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CMT2050: Object Oriented Programming and Design

www.cs.mdx.ac.uk/staffpages/xiaohong/cmt2050.html

Data Polymorphism

Dr. Xiaohong Gao

Room 2C23,BG

Ext. 2252

E-mail: x.gao@mdx.ac.uk

(Week 5)

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1. Review of Lectures 1-4

• Program = class.h + class.cpp + classMain.cpp

• class =

private:

public:

data members

member functions

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Note: C++ Builder only runs through local spaces

• C drive ---- e.g., desktop

• E drive ---- e.g., zip disk

• A drive ---- e.g., floppy disk

But NOT through networked spaces (linking errors) such as

• H: drive --- your personal storage spaces

• L: drive --- lecturers’ spaces

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Copy your programs from H: to C (desktop):

Run C++ builder on the desktop

Before you logout, remember to copy your programs back to your H: drive to save them

Steps to run your program saved in H drive

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1.1 Some key words related to a class

• pre-processor -- #ifndef - #define - #endif

• const

• default constructor

• destructor

• dynamic memory allocation (pointer)

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class Email_Sent

{ private:

char name [40]; // the name of recipientchar message [120]; // message to the recipient

public:

void set_all(char* , char*);char * get_name();char * get_message();

};

1.2 Example: an Email_sent class keeps records the emails you have sent

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1.3 Exercises : modify the Email_Sent class to make sure:

1). all the data are initialised

2). “message” can be any length

3). “message” can not be changed before the “get_message”

4) “message” is a read-only file.

5) Email_Sent class is only included once in the main program

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void main() // A running problem{ Student student1; char aName[40],aCampus[40]; int newAge, newID; cout << "Input student's name:" << endl; cin.getline(aName,40);

cout << "input ID:” << endl; cin >> newID;

cin.ignore(256,'\n'); cout << "input student's campus:" << endl; cin.getline(aCampus,40);

student1.set_all(aName, aCampus,newID,18); student1.print_all();

getch(); return; }

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The deadline for coursework 2 is extended to Week 7

The deadline for coursework 1 is extended to Week 6

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2. The features that Object Oriented Programming Supports

2.1 Four data features are supported:

• Data encapsulation

• Data abstraction

• Data polymorphism

• Data inheritance

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2.2. Polymorphism

• Some definitions

• Poly ---- many (from Greek)

• Morph ---- Form (from Greek)

• Polymorphism ---- having many forms.

• polymorphism refers to many functions sharing the same name.

• In C++, polymorphism is realised using overloading functions.

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Example 1: function overloading (1) -- video.h

// video.h

#ifndef VIDEO_H //pre-processor

#define VIDEO_H

class Video

{

private:

char name[40];

char producer[40];

short quantity;

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Example 1: function overloading (2) -- video.h

public:

Video(); //Default constructor

void set_all(); //no arguments

void set_all(char *); //1 argument

void set_all(char*, char*); //2 arguments

void set_all(char*, char*, short); //3 arguments

void print();

char * get_name();

char * get_producer();

short get_quantity();

};

#endif

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2.3 Overloading

• Is used to achieve polymorphism.

• Function overloading ---- using the same name for several functions

• Signature ---- consists the function name and the argument list (but not function return type)

• Signature is used to identify which function is called.

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Example 1: function overloading (3) -- videoMain.cpp

#include <iostream.h>

#include “video.h”

int main()

{

Video vid1; vid1.set_all(); // Uses 0 argument function

vid1.print(); vid1.set_all("Titanic"); // Uses 1 argument function

vid1.print(); vid1.set_all("Titanic", "Cameron Co."); vid1.print(); // Uses 2 argument function

vid1.set_all("Titanic", "Cameron Co.", 56); vid1.print(); // Uses all 3 argument function

return 0;}

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Example 1: function overloading (4) -- output

// The output of the programme:

__ __ 0

Titantic __ 0

Titantic Cameron Co. 0

Titantic. Cameron Co. 56

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Example 1: function overloading (5) -- video.cpp (1)

#include "video.h"#include <iostream.h>#include <string.h>

// Default constructor, invoked with no arguments;

Video::Video() // initializes all data members to 0 values.{strcpy(name,"");strcpy(producer, ""); quantity = 0;}

