Understanding Virtual Functions and Inheritance in C++: A Guide to Creating a Square Class, Slides of Construction

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• Introduction to C++: Part

Tutorial Outline: Part 3

 Defining Classes

 Class inheritance

 Public, private, and protected access

 Virtual functions

Using Eclipse

 An IDE is very useful for setting up code that follows patterns and configuring the build

system to compile them.

 This saves time and effort for the programmer.

 Right-click on the Basic_Rectangle project and choose New  Class

 Give it the name Rectangle and click the Finish button.  Open the new files Rectangle.h and Rectangle.cpp

Default declared methods

 Rectangle();

 A constructor. Called when an object of this class is

created.

 ~Rectangle();

 A destructor. Called when an object of this class is

removed from memory, i.e. destroyed.

 Ignore the virtual keyword for now.

Rectangle.cpp

Header file included Class_name:: pattern indicates the method declared in the header is being implemented in code here. Methods are otherwise regular functions with arguments () and matched curly braces {}.

Encapsulation

 Bundling the data and area calculation for a rectangle into a

single class is an example of the concept of encapsulation.

The code for the two methods is needed

 Right-click in the Rectangle.h

window and choose

SourceImplement Methods

Fill in the methods

 Member variables can be accessed as though they were passed to the method.

 Methods can also call each other.

 Fill in the Area() method and then write your own ScaledArea(). Don’t forget to compile!

 Step 1: add some comments.

 Step 2: add some code.

Using the new class

 Open Basic_Rectangle.cpp

 Add an include statement for

the new Rectangle.h

 Create a Rectangle object

and call its methods.

 We’ll do this together…

Construction and Destruction

 The constructor is called when an

object is created.

 This is used to initialize an object:

 Load values into member variables

 Open files

 Connect to hardware, databases,

networks, etc.

 The destructor is called when an

object goes out of scope.

 Example:

 Object c1 is created when the

program reaches the first line of

the function, and destroyed when

the program leaves the function.

void function () {

ClassOne c1 ;

When an object is instantiated…

 The rT object is created in memory.

 When it is created its constructor is called to

do any necessary initialization.

 The constructor can take any number of

arguments like any other function but it

cannot return any values.

 What if there are multiple constructors?

 The compiler follows standard function overload rules. Note the constructor

has no return type!

Member Initialization Lists

 Syntax:

MyClass (int A , OtherClass & B , float C ): m_A ( A ), m_B ( B ), m_C ( C ) { /* other code can go here */ }

Colon goes here

Members assigned

and separated with

commas. The order

doesn’t matter.

Additional code can be

added in the code

block.

And now use both constructors

 Both constructors are now used.

The new constructor initializes the

values when the object is created.

 Constructors are used to:

 Initialize members

 Open files

 Connect to databases

 Etc.

#include

using namespace std ;

#include "Rectangle.h“

int main (int argc, char** argv)

Rectangle rT ;

rT. m_width = 1.0 ;

rT. m_length = 2.0 ;

cout << rT. Area () << endl ;

Rectangle rT_2 ( 2.0 , 2.0 ) ;

cout << rT_2. Area () << endl ;

return 0 ;