Pure Virtual Method

C++ pure virtual Method also known as pure virtual function is same as that of virtual method. The only difference here in Pure virtual function is we need not to write any function definition, only we have to declare it. Pure virtual method don't have a body, it will have = 0 in the end.

Syntax
virtual returnType methodName() = 0;

Before entering coding part, we need to be clear with Virtual Method Concept where we clearly studied a concept called Method Overriding. In Pure virtual method also same method overriding takes place.

NOTE
In c++, If a normal class having atleast one pure virtual function inside it than that class will be called as Abstract class.
Abstract class can have normal functions and variables along with a pure virtual function. If an Abstract Class has derived class, they must implement all pure virtual functions, or else they will become Abstract too. Abstract class cannot be instantiated, but pointers and references of Abstract class type can be created.

CODE

EG 1 . Creating an abstract class called Shape with 3 Pure virtual methods and inheriting it to a child class called Rectangle.

#include <iostream>
using namespace std;

class Shape {
  public:
    virtual double Area() = 0;
    virtual double Perimeter() = 0;
    virtual double Volume() = 0;
};

class Rectangle : public Shape {
  public:
    double length, width, height;
    double Area() {
        return length * width;
    }
    double Perimeter() {
        return 2 * (length+width);
    }
    double Volume() {
        return Area() * height;
    }
};

int main() {
    //UPCASTING
    Shape* s1 = new Rectangle();
}

CODE EXPLANATION
In the above code, we created an abstract class called Shape with 3 pure virtual methods called Area(), Perimeter(), Volume(). We created another normal class called Rectangle where we inherited Shape class. If we are inheriting abstract class to any class, that child class must override all the pure virtual functions present inside abstract class. In virtual function it is not mandatory, if child class wants to override it can override or we can leave as it is but in Pure virtual function it is mandatory that child class must override all pure virtual functions. Hence we overridden all the pure virtual function present inside Shape abstract class inside Rectangle child class.


Where to use Virtual function and Pure virtual function?
Consider the simple example of drawing shapes. Imagine circles, triangles, and squares. All are shapes, but each is subtly different. Let's take a bunch of different shapes and put them all together in one collection. Now let's say, “draw each shape.” As you go through the collection drawing the shapes, you need to ask, “What sort of shape is this one?” If all you knew was that it was a shape, how would you draw that? You need to know what kind of shape it is. Virtual functions essentially mark each instance of a class with its exact type. This allows the system to later identify what type it was.

Exercises
1. Use above Shape abstract class from EG 1 , Try to create multiple child classes like Circle, Triangle etc

(Click Here, For more exercises)