5. Basic Object Oriented Programming What is an Object?

1

What is an Object?• The real world is composed of different kinds of

objects: buildings, men, women, dogs, cars, etc.• Each object has its own states and behaviors.

Color = Red Brand = Ferrari Speed = 200 mph

Gear = 4States

BrakingAccelerating

Changing Gear

Steering

Behaviors

Computer Programming

Variables and functions

5. Basic Object Oriented Programming

2

What is a Software Object?• Software designers use the same idea to ease

programmers to develop their software• Software is also composed of different kind of

software objects• Each software object also has its own states and

behaviors.

Variables (States)Color = Grey

Size = 2cm x 2cm Shape = Rectangular

Method(Behavior)

(protruded)

Press( )


Button


3

Encapsulation• Hiding information within an object’s nucleus• Provide a public interface for interacting with it• Advantages to software developers

Modularity: An object can be easily passed aroundin the system (Otherwise, you need to think about how many files you need to bundle together to pass to your friends)

Information hiding: Users need not go into detailsof the object before using the object (E.g., you don’t need to know the circuit of a TV set if you want to watch TV)

Safety:Users of the object may not directly access the internal state of the object. This reduces the possibility of erroneous situations.


Library Reusability


4

Objects of class button

What is a Class?• A Class is a blueprint or prototype that defines

the variablesand methodscommon to all objects of a certain kind

• Every object of a Class is an instanceof that class• Benefit - Reusability

This arrangement saves effortin developing a number of objects of the same kind.

Usually objectsof a class are used many times in an application

Computer Programming5. Basic Object Oriented Programming

5

VariablesColor (e.g. Grey )

Size (e.g. 2cm x 2cm )Shape (e.g. Rectangular )

(protruded)

Method

The Button Class

Instance of Button Class

Instance of Button Class

Instantiation

Instantiation

Press( )


Attributes:AgeHeightWeight

Methods:Move

Attributes:AgeHeightWeight

Methods:Move

PersonPerson

Class

larrylarry

Attributes:Age = 54Height = 179 cmWeight = 67 kg

Methods:Move


Methods:Move

billbill


Methods:Move


Methods:Move

scottscott

Attributes:Age = 52Height = 182 cmWeight = 75kg

Methods:Move

Attributes:Age = 52Height = 182 cmWeight = 75kg

Methods:Move

stevesteve


Methods:Move


Methods:Move

Instances

Many Different Objects from a Single Class


6

7

Classes are implementations of Data Types

� A class is a programmer-defined data type and objects are variables of that type

– Just like in int x; , x is a variable of type int

� Class : in some sense, a data type that defines both data and functions

� When a data type has been created, programmers can use it without knowing how it is implemented internally

� Example: a date object might include operations for various display formats, comparing two dates, extracting the month, day, and year numbers, etc.

� A class generally contains private data and public operations (called member functions)


8

• To declare a class, use the class keyword as follows

• Declaring this class doesNOT allocate memory for a Cat

• Only tell the compiler what a Cat is, how big a Cat is (by member variables, e.g.itsAge , itsWeight ), what a Catwould do (by member functions, e.g.Meow() )

• Declaration of classes should be placed in the headerfile and included into your program.

Declaring Classes in C++

class Cat{

unsigned int itsAge; // Member variableunsigned int itsWeight; // Member variablevoid Meow(); // Member function

} ;


#include

Definition of Meow() should follow somewhere


9

• When a class is defined, we can further define the objectsof that class:Cat Frisky; // define one of the Cats call Frisky

• It states that we are going to handle a Cat called Frisky

• It is similar to declaring a number of the type integerint xyz; // define one of the integers call xyz

• Obviously, if one declares two cats as followsCat Frisky, Felix; // define two Cats

• Frisky is never the same as Felix , although they both belong to the class Cat , i.e. they are cats.

Declaring an Object of a Class


10

• When an object is defined, we can access the members of that object based on its class definition

• For example, if we want to know the weight of Frisky :unsigned int weight = Frisky . itsWeight;

// Get the weight of Frisky

• The operator ‘. ’ allows us to access the members of the object

• Similarly, if we want to know the age of Frisky :unsigned int age = Frisky.itsAge;

// Get the age of Frisky

• If we want to ask Frisky to meow:Frisky.Meow(); // Ask Frisky to execute meow()

Accessing Class Members


Variable and methods


11

• Never access directly to class

Cat.itsAge = 5; // Don’t do that! Cat is class

• It is because Cat is only a template of all cats. A different cat may have a different age

• If we want to assign the age of Frisky , we write:

Frisky.itsAge = 5; // Set the age of Frisky to 5

• Also, if the class doesn’t define a member, you can’t use it

Frisky.bark(); // Frisky has no bark() defined

Frisky.itsColor = 5; // Also error, because the class

// Cat does not have itsColor

Accessing Class Members


12

• Members can be divided into public members or privatemembers

• Private members of a class are those members that can only be accessed by methods of that class

• By default, all members are private

• Public members of a class are those members that can be accessed by other class objects and functions

• This mechanism provides the privacy or security to the information of a class that requires protection.

Private Versus Public


Typically, access private members by public methods.


13

#include <iostream> // for coutusing namespace std;class Cat // declare the class {

int itsAge;int itsWeight;

};

int main(){

Cat Frisky;Frisky.itsAge = 5; // assign to the member variablecout << "Frisky is a cat who is ";cout << Frisky.itsAge << " years old.\n";return 0;

}

Private Versus Public

An error is generated since by default itsWeight and itsAge

are private. They cannot be accessed even by main()


14

Private Versus Public#include <iostream> // for coutusing namespace std;class Cat // declare the class{public:

int itsAge;int itsWeight;

};

int main(){

Cat Frisky;Frisky.itsAge = 5; // assign to the member variablecout << "Frisky is a cat who is ";cout << Frisky.itsAge << " years old.\n";return 0;

}

Now itsWeight and itsAgeare public members. They can

be accessed by main()


15

How Private Variables are used?#include <iostream> // for coutusing namespace std;class Cat // declare the class Cat{public: void SetAge (int age);

int GetAge();void SetWeight (int weight);int GetWeight();

private: int itsAge;int itsWeight;

};int Cat :: GetAge(){ return itsAge;}void Cat::SetAge(int age){ itsAge = age;}

Public Accessor Methods

Accessor methods provide access to private members since they are in the same class


Read/write

Private member variables


16

Scope Resolution Operator ( :: ) � C++ programs typically use several class types

� different classes can have member functions with th e same identifier, like Write( )

� member selection operator is used to determine the class whose member function Write( ) is invoked

currentTime .Write( ) ; // class TimeTypenumberZ .Write( ) ; // class ComplexNumberType

� in the implementation file, the scope resolution op erator is used in the heading before the function member’s name to sp ecify its class

void TimeType :: Write ( ){ . . .

}


17

Implementing Class Methods• Member functions declared in a class are only prototypes

of these functions

• Actual implementation (the operations performed by the function) should be separatelydescribed.

