Abstract Classes And Pure Virtual Functions

Pure Virtual Function

Pure Virtual Function are virtual functions that are assigned value of zero. They make for the characteristics of the abstract classes.

Abstract Class

Abstract Class is a class which contains at least one pure virtual function in it.

Syntax:

class AbstractClassName { // class body // pure virtual function: virtual funcType funcName(args) = 0; };

Due to the virtual functions in the abstract class:

• Instances of abstract classes cannot be created.
• If the child of abstract class doesn't extend all pure virtual functions in the parent, then instances of the child class cannot be created.

Let's look at the following example:

#include <iostream> using namespace std; class Vehicle { public: // pure virtual function: virtual double getInsuranceCost(int months) = 0; }; class Car : public Vehicle { public: // extending class that extends // the pure virtual function double getInsuranceCost(int months) { return months * 15; } }; class Truck : public Vehicle { // extending class that doesn't // extend the pure virtual function }; void main() { // create instances: Car carObj; // Vehicle vehicle; // Truck truck; cout << carObj.getInsuranceCost(12); }

Couple of things to notice:

• Both classes Car and Truck extend class Vehicle.
• If we uncomment lines 33 and 34 the the code won't compile. That's because objects of those classes cannot be created.
• If Truck class overwrites the pure virtual function from the parent class (like in the Car class), then Truck object can be created (by uncommenting line 34).