TUTORIAL · STEP 5 OF 7

Inheritance

A class hierarchy keeps its shape in JavaScript: a derived object goes wherever its base class is expected.

Circle and Square derive from Shape, whose methods are virtual. describe takes a const Shape &; handed a Circle, its call to area() runs Circle::area(), as it would in C++.

instanceof follows the hierarchy too: a Circle is a Module.Shape. Single inheritance with virtual methods binds; multiple inheritance does not.

Try it

Add a Triangle with a base and a height, and describe one from main.js.

#pragma once
 
#include <string>
 
// A Circle or a Square goes wherever a Shape is expected,
// and a virtual call runs the derived class's code.
class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;
    virtual std::string name() const = 0;
};
 
class Circle : public Shape {
public:
    explicit Circle(double radius);
    double area() const override;
    std::string name() const override;
 
private:
    double radius;
};
 
class Square : public Shape {
public:
    explicit Square(double side);
    double area() const override;
    std::string name() const override;
 
private:
    double side;
};
 
std::string describe(const Shape &shape);
 
 
#pragma once

#include <string>

// A Circle or a Square goes wherever a Shape is expected,
// and a virtual call runs the derived class's code.
class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;
    virtual std::string name() const = 0;
};

class Circle : public Shape {
public:
    explicit Circle(double radius);
    double area() const override;
    std::string name() const override;

private:
    double radius;
};

class Square : public Shape {
public:
    explicit Square(double side);
    double area() const override;
    std::string name() const override;

private:
    double side;
};

std::string describe(const Shape &shape);

Runs without a compile until you edit the C++.

Console

Press Run, or Ctrl+Enter, to run main.js.