#include <iostream> // for coutusing namespace std;class Cat // declare the class Cat{public:

int GetAge();private: int itsAge;};

int Cat :: GetAge(){ return itsAge;}

To indicate GetAge() is a function of class Cat

That’s the implementation


Publicly known

Only the developer

knows


18

Two Separate Files used// SPECIFICATION FILE ( timetype .h )// Specifies the data and function members.class TimeType{

public:. . .

private:. . .

} ;

// IMPLEMENTATION FILE ( timetype.cpp )// Implements the TimeType member functions.

. . .


19

Exercise 5.2aBased on the program in page 15, write a program that will first print the age and weightof a cat called Felix , and then it asks the user to input the age and weightof that cat.

To do that, you need to do the following things:

a. Complete the implementation of the member functions GetWeight() and SetWeight() .

b. Add the main() function that prints the current statusof a cat Felix , then ask for user’s input to modify the status.

c. Build the result program and note the result.

d. Before you ask the user to input the age and weight, what are their initial values? Are they the ones you expect?


Age and weight


20

Constructors and Destructors• How do we initialize an object?

⇒ By means of the constructor of the class

• Every class should have a constructor

• User can define its own constructor for the class

• Otherwise, the compiler will make one for the user although it does nothing

• Constructor is a function of which the compiler will call if an object of this class is constructed (created)

• Besides constructor, every classhas also a destructor that will be called when the object of that class is destructed (removed).


In memory


21

Class Constructors

� a class constructor is a member function whose purpose is to initialize the private data members of a class object

� the name of a constructor is always the name of the class, and there is no return type for the constructor

� a class may have several constructors with different parameter lists. A constructor with no parameters is the default constructor

� a constructor is implicitly invoked when a class object is declared--if there are parameters, their values are listed in parentheses in the declaration


22

class Cat{public:

Cat(int initialAge); // Constructor of Cat~Cat(); // Destructor of Catint GetAge();void SetAge(int Age);void Meow(); //public function

private:int itsAge;

};Cat::Cat (int initialAge){ itsAge = initialAge;}Cat::~Cat(){}void Cat::Meow() {}

A typical Class definition with user-definedconstructor and destructor

Implementation of constructor

When any object of the class Cat is constructed, this function is called and in

effect it will set itsAge to the parameter passed

Implementation of destructorWhen any object of the class Cat is

destructed, this function is called which in effect does nothing


23

Constructors and Destructors• A possible main() function for the class Cat above is as

follows:// some lines of p.22// some lines of p.15int main(){ Cat Frisky(5) ;

Frisky.Meow();cout << "Frisky is a cat who is ";cout << Frisky.GetAge() << " years old.\n"; Frisky.Meow(); //Meow() does nothing hereFrisky.SetAge(7);cout << "Now Frisky is ";cout << Frisky.GetAge() << " years old.\n";Frisky.Meow();return 0;

}

A Cat Frisky is constructed hereThe constructor is called and the parameter 5is passed to the constructor and in turn initializes the private member variable itsAge to 5


24

Destructor

� Member function automatically called when object goes out of scope

� Name of destructor function required to be class name preceded by tilde (~)

� Cannot return a value or contain arguments� General destructor declaration:

~Class ( );Class :: ~Class ( )

Declaration in class definition

Declaration in function definition


25

• It is possible that some functions will never change the value of any member

• It is desirable to declare them as const member function

• Then, the compiler will automatically check if there is any inconsistencyin the program

• It helps to debugthe program

• In the example above, obviously GetAge() and GetWeight() will not change any member

• Hence they can be declared as const as follows:int GetAge() const ; // add the keyword const when

int GetWeight() const; // you declare the functions

const Member Functions


Read only


26

Exercise 5.2b

From the program you wrote in exercise 5.2a

a. Add the constructor and destructor such that the initial ageand weight of Felix is 5 and 10 respectively.

b. Use the const keyword to define the class methods that will not modify the member variables.


27

Exercise 5.2cIdentify the errors in the following program. The program is divided into 3 parts: class declaration, member functions implementation and the main() . Fix the errors and verify your results by building the program.

#include <iostream> // for coutusing namespace std;class Cat // begin declaration of the class{public: // begin public section

Cat(int initialAge); // constructor~Cat(); // destructorint GetAge() const; // accessor functionvoid SetAge(int Age); // accessor functionvoid Meow(); // general function

private:int itsAge; // member variable

};


28

Cat::Cat(int initialAge) { itsAge = initialAge;

cout << "Cat Constructor\n";}

Cat::~Cat(){ cout << "Cat Destructor\n";}

int Cat::GetAge(){ return (itsAge++);}

void Cat::SetAge(int age){ itsAge = age;}

void Cat::Meow(){ cout << "Meow.\n";}

int main(){ Cat Frisky;

Frisky.Meow();Frisky.Bark();Frisky.itsAge = 7;return 0;

}


29


• The development of Object Oriented Programming has enabled a new business in software development

� People develop different classesand sell to other people

• These classes often provide functions that are popularly used in applications

� For example, the java calculator used in some Web pages

• People post the advertisement of their classes on the Web. Those who are interested in their classes can directly download it and perhaps pay by credit card.

Doing Business by Selling Classes5. Basic Object Oriented Programming

30


• Assume that you have designed the following class CATand would like to sell it to a customer

#include <iostream> // for coutusing namespace std;class Cat // declare the class object{public: void SetAge (int age);

int GetAge();void SetWeight (int weight);int GetWeight();

private: int itsAge;int itsWeight;

};

• You do not want to give him the source codesof the member functions since your customer may copy your codes

• They may modify it and re-sell it to somebody else

• You do not want to give him the source codesof the member functions since your customer may copy your codes

• They may modify it and re-sell it to somebody else

int Cat::GetAge(){ return itsAge;}void Cat::SetAge(int age){ itsAge = age;}


31


• If the customer has the source code, he may try to modify it by himself

• It may introduce a lot of hidden errors as he knows very little about your program

• The resulting program can be very difficult to debug.

Conclusion: It is better to give your customer executable codessuch that they cannot reador modify it

Conclusion: It is better to give your customer executable codessuch that they cannot reador modify it

Problem 1: A program without main() cannot be built. No executable codes can be generated without main()

Problem 2: If your customer only has the executable codes, how can he know the way to use your class?

Problem 1: A program without main() cannot be built. No executable codes can be generated without main()

Problem 2: If your customer only has the executable codes, how can he know the way to use your class?


32


Static Library• Rather than giving your customer the

executable codes, you can give him a (static) library

• Inside a library, it does not contain the executable codes, but the object codes– machine code waiting forlinking

• Since object codescannot be read directly, your customer cannot modify your codes

• To enable your customer know how to use your class, give also your customer a headerfile that contains the definition of your class.

