Abstract Classes & Interfaces · C++ OOP

Pure virtual functions

Sometimes a base class cannot sensibly implement a function. What is the area of a generic "Shape"? Declare it pure virtual with = 0: it has no body in the base, and every concrete derived class must override it.

C++

virtual double area() const = 0;   // pure virtual

Abstract classes

A class with at least one pure virtual function is an abstract class:

You cannot create objects of it: Shape s; is a compile error.
You can have pointers and references to it.
A derived class that does not override every pure virtual function is abstract too.

C++

#include <iostream>
#include <cmath>
using namespace std;

class Shape {                                    // abstract base class
public:
    virtual double area() const = 0;
    virtual double perimeter() const = 0;
    virtual const char *name() const = 0;
    void describe() const {                      // concrete function using the abstract ones
        cout << name() << ": area " << area() << ", perimeter " << perimeter() << endl;
    }
    virtual ~Shape() = default;
};

class Circle : public Shape {
    double r;
public:
    explicit Circle(double r) : r(r) {}
    double area() const override { return M_PI * r * r; }
    double perimeter() const override { return 2 * M_PI * r; }
    const char *name() const override { return "Circle"; }
};

class Square : public Shape {
    double s;
public:
    explicit Square(double s) : s(s) {}
    double area() const override { return s * s; }
    double perimeter() const override { return 4 * s; }
    const char *name() const override { return "Square"; }
};

int main() {
    // Shape s;              // error: cannot declare variable 's' to be of abstract type
    Circle c(1);
    Square q(2.5);
    const Shape &a = c, &b = q;
    a.describe();
    b.describe();
    return 0;
}

Output

Circle: area 3.14159, perimeter 6.28319
Square: area 6.25, perimeter 10

Try it Yourself

Interfaces

C++ has no interface keyword (Java does). An interface in C++ is simply an abstract class with only pure virtual functions and a virtual destructor. A class can implement several interfaces through multiple inheritance:

C++

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

class Printable { public: virtual void print() const = 0; virtual ~Printable() = default; };
class Payable   { public: virtual double amountDue() const = 0; virtual ~Payable() = default; };

class Invoice : public Printable, public Payable {
    string client;
    double hours, rate;
public:
    Invoice(string c, double h, double r) : client(c), hours(h), rate(r) {}
    double amountDue() const override { return hours * rate; }
    void print() const override { cout << "Invoice for " << client << ": Rs " << amountDue() << endl; }
};

void printAll(const Printable *items[], int n) { for (int i = 0; i < n; i++) items[i]->print(); }

int main() {
    Invoice a("Acme Labs", 12, 1500), b("Nova Foods", 4.5, 2000);
    const Printable *docs[] = {&a, &b};
    printAll(docs, 2);
    const Payable &p = a;
    cout << "Total due from Acme: " << p.amountDue() << endl;
    return 0;
}

Output

Invoice for Acme Labs: Rs 18000
Invoice for Nova Foods: Rs 9000
Total due from Acme: 18000

Try it Yourself

final

final stops further overriding or inheritance:

C++

class Base { virtual void f(); };
class Mid : public Base { void f() override final; };   // no class may override f again
class Leaf final : public Mid { };                       // no class may inherit from Leaf

Abstract class vs concrete class

Abstract class	Concrete class
has at least one pure virtual function	all functions have bodies
cannot be instantiated	can create objects
defines a contract for derived classes	provides a full implementation
used through pointers/references	used directly or through pointers

Test yourself

4 questions · pick an answer to check it instantly

1
What makes a class abstract?
2
Which statement about an abstract class Shape is true?
3
A class derives from an abstract class but overrides only some of its pure virtual functions. It is...
4
How is an "interface" usually written in C++?

#Pure virtual functions

#Abstract classes

#Interfaces

#final

#Abstract class vs concrete class