dog : public Animal { };

This sentence is true:

A dog is an animal.

So public inheritance can make sense.

Has-A Relationship

Use composition or another object relationship when one class contains, uses, or refers to another class.

class Engine { };

class Car {
private:
    Engine engine;
};

This sentence is true:

A car has an engine.

This sentence is false:

A car is an engine.

So Car : public Engine would be bad design.

Complete Example

#include <iostream>
using namespace std;

class Engine {
public:
    void start() const {
        cout << "Engine started" << endl;
    }
};

class Car {
private:
    Engine engine;

public:
    void drive() const {
        engine.start();
        cout << "Car is driving" << endl;
    }
};

class Animal {
public:
    void breathe() const {
        cout << "Animal is breathing" << endl;
    }
};

class Dog : public Animal {
public:
    void bark() const {
        cout << "Dog is barking" << endl;
    }
};

int main() {
    Car car;
    car.drive();

    Dog dog;
    dog.breathe();
    dog.bark();

    return 0;
}

Quick Decision Test

Say the relationship out loud:

Viva Answer

Use inheritance for an is-a relationship where the derived class is a specialized form of the base class. Use composition or another object relationship for has-a or uses-a relationships, where one class owns, contains, refers to, or temporarily uses another object.

Quick Check

1. Is Car : public Engine good design?
2. What relationship does Dog : public Animal represent?
3. What question should you ask before using inheritance?