Header

library


33


Main Program

main() of the Customer

#include "CatClass.h"

void main(){

CAT Frisky; Age = Frisky.GetAge();

}

MainProject.cpp

:

:

{

public:int GetAge();

};

CatClass.h

:

:

class CAT

Library

10 20 2d 35 5f 43 2343 23 …

… 22 6f… 21 44 dd 23

Contain the implementationof GetAge() , however, the source code

cannot be seen by the customer

Show that the class CAThas a public function

called GetAge()It will return an

integer

:

Explain why

public and private

When the customer build the main() , it

links with CatClass.h and the library


34

Modular Program Design


• Besides doing business with classes, the use of static library also facilitates modular program design

• The system analyst analyses the requirement of a program and divides it into a number of modules

• The specificationsof each module, such as its functions, the calling methods, the parameters returned, are well defined

• Based on the specifications, the development of each module will be done by different programmers.

Very useful for teamwork


35

Example– step 1: obtain the requirements

• A program is to be designed to show a zoo that contains different kinds of animals, including dog, cat, sheep, and horse

• At the beginning of the program, all animals are drawn on the screen

• When user clicks on an animal, the sound of it is played.

• The system analyst talks with the customer and obtains the following program requirements:


36

Example– step 2: design the program flow


• Based on the requirements, the system analyst designs the program flowusing, for example, a flow chart.

Start

All animal drawn?

User click on animal?

No

Yes

A

No

Yes

B

Draw animal


37


A Click Dog?

Click Cat?

Click Sheep?

Click Horse?

Click End?

BPlay dog sound

Play cat sound

Play sheep sound

Play horse sound

End

Yes

Yes

Yes

Yes

Yes

No

No

No

No

No


38


Example– step 3: divide into modules

• From the flow chart, identify the classes required and their functions

{

public:int DrawShape();int PlaySound();

};

DogClass.h

:

:

class DOG{


};

CatClass.h

:

:

class CAT

{


};

SheepClass.h

:

:

class SHEEP{


};

HorseClass.h

:

:

class HORSE


39


Example– step 4: ask programmers to

develop the modules• For each module, the system analyst will ask a programmer

to develop the codes required (to implement the class)

• To speed up the process, the system analyst may ask 4 programmers to develop the modules simultaneously

• The codes developed by the 4 programmers may be different

• If the specifications are correct, all modules are correct irrespective to the difference in the codes

• Hence facilitate teamwork.


40

Example– step 5: integrate the modules

{


DogClass.h

:class CAT

};:


Main Program

Integrating all modules by the system analyst

#include “DogClass.h"

void main(){

DOG Bobby; CAT Frisky; Bobby.DrawShape();Frisky.DrawShape();

}

MainProject.cpp

:

:Frisky.PlaySound();

:

#include “CatClass.h" {


CatClass.h

:class CAT

Library for CAT

10 20 2d 35 5f 43 2343 23 …

… 22 6f… 21 44 dd 23

:

};:

Library for DOG

10 20 2d 35 5f 43 2343 23 …

… 22 6f… 21 44 dd 23

:

:

Skeleton


41

Interface versus Implementation• The declaration of a classstored in the header file is in fact

a contract between the System Analyst and Programmer, and between the developer and customer

• System Analyst who wants to use the object of this class should follow the rules defined in the declaration

• Class should be implemented exactly the same way it is declared; otherwise the customer using it will have error

• C++ is strongly typed, i.e., the compiler will enforce the contracts and set the error flag if someone violates them

• In the language of OO programming, this contract is named as interface.


42


Create a static library with Visual C++• Suppose you are one of the programmers and is asked to

develop the class CAT

• Now, the system analyst sets out two specificationson the class CAT:

1. It should have a function that allows the setting of CAT’s age. The function is

void SetAge(int age);

2. It should have another function that allows the reading of CAT’s age. The function is

int GetAge();


43


Step 1: Create the class required

class CAT // declare the class{public: void SetAge (int age);

int GetAge();private: int itsAge;}; • Based on the

specifications, you may design the above class CAT

• Based on the specifications, you may design the above class CAT


44


Step 2: Implement the classclass CAT // declare the class{public: void SetAge (int age);


int CAT::GetAge(){ return itsAge;}void CAT::SetAge(int age){ itsAge = age;}

• For each member function, develop the required codes for building the library

• Keep this restricted

• For each member function, develop the required codes for building the library

• Keep this restricted

To be put to header file


45


Step 3: Test the class

class CAT{public: void SetAge (int age);


• Create the header fileand a main() for testing the functionality of the class

• This main() is only for testing purpose, not supposed to be used in real application

• Create the header fileand a main() for testing the functionality of the class

• This main() is only for testing purpose, not supposed to be used in real application

#include <iostream>using namespace std;#include "CatClass.h"// --- put implementation hereint main(){ CAT Frisky;

Frisky.SetAge(7);int Age = Frisky.GetAge();cout << Age << endl;return 0;

}

CatClass.hMainTest.cpp


46

Step 4: Create library with Visual C++


• Assume your class is fully tested. You can create your library using Visual C++

• Start your Visual C++ 2017

• In Visual C++ 2017, start a new project as usual

• Set the location to store your library project to, e.g. e:\VS_Proj\ENG236\Ch5

• Set the project name, e.g. LibCat

• However, when choosing the Application type in the Application Settings window, choose Static library and uncheck the option Precompiled header



48

48


• In Visual C++ 2017 Solution Explorer, under Header Files , right-click Add → New Item ... and choose Header File (.h )

• Set the Header File name to, e.g. CatClass

• Click Add and the header file CatClass.h will be added to the project LibCat

• Type the class declaration


49


• Type your class declaration here• Remember to comment your codes• Type your class declaration here• Remember to comment your codes


50


• In Visual C++, click Project/Add New Item... and choose C++ File (.cpp )

• Set the C++ source file name, e.g. CatCodes

• Click OKand the C++ source file CatCodes.cpp will be added to the project LibCat

• You may verify it by checking the Solution Explorer

• Type the codesfor the implementation of the member functions

• Remember to include your header fileand be careful of its location

• Build the library by clicking Build Solution .Give LibCat.lib


51



• You need to enter the correct path of the header file• It shows that the CatClass.h is in the current

folder of CatCodes.cpp

• You need to enter the correct path of the header file• It shows that the CatClass.h is in the current

folder of CatCodes.cpp

• After building the library, the result is shown • After building the library, the result is shown

Solution Explorerwindow

52


Step 5: Send the library and header file to the System Analyst

• Locate the files CatClass.h and LibCat.lib and send to the System Analyst

• CatClass.h shows the class definition

• From its contents, the System Analyst knows how to use the class

• LibCat.lib contains the codes for the implementation of the member functions

• It is in object code formsuch that even the System Analyst cannot interpret or modify the codes.


