We cover a lot in this tutorial. We cover Polymorphism, Structs, Friend Classes, Abstract Classes, Override, Final, Virtual and much more.

All of the code used follows the video below. For best results print it and take notes as you watch. Please leave any questions that you have. I hope this tutorial clears up Polymorphism for everyone.

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Code & Transcript

// ---------- C++ TUTORIAL 11 ----------

// ---------- STRUCT EXAMPLE ----------

#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>
#include <ctime>
#include <numeric>
#include <cmath>

// Classes have default private fields and methods
// while structs have public
// Structs are used to model new data types, while
// classes model more complex real world objects
struct Shape{
    // Variables are public by default
    double length, width;
    
    // Constructors are public by default
    Shape(double l = 1, double w = 1){
        length = l;
        width = w;
    }
    
    // Structs can contain functions
    double Area(){
        return length * width;
    }
    
    // Structs can contain private members
private:
    int id;
};

// You can inherit from a struct
struct Circle : Shape{
    // Override the constructor
    // You also use this with structs
    Circle(double width){
        this->width = width;
    }
    
    // Override Area()
    double Area(){
        return 3.14159 * std::pow((width / 2), 2);
    }
};
 
int main()
{
    // Create a struct
    Shape shape(10, 10);
    
    // Call a Struct function
    std::cout << "Square Area : " << shape.Area() 
            << "\n";
    
    // Create a struct
    Circle circle(10);
    
    // Call a Struct function
    std::cout << "Circle Area : " << circle.Area() 
            << "\n";
    
    // You can initialize a struct or class using
    // an aggregate
    Shape rectangle{10,15};
    std::cout << "Rectangle Area : " << 
            rectangle.Area() << "\n";
    
    return 0;
}

// ---------- END STRUCT EXAMPLE ----------

// ---------- FRIEND CLASS EXAMPLE ----------

#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>
#include <ctime>
#include <numeric>
#include <cmath>

// Private members of a class are not accessable
// to outside code unless a class is marked as
// a friend
class Customer{
private:
    friend class GetCustomerData;
    std::string name; 
public:
    Customer(std::string name){
        this->name = name;
    }
};

class GetCustomerData{
public:
    static std::string GetName(Customer& customer){
        return customer.name;
    }
};

 
int main()
{
    Customer tom("Tom");
    GetCustomerData getName;
    std::cout << "Name : " << 
            getName.GetName(tom) << "\n";
    
    return 0;
}

// ---------- END FRIEND CLASS EXAMPLE ----------

// ---------- POLYMORPHISM EXAMPLE ----------

// Polymorphism is a feature in which similar 
// objects can be treated the same, but also
// utilize their differences automatically
// for any methods marked as virtual

class Shape{
protected:
    double height;
    double width;
public:
    // Assume that if only 1 value is passed
    // that height and width are equal
    Shape(double length){
        height = length;
        width = length;
    }
    Shape(double h, double w){
        height = h;
        width = w;
    }
    virtual double Area(){
        return height * width;
    }
};

class Circle : public Shape{
public:
    Circle(double w) :
    Shape(w){

    }
    
    // Override Area()
    double Area(){
        return 3.14159 * std::pow((width / 2), 2);
    }
};

// This function receives Shapes but uses the 
// correct Area() automatically
void ShowArea(Shape& shape){
    std::cout << "Area : " << shape.Area() << "\n";
}
 
int main()
{
    Shape square(10,5);
    Circle circle(10);
    ShowArea(square);
    ShowArea(circle);
    
    return 0;
}

// ---------- END POLYMORPHISM EXAMPLE ----------

// ---------- ABSTRACT CLASS / OVERRIDE EXAMPLE ----------

// Shape here has no purpose except for
// being able to to group similar objects
// so it should be instead an Abstract
// Class

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

class Circle : public Shape{
protected:
    double width;
public:
    Circle(double w){
        width = w;
    }
    
    // Override Area()
    // You should use override to force the
    // compiler to check if the base class
    // virtual fucntion is the same as
    // the subclass
    double Area() override{
        return 3.14159 * std::pow((width / 2), 2);
    }
};

class Rectangle : public Shape{
protected:
    double height, width;
public:
    Rectangle(double h, double w){
        height = h;
        width = w;
    }
    // Override Area()
    // Marking a method as final means
    // that subclasses that inherit from
    // Rectangle can't override Area()
    double Area() override final{
        return height * width;
    }
};

class Square : public Rectangle{
public:
    Square(double h, double w) :
    Rectangle(h,w){
        
    }
    
    /* This would trigger an error
    double Area() override{
        return height * 2;
    }
    */
};

// This function receives Shapes but uses the 
// correct Area() automatically
void ShowArea(Shape& shape){
    std::cout << "Area : " << shape.Area() << "\n";
}
 
int main()
{
    Rectangle rectangle(10,5);
    Circle circle(10);
    ShowArea(rectangle);
    ShowArea(circle);
    
    Square square(10,10);
    ShowArea(square);
    
    return 0;
}

// ---------- END ABSTRACT CLASS / OVERRIDE EXAMPLE ----------

// ---------- END C++ TUTORIAL 11 ----------