Abstract Classes

Sometimes the parent can't give a sensible default. What is the area of “a shape”? We don't know until we know which shape. Write = 0 to make the method pure virtual: it has no body, and every child must provide one:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

class Shape {
public:
    virtual double area() const = 0;        // pure virtual
    virtual string name() const = 0;
    virtual ~Shape() {}
};

class Square : public Shape {
    double side;
public:
    Square(double s) : side(s) {}
    double area() const override { return side * side; }
    string name() const override { return "Square"; }
};

class Circle : public Shape {
    double r;
public:
    Circle(double radius) : r(radius) {}
    double area() const override { return 3.14159 * r * r; }
    string name() const override { return "Circle"; }
};

int main() {
    Square s(3);
    Circle c(1);
    vector<const Shape*> shapes = {&s, &c};
    for (const Shape* sh : shapes) {
        cout << sh->name() << ": " << sh->area() << "\n";
    }
    return 0;
}

Output

Square: 9
Circle: 3.14159

A class with at least one pure virtual method is abstract. You can't make an object of it directly – it only exists to be inherited from:

class Shape {
public:
    virtual double area() const = 0;
};

int main() {
    Shape s;
    return 0;
}

Output (the program does not compile)

main.cpp:7:11: error: variable type 'Shape' is an abstract class
    Shape s;
          ^
main.cpp:3:20: note: unimplemented pure virtual method 'area' in 'Shape'
    virtual double area() const = 0;
                   ^

Remember: An abstract class is a contract: “anything that wants to be a Shape must be able to tell me its area and its name.” Code that uses shapes only needs to know the contract.

Tip: In other languages this idea is called an interface. In C++ an interface is just an abstract class whose methods are all pure virtual.