53


e:\Vs_Proj\ENG236\Ch5\LibCat\LibCate:\Vs_Proj\ENG236\Ch5\LibCat\LibCat

e:\Vs_Proj\ENG236\Ch5\LibCat\debuge:\Vs_Proj\ENG236\Ch5\LibCat\debug

54

Step 6: Integrating into the application


• Open a Win32 Console project, e.g. MainProject in the folder e.g. e:\VS_Proj\ENG236\Ch5\

• Enter the codes for the main() and other functions if necessary to implement the application

• Copy CatClass.h and LibCat.lib sent from the programmer to current folder e:\VS_Proj\ENG236\Ch5\MainProject\MainProject

• In Visual C++, click Project/ Add Existing Item...under MainProject . Select All Files to show all the files in the folder

• Add CatClass.h and LibCat.lib to the project

• Build Solution and run the application.

Prog.cpp


55


See the added CatClass.h and LibCat.lib here

See the added CatClass.h and LibCat.lib here

After building the application, the result is shown

After building the application, the result is shown


56


You may double-click LibCat.lib . You can only see some hex codes. No source codes can be found

You may double-click LibCat.lib . You can only see some hex codes. No source codes can be found


57


To see how to use the class CAT, double-click CatClass.hTo see how to use the class CAT, double-click CatClass.h


58

� often several program files use the same header file containing typedef statements, constants, or class type declarations--but, it is a compile-time error to define the same identifier twice

� this preprocessor directive syntax is used to avoid the compilation error that would otherwise occur from multiple uses of #include for the same header file

#ifndef Preprocessor_Identifier#define Preprocessor_Identifier

.

.

.

#endif

Avoiding Multiple Inclusion of Header Files


59

Using Preprocessor Directive #ifndef

// timetype .h FOR COMPILATION THE CLASS DECLARATION IN

// SPECIFICATION FILE FILE timetype.h WILL BE INCLUDED ONLY ONCE

#ifndef TIME_H #define TIME_H // timetype .cpp // client.cpp

// IMPLEMENTATION FILE // Appointment programclass TimeType{ #include “timetype.h” #include “timetype.h”

public:. . . . . . int main ( void )

{private: . . .

. . . }} ;#endif


60


Exercise 5.3 - Requirements• Write a program using Visual C++ to do the following:

• A class CATmust be created and your program will repeatedlyask user to choose one of the following

a. Set the weight of a catb. Get the weight of a catc. Ask the cat to Meow!d. Quit

• If the user chooses (a), the user will be asked to input the weight of the cat and the program will store it up

• If the user chooses (b), the weight of the cat will be shown on the screen

• If the user chooses (c), show the following message on the screen “ Meow, Meow ... Meow ”

• If the user chooses (d), quit the program.


61


Exercise 5.3 (cont.) - Activity

1. Two students in a group. One plays the role of a System Analyst. One plays the role of a Programmer

2. The System Analystshould design the program structure using flow chart and define the specifications of the class required

3. The Programmer should develop the classand build a library (with the header file required). You may email your library file and header file to the System Analyst

4. After receiving the files from the Programmer, the System Analyst should integrate them into the application

5. Show the result to your tutor.


62


Exercise 5.3b - Requirements• Write a program using Visual C++ to do the following:

• A class PHONEmust be created and your program will repeatedly ask user to choose one of the following

a. Set the serial no. of a phoneb. Get the serial no. of a phonec. Ask the phone to ring!d. Quit

• If the user chooses (a), the user will be asked to input a 6-digit serial no. of the phone and the program will store it up

• If the user chooses (b), the 6-digit serial no. of the phone will be shown on the screen

• If user chooses (c), show the following message on the screen “ Ring ... Ring, Ring ... Ring ”

• If user chooses (d), quit the program.


63

Exercise 5.3b (cont) - Activity


1. For the same group,interchange the role of group members. The Programmer will play the role of System Analyst now; and the System Analyst will play the role of Programmer now

2. The System Analystshould design the program structure using flow chart and define the specifications of the class required

3. The Programmer should develop the classand build a library (with the header file required). You may email your library file and header file to the System Analyst

4. After receiving the files from the Programmer, the System Analyst should integrate them into the application


64

How memory is used in C++?

• The whole big piece of memory is divided into 4 areas:

Code Space - for the storage of program code

Stack - for the storage of local variables, passed parameters.

Global Name Space- for the storage of global variables

Free store- for the storage of dynamically createddata

Computer Programming6. Pointers and Arrays

65

Free Storeor

the heap

Global Name SpaceCode SpaceThe Stack

How memory is used in C++?


funcB(){ Cat Frisky;

return;}

funcA(){ int a;

return;}

main(){ Statements;

funcA();Statements;funcB();Statements;

}

66

• A variable is a storage spacein memory

• Every variable has amemory address

What is the Address of a Variable?

0100 0101 0102 0103 0104 0105 0106 0107 0108 0109Address

Memory

Variables char a int b short int c bool d

30 0A 21 3A 51 44 20 00

a = 30 Address of a = 0100

b = 0A 21 3A 51 Address of b = 0101

c = 44 20 Address of c = 0105


All values written in

hexadecimal but

binary in reality

The character '0'

Each byte has an address

Each variable has the

starting-byte address

67

• In C++, the symbol & is used to indicate the address of a variable #include <iostream>

using namespace std;int main(){ unsigned short shortVar = 5;

unsigned long longVar = 65535;long sVar = -65535;cout << "shortVar:\t" << shortVar;cout << "\tAddress of shortVar:\t";cout << &shortVar << "\n";cout << "longVar:\t" << longVar;cout << "\tAddress of longVar:\t";cout << &longVar << "\n";cout << "sVar:\t\t" << sVar;cout << "\tAddress of sVar:\t";cout << &sVar << "\n";return 0;

}

The addresses of shortVar ,

longVar and sVar



68

• Variable and address of a variable are different



69


What is a Pointer?• In many situations, we want to store the address of a

variable into a particular memory location

0100 0101 0102 0103 0104 0105 0106 0107 0108 0109Address

Memory

Variables char a int b (address of a) pa

10 0A 21 3A 51 00 0100 00

• pa is a variable that can store the address of a

• In C++, every address has 4 bytes. So we need to reserve 4 bytes of memory to store the address of a .

pa is the pointer of a

70


What is a Pointer?• In C++, every variable needs to have its type declared

� int abc; // means abc belongs to the type of integer

�CAT Felix; // means Felix belongs to the class CAT

• If we want to declare the type of pa , which stores the address of a variable a, how should we do it?

� How about address pa;

Not good enough, since it does not tell the nature of aNot good enough, since it does not tell the nature of a

� How about (address of a character) pa;

Better, but look too clumsy Better, but look too clumsy

71


What is a Pointer?• C++ uses an elegant way to solve the problem (but need

some time to understand!)

• It introduces a symbol *

� means the contentof an address

0100 0101 0102 0103 0104 0105 0106 0107 0108 0109Address

Memory

Variables char a int b char *pa

10 0A 21 3A 51 00 0100 00

• char *pa indicates that the memory contentof the address stored inpa is a character. pa is indirectlydeclared to be an address of character.

pa 's content is an address, the memory

content of that address is a character

72

What is a Pointer?

char *pa, a= 0x30 ; // 48

cout << a; // a = '0'

pa = &a; // pa = 0100

cout << *pa; // *pa = '0'

*pa = 49; // a = '1'

cout << a;


0100 0101 0102 0103 0104 0105 0106 0107 0108 0109Address

Memory

Variables char a int b char *pa

30 0A 21 3A 51 00 0100 00

• We can modify the content of a memory location using pointer

We modify a indirectly by using its address

73

Pointers� Special variables that only hold memory addresses� Hold the location (address) of information, not the

information itself� Pointers are declared using a “*” after the type

int a = 10 ;

int * p; // A POINTER VARIABLE!!!!

p = &a;

You have to specify the pointer type!


