Classes and OOP · Lesson 16 of 21

Inheritance and virtual functions

One class building on another, polymorphism, and why destructors must be virtual.

12 minutes2 games

class Quiz : public Lesson means Quiz gets everything in Lesson and adds more. The real power is polymorphism: a pointer or reference of the parent type can refer to any child, and calling a virtual function runs the version of the actual type.

C++

#include <iostream>
#include <memory>
#include <string>
#include <vector>

class Lesson {
public:
    explicit Lesson(std::string title) : title_(std::move(title)) {}
    virtual ~Lesson() = default;
    virtual int minutes() const = 0;            // pure virtual: every lesson must say
    std::string describe() const { return title_ + " (" + std::to_string(minutes()) + " min)"; }
protected:
    std::string title_;
};

class Video : public Lesson {
public:
    Video(std::string t, int seconds) : Lesson(std::move(t)), seconds_(seconds) {}
    int minutes() const override { return (seconds_ + 59) / 60; }
private:
    int seconds_;
};

class Quiz : public Lesson {
public:
    Quiz(std::string t, int questions) : Lesson(std::move(t)), questions_(questions) {}
    int minutes() const override { return questions_ * 2; }
private:
    int questions_;
};

int main() {
    std::vector<std::unique_ptr<Lesson>> course;
    course.push_back(std::make_unique<Video>("Pointers", 610));
    course.push_back(std::make_unique<Quiz>("Pointers quiz", 5));

    int total = 0;
    for (const auto& l : course) {
        std::cout << l->describe() << "\n";
        total += l->minutes();
    }
    std::cout << "total " << total << " min\n";
    return 0;
}

Lesson (abstract)
- virtual int minutes() const = 0
- Video : minutes() = ceil(seconds / 60)
- Quiz : minutes() = questions × 2

describe() is written once in the parent, yet each child reports its own time.

virtual in the parent lets children replace the function.
override in the child makes the compiler check you're really replacing one (a typo in the name or const is reported).
= 0 makes the function pure virtual and the class abstract: you can't create one, only inherit from it.

What's the output?

#include <iostream>
struct A {
    void hi() const { std::cout << "A"; }
    virtual void bye() const { std::cout << "a"; }
    virtual ~A() = default;
};
struct B : A {
    void hi() const { std::cout << "B"; }
    void bye() const override { std::cout << "b"; }
};
int main() {
    B b;
    const A& r = b;
    r.hi();
    r.bye();
    std::cout << "\n";
}

What will this code print?

Exercise

Finish Circle and Square so each overrides area(). The program must print 28.26 then 16.

Expected output:

28.26
16

C++

#include <iostream>
#include <memory>
#include <vector>

struct Shape {
    virtual ~Shape() = default;
    virtual double area() const = 0;
};

struct Circle : Shape {
    double r;
    explicit Circle(double radius) : r(radius) {}
};

struct Square : Shape {
    double side;
    explicit Square(double s) : side(s) {}
};

int main() {
    std::vector<std::unique_ptr<Shape>> shapes;
    shapes.push_back(std::make_unique<Circle>(3));
    shapes.push_back(std::make_unique<Square>(4));
    for (const auto& s : shapes) std::cout << s->area() << "\n";
    return 0;
}

Learn more from

Played 0 of 2 games

#include <iostream> #include <memory> #include <string> #include <vector> class Lesson { public: explicit Lesson(std::string title) : title_(std::move(title)) {} virtual ~Lesson() = default; virtual int minutes() const = 0; // pure virtual: every lesson must say std::string describe() const { return title_ + " (" + std::to_string(minutes()) + " min)"; } protected: std::string title_; }; class Video : public Lesson { public: Video(std::string t, int seconds) : Lesson(std::move(t)), seconds_(seconds) {} int minutes() const override { return (seconds_ + 59) / 60; } private: int seconds_; }; class Quiz : public Lesson { public: Quiz(std::string t, int questions) : Lesson(std::move(t)), questions_(questions) {} int minutes() const override { return questions_ * 2; } private: int questions_; }; int main() { std::vector<std::unique_ptr<Lesson>> course; course.push_back(std::make_unique<Video>("Pointers", 610)); course.push_back(std::make_unique<Quiz>("Pointers quiz", 5)); int total = 0; for (const auto& l : course) { std::cout << l->describe() << "\n"; total += l->minutes(); } std::cout << "total " << total << " min\n"; return 0; }

#include <iostream> struct A { void hi() const { std::cout << "A"; } virtual void bye() const { std::cout << "a"; } virtual ~A() = default; }; struct B : A { void hi() const { std::cout << "B"; } void bye() const override { std::cout << "b"; } }; int main() { B b; const A& r = b; r.hi(); r.bye(); std::cout << "\n"; }

#include <iostream> #include <memory> #include <vector> struct Shape { virtual ~Shape() = default; virtual double area() const = 0; }; struct Circle : Shape { double r; explicit Circle(double radius) : r(radius) {} }; struct Square : Shape { double side; explicit Square(double s) : side(s) {} }; int main() { std::vector<std::unique_ptr<Shape>> shapes; shapes.push_back(std::make_unique<Circle>(3)); shapes.push_back(std::make_unique<Square>(4)); for (const auto& s : shapes) std::cout << s->area() << "\n"; return 0; }