Lesson 152 +10 XP

Pure Virtual Functions and Abstract Classes

Sometimes a base class's job is to define the shape of a method without supplying the body. Every derived class must fill it in. For that, C++ gives pure virtual functions and abstract classes.

Pure virtual syntax

A pure virtual function is a virtual function with the text = 0 instead of a body:

class Animal {
public:
  virtual void sound() = 0;   // pure virtual
};

Note: virtual void sound() = 0; - the = 0 signals that the base provides no implementation.

The class becomes abstract

Any class with at least one pure virtual function is abstract. You cannot create an object of it:

Animal a;   // ERROR: cannot instantiate an abstract class

You can still hold a pointer or reference to it, which keeps polymorphism alive.

Derived classes must override

A concrete derived class must implement every pure virtual function. If it skips one, the derived class is abstract too:

class Dog : public Animal {
public:
  void sound() override { cout << "Woof"; }   // now Dog is concrete
};

Once sound() is supplied, Dog can be instantiated.

Interface classes

A class that is all pure virtual functions is often called an interface class. It is a contract: every class that inherits it must provide the required behaviors. The compiler enforces the contract.

TL;DR

virtual void f() = 0; declares a pure virtual function.
It has no body; the = 0 marks it pure.
The class becomes abstract and cannot be instantiated.
Derived classes must override every pure virtual function.
Interface classes are contracts of required behaviors.

Pure Virtual Functions and Abstract Classes