74

Don’t get confused

� pointer p does not hold the value of a� it points to the memory location of a� pointer p is a variable

– and it has its own memory location

1 0 0 0 2

1048575

1048574

1048573

Pointer Constants

0

a p

1048572

1048571

1048570


75

Examplesint x=7; int *p, *q; p = &x; q = &x;


76

Pointer Indirection (Pointer to Pointer)

What is the result?

58 58 58


77

Computer Programming6. Pointers and Arrays#include <iostream>

using namespace std;typedef unsigned short int USHORT;int main(){ USHORT myAge; // a variable

USHORT * pAge = 0; // a null pointer , pAge=0, not *pAge=0// Don’t let it become wild pointer (point to unknown)myAge = 5;cout << "myAge: " << myAge << "\n";pAge = &myAge; // assign address of myAge to pAgecout << "*pAge: " << *pAge << "\n\n";cout << "*pAge = 7\n";*pAge = 7; // *pAge=7, not pAge=7 , sets myAge to 7cout << "*pAge: " << *pAge << "\n";cout << "myAge: " << myAge << "\n\n";cout << "myAge = 9\n";myAge = 9;cout << "myAge: " << myAge << "\n";cout << "*pAge: " << *pAge << "\n";return 0;

}

78


79

80

Why Pointer? - Using Free Store• Pointer allows us to handle the memory in Free Store

• The memory Free Store is opened to all functions

• Pointer helps us to identify the part of memory in Free Store that is being used by a particular function or object

• To use the memory in Free Store:

1. Claim to the systemhow much memory is required

2. System allocates a memory space with size big enough

3. System returns a pointer valuewhich is the startingaddress of that memory space

4. When the memory space is not required, release it back to the systemfor other functions.


81

new operator� General form:

new type [num_elements]– type is data type of array elements– num_elements is number of array elements

� Reserves memory but does NOT fill with values– new returns address of memory it reserves– Value assigned to pointer variable of same type

int* array_address;array_address = new int [10];

array_address[0], … array_address[9]*array_address, … *(array_address+9)


82

new and delete

unsigned short int * pPointer;pPointer = new unsigned short int;:// after using the memory space:delete pPointer; // return it to system

Claim a piece of memory in Free Store with size that is equal to an unsigned short

integer

The keywords new and delete help us claim and release memory in Free Store

int * pPointer2;pPointer2 = new int[2];:// after using the memory space:delete [] pPointer2; // return it to system

We claim memory with size equals to 2 integers.

pPointer2 now points to the starting address of this

memory space


Results unpredictable if no []

83

delete operator

� General form:delete [ ] array_address;

– array_address is pointer variable

� Releases memory stored at address indicated� Knows size of memory reserved by new and

releases memory for all the array� delete address;

– deletes memory for single element


84

8003 8004 8005 8006 8007 8008 8009 800A 800B 800CAddress

Memory

Free Store

int * pPointer;unsigned short int * pPointer2;pPointer = new int;:pPointer2 = new unsigned short int [2];:delete pPointer; // return it to systemdelete [] pPointer2; // return it to system

pPointer = 8004pPointer = 8004

pPointer2 = 8008pPointer2 = 8008


85

Null Address

� 0 is a constant pointer that represents the empty or null address

– Its value indicates that pointer is not pointing to any valid object

– Cannot dereference a pointer whose value is null

int *ptr = 0;

cout << *ptr << endl; // invalid, ptr

// does not point to

// a valid int


86

Dangling Pointers and Memory Leaks

� A pointer is dangling if it contains the address of memory that has been freed by a call to delete .– Solution: set such pointers to 0 as soon as memory

is freed.

� A memory leak occurs if no-longer-needed dynamic memory is not freed. The memory is unavailable for reuse within the program.– Solution: free up dynamic memory after use

10-86


Dangling Pointer

int *A = new int[5]; //int *A; A=new int[5];

for (int i = 0; i < 5; ++i) A[i] = i;

int *B = A; //int *B; B=A;

delete [] A;

A

B0 1 2 3 4

A

B

Locations do not belong to program

—

?


Memory Leak

int *A = new int [5];

for (int i = 0; i < 5; ++i) A[i] = i;

A = new int [5];

A 0 1 2 3 4

— — — — —

These locations cannot be

accessed by program

A 0 1 2 3 4


89

Stray (Wild or Dangling) Pointers• When one deletes a pointer, the associated memorywill

be given backto system

• If one tries to use that pointer again without reassigningit, the result is unpredictable

• To ensure one will not use the deleted pointer again, always assign the pointer with the value 0 after delete

• A stray (or wild , dangling) pointer is the pointer that has been deleted but without assigning to null int *pNum = new int (5) ; // Initialize *pNum to 5

delete pNum;

pNum = 0; // To ensure the program will crash rather

// than unpredictable if one reuses it


NULLpoints to ROM

delete a pointer ≠ remove a pointer, it still exists

90

Exercise 6.1The program on the next page will introduce the problem of memory leaks(the system cannot get back the memory allocated to the program) and execution error. Build the program and step over each line of code using the Run-time Debugger. Answer the following questions:


1. What is the address of localVariable ?2. What is the value of pHeap after executing line 6 ?3. What is the value of pHeap after executing line 11 ?4. Assume that you can finish executing the program. Do

you think you can free the memory claimed by the newstatement in line 6 ? If no, why not?

Modify the program such that we can free the memories claimed by both new statements in line 6 and line 11 .

91

#include <iostream>using namespace std;int main(){ int localVariable = 5;

int * pLocal = &localVariable;int * pHeap = new int; //line 6*pHeap = 7;cout << "localVariable: " << localVariable << "\n";cout << "*pLocal: " << *pLocal << "\n";cout << "*pHeap: " << *pHeap << "\n";pHeap = new int; //line 11*pHeap = 9;cout << "*pHeap: " << *pHeap << "\n";delete pHeap;delete pHeap;return 0;

}


92

Creating Objects in the Free Store• Similar to integer, we

can create objectsin the Free Store

Cat *pCat = new Cat;

Free Storeor

the heap


Size enough for a Cat

pCat

• pCat stores the beginning addressof the memory allocated

• When the object is created, its constructor is called


Object identified by a pointer.

93

Deleting Objects• Objects in the Free Store can also be deleted

Cat *pCat = new Cat;

delete pCat;

• pCat becomes an identifier of the object created

• When an object is deleted, its destructor will be called

• Hence the destructor of Cat will be called when the keyword delete is used in the above

• (The destructor will also be called if the function that creates the object finishes.)