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void Video::print() // Method - prints data members{ cout << name << " " << producer << " " << quantity; cout << endl; }

// Invoked with no arguments; assigns 0 values to all data members

void Video::set_all() { strcpy(name,""); strcpy(producer,""); quantity = 0; }

// video.cpp (2)

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// data member and assigns 0 values to the other data members. void Video::set_all(char* name_in) { strcpy(name, name_in); strcpy(producer, ""); //option quantity = 0; //option }

//0 to the quantity data member.

void Video::set_all(char* name_in, char* prod_in) { strcpy(name, name_in); strcpy(producer, prod_in); quantity = 0; //option }

// video.cpp (3)

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// Invoked with three arguments .

void Video::set_all(char* name_in, char* prod_in, short qty) { strcpy(name, name_in); strcpy(producer, prod_in); quantity = qty; } // Return name data member

const char* Video::get_name(){ return name;}

// video.cpp (4)

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// Returns producer data member

const char* Video::get_producer(){ return producer;}

// Return quantity data member

short Video::get_quantity(){ return quantity;}

// video.cpp (5)

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2.4 Default function arguments

• Instead of using 4 set_all methods in Example 4, a single method can be applied using default function arguments.

void set_all (char * = “”, char* = “”, short =0);

void set_all();

void set_all(char *);void set_all(char*, char*);void set_all(char*, char*, short);

• A single prototype and the implementation showing the values that can be defaulted.

• To cover the variations where parameters are defaulted.

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•Argument subset must be (disadvantages):

• in sequence

• continuous

• starting from left to right

• Advantage ---- less functions to program

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//video.h

void set_all (char* = “”, char* = “”, short =0);

//video.cpp

void Video::set_all(char* name_in, char* prod_in, short qty) { strcpy(name, name_in); strcpy(producer, prod_in); quantity = qty; }

Implementation of default argument function

space

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// video.h#ifndef VIDEO_H#define VIDEO_Hclass Video{

private:char name[40];char producer[40];short quantity;

public:Video();void set_all(char* = “”, char* =“” , short = 0);

void print();char * get_name();char * get_producer();short get_quantity();

}; #endif

Example 5 (1)

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//main.cpp

#include <iostream.h>

#include “video.h”

int main()

{

Video vid1; vid1.set_all(); // Uses 0 default argument values vid1.print(); vid1.set_all("Titanic"); // Uses 1 default argument value

vid1.print(); vid1.set_all("Titanic", "Cameron Co."); vid1.print(); // Uses 2 default argument values

vid1.set_all("Titanic", "Cameron Co.", 56); vid1.print(); // Uses all 3 default argument values

return 0;}

Example 5 (2)

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2.5 The Advantages of Function Overloading

• Giving meaningful names to functions.

• making an application programmer interface (API) easy to remember.

e.g., print in MsWord, MsPowerPoint, etc..

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2.6 Overloading Constructors

• A constructor is a special member function.

• No return values.

• Share the same name with the class.

• A constructor is called whenever an object is created.

• Default constructor ---- no arguments.

• Overloaded constructors:

Video();

Video(char*, char*, short);

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Example 2: overloading constructors (1)

class Classroom{private:

char campus[50];char room[50];float floorarea;

public:Classroom ( ); // DEFAULT CONSTRUCTORClassroom (char *, char *, float );

};

Classroom::Classroom ( ){

strcpy (campus, "");strcpy (room, "");floorarea = 0.0;

}

Classroom::Classroom (char *acampus, char *aroom, float anarea);{

strcpy (campus, acampus);strcpy (room, aroom);if (anarea >= 0.0)

floorarea = anarea;else

floorarea = 0.0;}

// classroom.h

// classroom.cpp

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Example 2: overloading constructors (2)

int main(){

Classroom clr1;Classroom clr2( “BG” , ”1C2” , 1);

return 0;}

// classroommain.cpp

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3. Differences between overloaded constructors and overloaded functions

• No return types for constructors

e.g., Video::Video(char * aname);

void Video::set_all(char *aname);

• The object can be declared by constrctors.

e.g., Video vid1(“ET”, “Unknown”, 30);

Video vid2;

vid2.set_all((“ET”, “Unknown”, 30);

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Summary:

1. Polymorphism

2. Function overloading

3. Constructor overloading