Key Concepts:

Here we are going to briefly look at some of the C++ code for the advanced tutorial about creating a geometry library.

First we look at the shape class:

#pragma once
#ifndef _H_SHAPE
#define _H_SHAPE

class shape {
protected:
	int width, height;

public:
	shape() {};
	~shape() {};

	void set_values(int a, int b);

	virtual int area() = 0;

};

#endif

The shape class as a class constructor: shape() and destructor ~shape() which creates and destroys a new shape object.

We have one function : set_values that takes 2 input arguments. We also have an area function which we are setting to 0. This will become a virtual member function.

This example really brings out object oriented programming. We are creating a shape class that will have children rectangle and triangle.

The #pragma once and #if statements just protect the class from only being compiled once by the c++ compiler.

Now let's look at the shape.cpp source file:

#include "shape.h"

void shape::set_values(int a, int b)
{
	width = a; height = b;
}

First we include the proper header file. Then provide the functionality for the set_value function which sets the class variables from the two inputs to the function.

Now that we have shapes, we can create different kinds of shapes. So let us now create a rectangle:

#ifndef _H_RECTANGLE
#define _H_RECTANGLE

#include "shape.h"

class rectangle : public shape {
public:
	rectangle();
	rectangle(int a, int b);
	~rectangle() {};
	int area();
};

#endif

Here we tell the rectangle class to inherit all the functionality of the shape class. Now, inside the rectangle class we will provide the functionality for the area, since each shape has a unique area calculation.

#include "rectangle.h"

rectangle::rectangle()
{
	width = height = 0;
}

rectangle::rectangle(int a, int b)
{
	width = a; height = b;
}

int rectangle::area()
{
	return width*height;
}

The area calculation is what we expect: width multiplied by the height.

We do something similar for the triangle, expect the triangle has a unique area calculation.

The tester project is our main entry point to this example. The tester will provide some example how to test the geometry library for us.

#include <iostream>
#include <vector>
#include "rectangle.h"
#include "triangle.h"
#include "shape.h"

 
int main(int argc, char *argv[]){
   std::cout << "Hello Geometry Library Tester!" << std::endl;

   //Example of creating two objects and looking at area.
   shape *rect = new rectangle();
   shape *trgl = new triangle();

   rect->set_values(4, 5);
   trgl->set_values(4, 5);
   
   std::cout << rect->area() << '\n';
   std::cout << trgl->area() << '\n';
   std::cout << std::endl;

   delete rect;
   delete trgl;

   rect = NULL;
   trgl = NULL;

   //Example of creating vector of 'shapes' and looking at area.
   std::vector<shape *> myshapes;

   myshapes.push_back(new rectangle(4,5));
   myshapes.push_back(new triangle(4, 5));

   for(int i =0; i<myshapes.size(); i++)
	   std::cout << myshapes[i]->area() << '\n';

   std::cout << std::endl;

   // Just another example of a loop structure
   for (auto elem : myshapes)
	   std::cout << elem->area() << '\n';

   for (auto elem : myshapes)
   {
	   delete elem;
	   elem = NULL;
   }

   // all three prints should return same answers

   return 0;
}

The tester creates two shape objects of different types of shapes (lines 12-13).

   shape *rect = new rectangle();
   shape *trgl = new triangle();

This is useful if we ever want to create a vector (or array, list, etc) of shapes and we want to run an operation (like area, but could be intersection, color, draw, etc.) on every shape in the vector (line 29-32). I provide a 2nd way to create the different shapes:

   std::vector<shape *> myshapes;

   myshapes.push_back(new rectangle(4,5));
   myshapes.push_back(new triangle(4, 5));

The rest of the code provides various means to test the area, printing the output to the terminal and cleaning up the memory.