94

#include <iostream>using namespace std;class SimpleCat{public:

SimpleCat();~SimpleCat();int GetAge() const {return itsAge;}void SetAge(int age) {itsAge = age;}

private:int itsAge;

};SimpleCat::SimpleCat(){ cout << "Constructor called.\n";

itsAge = 1;}SimpleCat::~SimpleCat(){ cout << "Destructor called.\n";}

The class SimpleCat

Constructor

Destructor

Example

Computer Programming6. Pointers References and Arrays

95

int main(){

cout << "SimpleCat Frisky...\n";SimpleCat Frisky;cout << "SimpleCat *pRags = new SimpleCat...\n";SimpleCat * pRags = new SimpleCat;cout << "delete pRags...\n";delete pRags;cout << "Exiting, watch Frisky go...\n";return 0;

}

Output of the program


pRags in the stack, *pRags in the heap

96

Accessing Members of Objects• To access members of an object, the operator (. ) is used

SimpleCat *pCat = new SimpleCat;

(*pCat).SetAge(2);

The object pointed by pCat

The member function of the

object

Input parameter of the member

function

• In C++, a shorthand is provided for such member access

SimpleCat *pCat = new SimpleCat;

pCat -> SetAge(2); // The same as before

The Same


97

Pointers to Objects

� Special arrow operator ->– Negative and greater than symbol with no space between– Used to access members with object's address

� Declaring pointer Class_name* ptr_name;� Initializing pointer ptr_name = &object_name;� Calling member function (2 argument example)

ptr_name -> member_fun(arg1, arg2);

(*ptr_name).member_fun(arg1, arg2);ptr_name[0].member_fun(arg1,arg2);


98

6. Pointers and Arrays#include <iostream>using namespace std;class Object{public:

Object();~Object();int GetCnt()

const {return *count;}private:

int *count ;};Object::Object(){ count = new int(1);} // initialize *count to 1Object::~Object(){ delete count; }int main(){ Object Obj;

return 0;}

Question

If I declare an object in the stack that has member variables in the heap, what is in the stack and what is in the heap?


99

Answer4 bytes on the stack to hold Obj which contains a pointer count

4 bytes on the heap that is pointed by count of Obj

Free Storeor

the heap

Global Name SpaceCode SpaceThe Stack4 bytes: count

4 bytes: *count

Obj


100

Using the this PointerCan be used to access members that may be hidden by parameters with the same name: class SomeClass

{private:

int num;public:

void setNum(int num){ this->num = num; }

};


101

0000000100020003000400050006000700080009

Pointers Arithmetic• Pointers can be added or subtractedfrom one another

if they are of the same type

Address

Memory

Variables short int *a, *b

10 0A 21 3A 51 44 20

cout << "a = " << a << endl; // Assume a = 0000

b = a + 1;

cout << "b = " << b << endl; // b = 0002

cout << "b - a = " << b-a << endl; // b - a = 1


102

Pointer Variables and Arithmetic

� Declare variable as pointerdouble* b;

– Holds address

� Can perform addition and subtraction– Purpose of add is to advance to subsequent cell

b + 1 (double is 8 bytes, so adds 8 bytes)– Subtraction goes back to previous memory cell

b – 1 (goes back 8 bytes)


103

Pointers Arithmetic• The same applies to objects

0000 0001 0002 0003 0004 0005 0006 0007 0008 0009Address

Memory

Variables Cat *a = new Cat; //Assume Cat takes 6 bytes

Cat *a = new Cat;

Cat *b;

cout << "a = " << a << endl; // Assume a = 0000

b = a + 1; // Don't touch *b as the memory is not from new

cout << "b = " << b << endl; // b = 0006

cout << "b - a = " << b-a << endl; // b - a = 1


You should not DIRECTLY assign a

value to a pointer, e.g.

int *p=0x00110110; �

104

Exercise 6.1bFind out the errors in the following programs. Note the error messages when you build these program. Fix the errors and rebuild it to verify your corrections.

#include <iostream>using namespace std;int main(){

int *pInt;*pInt = 9;cout <<

"The value at pInt: " << *pInt << endl;

return 0;}

#include <iostream>using namespace std;int main(){

int SomeVariable = 5;cout << "SomeVariable: "

<< SomeVariable << "\n";int *pVar = & SomeVariable;pVar = 9;cout << "SomeVariable: " <<

*pVar << "\n";return 0;

}


105

Exercise 6.1cModify the program you wrote in exercises 5.2 a and b such that

a. The program will ask the user if he wants to create the object Felix . If yes, the object is created in the heap. If no, just quit .

b. As before, initialize the age and weight of Felix to 5 and 10 using the constructor. Display the age and weight of Felix .

c. Ask the user to enter the age and weight of Felix and displaythem again.

d. After printing the age and weight of Felix , the program will repeatedlyask the user whether he wants to (i) enter the age and weight again; (ii) destroy Felix and create again; or (iii) quit the program. Your program should be able to perform the task the user selected.

e. Whenever Felix is destroyed, print the current age and weight of Felix using the destructor

f. Comment your program appropriately.


106

What Is an Array?• An array consists of a collectionof data storage

locations, each holds the same type of data

• An array can be easily declared as follows:

short shortArray[25]; It states that there is a sequence of 25 short integer data. The whole

sequenceis named as shortArray

shortArray[ 0] 000B 000C ... ... 0037 0038 0039 003A

Memory

Variables short shortArray[25]; //declaration

10 0A 21 3A ... ... 20 2A 4B 40


shortArray[1] shortArray[ 24]

In consecutive memory

107

Array Element• In an array, an array elementis referred to by indicating

its index

• The first element has index 0, and then 1, …

• Hence shortArray[0] is the first element

shortArray[1] is the second element

:

shortArray[24] is the last element

• No shortArray[25] !!!

• Do not try to use shortArray[25], result unpredictable.

25 short 25 short

integers


108


short shortArray[25];

0009 000A 000B 000C ... ... 0037 0038 0039 003AAddress(Hex)

Memory

Variables short shortArray[25];

10 0A 21 3A ... ... 20 2A 4B 40

• So shortArray[0] = 0x100A; // 4106 in deciaml

shortArray[1] = 0x213A; // 8506 in decimal

:

shortArray[23] = 0x202A;

shortArray[24] = 0x4B40;

Declaration

Assignment statements

109

Assigning Values to Individual Array Elements

float temps[ 5 ] ; // allocates memory for array

int m = 4 ;temps[ 2 ] = 98.6 ;temps[ 3 ] = 101.2 ;temps[ 0 ] = 99.4 ;temps[ m ] = temps[ 3 ] / 2.0 ;

temps[0] temps[1] temps[2] temps[3] tem ps[4]

