Step 3 of 9

Abstract classes

Sometimes the base class can't give a sensible default. What's the area of "a shape"? There isn't one: only circles, squares and triangles have areas. So the base declares the function but provides no body, by writing = 0. That's a pure virtual function.

A class with at least one pure virtual function is abstract: you can't create an object of it, only of classes derived from it that implement every pure virtual function. An abstract class with only pure virtual functions is an interface: a promise of what every derived class can do.

#include <iostream>
#include <string>

class Payment {
public:
    virtual double fee(double amount) const = 0;      // no body: must be overridden
    virtual std::string name() const = 0;
    virtual ~Payment() = default;
};

class Card : public Payment {
public:
    double fee(double amount) const override { return 0.30 + amount * 0.029; }
    std::string name() const override { return "card"; }
};

class Transfer : public Payment {
public:
    double fee(double) const override { return 1.0; }  // flat fee; parameter unused
    std::string name() const override { return "transfer"; }
};

void quote(const Payment& p, double amount) {
    std::cout << p.name() << ": " << p.fee(amount) << "\n";
}

int main() {
    quote(Card{}, 100);
    quote(Transfer{}, 100);
    // Payment p;   // error: cannot declare variable 'p' to be of abstract type
}

card: 3.2
transfer: 1

How it works

• = 0 makes a function pure virtual. Payment itself can never be created.
• Card and Transfer implement both pure virtual functions, so they're concrete classes. If a derived class missed one, it would be abstract too, and creating it would be a compile error that names the missing function.
• quote works with any payment method, including ones written years later. That's the point of an interface.
• double fee(double) const leaves the parameter unnamed because it isn't used. That avoids an "unused parameter" warning.

Your task

Each shape stores its own dimensions and implements area(): a circle is pi times r squared, a rectangle is width times height. Remember const and override, and use the value of pi given.

Your turn: write Circle (radius) and Rect (width, height) implementing area(). Use 3.14159265358979 for pi.

Previous: virtual and override Next: Polymorphic containers