7000 7004 7008 7012 7016

99.4 ? 98.6 101.2 50.6


110

#include <iostream>using namespace std;int main(){ int myArray[5],i;

for (i=0; i< =5; i++)myArray[i] = 20;

for (i=0; i<5; i++)cout << myArray[i] << endl;

return 0;}

The kind of mistake people often make


No myArray[5] !!!Do not try to use myArray[5] ,result unpredictable

No myArray[5] !!!Do not try to use myArray[5] ,result unpredictable

111

Example more definitionsSuppose

const int N = 20;

const int M = 40;

const int MaxStringSize = 80;

const int MaxListSize = 1000;

Then the following are all correct array definitions

int A[10]; // array of 10 ints

char B[MaxStringSize]; // array of 80 chars

double C[M*N]; // array of 800 floats

int Values[MaxListSize]; // array of 1000 ints


112

Initializing Arrays• An array can be initialized during declaration

int IntegerArray[5] = {10,20,30,40,50};

int AnotherArray[] = {50,40,30,20,10};

int BiggerArray[5] = {10,20};

IntegerArray declares itself to have 5 integers

AnotherArray requests the memory space in stack just enoughto hold the

data defined in the listBiggerArray declares itself to have 5 integers but only the first 2 of them are initialized. The others are 0.It is different from :

int IncorrectArray[2] = {10,20,30};


int a[5]; NOT the same as int a[5]={};

int BiggerArray[] = {10,20};

113

Copying One Array to Another

� Cannot copy with an assignment statement:tests2 = tests; //won’t work

� Must instead use a loop to copy element-by-element:

for (int i=0; i < SIZE; i++)

tests2[i] = tests[i];

8-113


114

Passing Arrays to FunctionsTo pass an array argument to a function, specify the name of the array

without any brackets int myArray[ 24 ];...

myFunction( myArray, 24 );...

Array size usually passed to function Arrays passed call-by-reference

� The called functions can modify the element values in the caller’s original array

� Name of array is the address of first element of the array� Function knows where the array is stored. Therefore, when the called

function modifies array elements in its function body, it is modifying the actual elements of array in the original memory locations

Pass array name

Size is also often sent as an argument

…myArray

main()

myFunction()


115

Use of const

� because the identifier of an array holds the base address of the array, an & is never needed for an array in the parameter list

� arrays are always passed by reference

� to prevent elements of an array used as an argument from being unintentionally changed by the function, you place const in the function heading and prototype


116

int Minimum(const int A[], int size)

{int SmallestValueSoFar = A[0];

for (int i = 1; i < size; ++i)

if (A[i] < SmallestValueSoFar )

SmallestValueSoFar = A[i];

return SmallestValueSoFar ;

}

Passing An Array Notice brackets are empty

First element

int Number[6] ={3, 88, -7, 9, 1, 24};cout << Minimum(Number, 6) << endl;

int List[3];

List[0] = 9; List[1] = 12; List[2] = 45;

cout << Minimum(List, 3) << endl;


117

#include <iostream>using namespace std;

void Obtain ( int [ ], int ) ; // prototypes herevoid Print ( const int [ ], int ) ;

using namespace std ;

int main( ) {

int temp[31] ; // array to hold up to 31 temperaturesint numDays;

cout << “How many daily temperatures? ” ;cin >> numDays ;

Obtain( temp, numDays ) ; // call passes value of numDays and // address of array temp to function

cout << numDays << “ temperatures“ << endl ;Print ( temp, numDays ) ;return 0 ;

}

Example with Array Parameters


118

Array of Objects• Any object can be stored in an array

• Accessing member data in an array of objects is a two-step process

• Identify the member of array by []

• Access the member by .

CAT Litter[5]; //Litter[0] - Litter[4] are 5 objects

int i;

for (i=0; i<5; i++)

cout << Litter[i].GetAge() << endl;

To find out which CAT Call GetAge() of that CAT


119

Multi-dimensional Array• It is possible to have an array of more than 1 dimension

Two-dimensional arrayA Chess Board

int Board[8][8];

• A 2-dimensional array can be declared as

• Each element can be written or read as

Board[5][3] = 0;

int number = Board[5][3];

// number = 0

0 1 2 3 4 5 6 7

0


64 integers

120

Processing Two-Dimensional Arrays

� Two-dimensional arrays can be initialized in the declaration by listing values within braces, separated by commas – Braces can be used to distinguish rows, but are not

required

int val[ ][4] = { 8, 16, 9, 52, 3, 15, 27, 6, 14, 25, 2, 10 }


121

Initialize Multidimensional Arrays• Multidimensional Arrays can also be initialized during

declaration

int SomeArray[5][2] = { {0,0},{1,2},{4,6},{7,2},{4,4}};

int AnotherArray[5][2] = {0,0,1,2,4,6,7,2,4,4};

0 0

1 2

4 6

7 2

4 4

0 1

4

3

2

1

0


SomeArray[0]�{0, 0}SomeArray[1]�{1, 2}SomeArray[2]�{4, 6}SomeArray[3]�{7, 2}SomeArray[4]�{4, 4}

SomeArray[2][1] = *(SomeArray[2]+1)= *( *(SomeArray+2) + 1) = 6

122

const int NUM_STATES = 50 ;const int NUM_MONTHS = 12 ;int stateHighs [ NUM_STATES ] [ NUM_MONTHS ] ;

� In memory, C++ stores arrays in row order. The first row is followed by the second row, etc.

Base Address

. . .

12 highs for state 0 12 highs for state 1 etc.

first row second row

8000 8048 8096

STORAGE rows columns


123

Viewed another way . . .stateHighs[ 0 ] [ 0 ]stateHighs[ 0 ] [ 1 ]stateHighs[ 0 ] [ 2 ]stateHighs[ 0 ] [ 3 ]stateHighs[ 0 ] [ 4 ]stateHighs[ 0 ] [ 5 ]stateHighs[ 0 ] [ 6 ]stateHighs[ 0 ] [ 7 ]stateHighs[ 0 ] [ 8 ]stateHighs[ 0 ] [ 9 ]stateHighs[ 0 ] [10 ]stateHighs[ 0 ] [11 ]stateHighs[ 1 ] [ 0 ]stateHighs[ 1 ] [ 1 ]stateHighs[ 1 ] [ 2 ]stateHighs[ 1 ] [ 3 ]

.

.

.

To locate an element such asstateHighs [ 2 ] [ 7] the compiler needs to know that there are 12 columnsin this two-dimensional array.

At what address will stateHighs [ 2 ] [ 7 ] be found?

Assume 4 bytes for type int.

Base Address 8000


124

void FindAverages( /* in */ const int stateHighs [ ] [ NUM_MONTHS] , /* out */ int stateAverages [ ] )

{

int state;int month;int total;for ( state = 0 ; state < NUM_STATES; state++ ) {

total = 0 ;for ( month = 0 ; month < NUM_MONTHS ; month++ )

total += stateHighs [ state ] [ month ] ;stateAverages [ state ] = int ( total / 12.0 + 0.5 ) ;

}}


125

Matrix Addition Solution

void MatrixAdd(const float A[][MaxCols],

const float B[][MaxCols], float C[][MaxCols],

int m, int n)

{

for (int i = 0; i < m; ++i)

for (int j = 0; j < n; ++j)

C[i][j] = A[i][j] + B[i][j];

}

Notice only first brackets are empty


126

const NUM_DEPTS = 5 ; // mens, womens, childrens, electronics, furnitureconst NUM_MONTHS = 12 ; const NUM_STORES = 3 ; // White Marsh, Owings Mills, Towson

int monthlySales [ NUM_DEPTS ] [ NUM_MONTHS ] [ NUM_STORES ] ;monthlySales[ 3][ 7][ 0]

sales for electronics in August at White Marsh

12 MONTHS columns

5 D

EP

TS

row

s


127

Array and Pointer• C++ allows the flexibility for user to useArray and

Pointer interchangeably

• The name of an array is a constant pointerpointing to the first element of the array

int a,b;

int SomeArray[5] = {10,20,30,40,50};

a = *SomeArray; // a = 10

b = *(SomeArray+1); // b = 20, pointer arithmetic

• Compiler does all the calculation

• SomeArray+1 does not add 1 to SomeArray but add 4(1 integer needs 4 bytes for storage) and point to the next element in the array


128

0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The Stack

SomeArray[5]

10 20 30 40


50

x = *SomeArray;

will be internally converted to x = SomeArray[0]; // x = 10

px = SomeArray; will be internally converted to

px = &(SomeArray[0]); // px = 0104

x = *SomeArray;

will be internally converted to x = SomeArray[0]; // x = 10

px = SomeArray; will be internally converted to

px = &(SomeArray[0]); // px = 0104

• There is NOT a memory location to store the pointer SomeArray . Everything is done by an internal conversion

129


A variable declared as a pointer can also be used as an arrayint SomeArray[5] = {10,11,12,13,14};

int *pSomePointer = SomeArray; // It is a pointer but will

// later be used as array

cout << pSomePointer[0] << endl; // number 10 will be shown

cout << pSomePointer[1] << endl; // number 11 will be showncout << pSomePointer[2] << endl; // number 12 will be showncout << pSomePointer[3] << endl; // number 13 will be showncout << pSomePointer[4] << endl; // number 14 will be shown

0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The stack

pSomePointer = 0104

10 11 12 13 14

SomeArray[]SomeArray[1] andpSomePointer[1] are exactly the same.

0104

pSomePointer

130

De-referencing An Array Name� Using pointers can be faster than using array subscripts

Use a for loop to step through the array:for(int i=0; i<SIZE; i++) {

cout << *(a + i) << endl;

}


131

Relationship Between Pointers and ArraysArrays and pointers are closely related

Array name like a constant pointerPointers can do array subscripting operationsExample: Declare an array b[ 5 ] and a pointer bPtr

int b[5], *bPtr;bPtr = b;// To set them equal to one another// The array name (b) is actually the address of first element of the array

bPtr = &b[ 0 ];// Explicitly assigns bPtr to address of first element of b

To access element b[ 3 ]:x=*( bPtr + 3 ) // Where n is the offset. Called pointer/offset notation

x=bptr[ 3 ] // Called pointer/subscript notation// bPtr[ 3 ] same as b[ 3 ]

x=*( b + 3 ) // Performing pointer arithmetic on the array itself


132

– Pointers and arrays can be used interchangeably.– The array name is equivalent to the address of the first

element in the arrayExample:

int a[10];int *pa;pa = &a[0]; /* is equivalent to pa = a */

So, a[1] ⇔ *(pa+1) ⇔ pa[1] ⇔ *(a+1)&a[1] ⇔ pa+1 ⇔ a+1a[i] ⇔ *(pa+i) ⇔ pa[i] ⇔ *(a+i)&a[i] ⇔ pa+i ⇔ a+ia[i]+=5 ⇔ *(pa+i)+=5 ⇔ pa[i]+=5

Example:f(int s[] ){ … }

f(int *s ){ … }

⇔

Relationship Between Pointers and Arrays


133

Array of Pointers

• So far the arrays are declared to store data in stack

• Usually the memory space of stack is very small

• If a large array is required, it is better to store the elementsof arrays in Free Store

• In this case, for every element in an array, a pointer is assigned to indicate its location in Free Store

• This becomes an array of pointers.


The array itself is still in the stack.

134

6. Pointers and Arrays

#include <iostream>using namespace std;class CAT{public:

CAT() {itsAge = 1;}~CAT() {}int GetAge() const {return itsAge;}void SetAge(int age) {itsAge = age;}

private:int itsAge;

};int main(){ CAT *Family[500];

int i;for (i=0; i<500; i++){ Family[i] = new CAT; }Family[255]->SetAge(1);for (i=0; i<500; i++){ delete Family[i] ; }return 0;

}

Creating 500 CATobjects may use up all memory in the stack

Hence only an array of 500 pointers of CATobjects are created in the stack

Each CATobject is located in the Free Store by the keyword new

To access member of a particular CATobject, use the corresponding

pointer in the array


135

Free Storeor

the heap


CAT 0 CAT 1

CAT 2

CAT 499

...

Array of Pointers

An array of pointers called Family is kept to point to

different elements in Free Store


(*Family[0])

136

Pointer of Array • If one feels that 500 pointers in the previous example are

still too many, we can put the whole array into Free Store

• Hence one pointer is enoughto point to the Array itself but not an individual element

• This becomes a pointer of array.

CAT *Family = new CAT[500];

int a = 0, b=0;

(Family+255)->SetAge(10);

a = (Family+255)->GetAge(); // a = 10

b = Family[255].GetAge(); // b = 10

delete [] Family;

Family is the pointer of array and points to

Family[0] in Free Store

Family is the pointer of array and points to

Family[0] in Free Store

Individual element of the array of the CATobjects

can be accessed by pointer arithmetic

Individual element of the array of the CATobjects

can be accessed by pointer arithmetic


137

Pointer of Array

Free Storeor

the heap


CAT 0 CAT 1 ... CAT 499

A pointer Family is kept to point to the beginning memory location of an array

of CATobjectsin Free Store


Family[0] Family[499]

138


#include <iostream>using namespace std;class CAT{public:

CAT() {itsAge = 1;}~CAT(){;}int GetAge() const {return itsAge;}void SetAge(int age) {itsAge = age;}

private:int itsAge;

};int main(){ CAT *Family = new CAT[10];

for (int i=0; i<10; i++){ Family[i].SetAge(2*i+1);

cout <<' '<< Family[i].GetAge();}delete [] Family;return 0;

}

Arrays created in Free Store can also be deleted

The [] symbol after deletelets the system know the whole array is to be deleted


139

Exercise 6.2

• Based on the program in the last page, add a destructor to CATsuch that when it is called, the age of the cat will be shown on the screen.

• How many times the destructor will be called when the “delete [] Family; ” statement is executed? Why?

• What will happen if we use the statement “delete Family ” instead? Can it be executed in Visual C++?


140


Exercise 6.2b

• Modify the program in Ex.6.2 such that the “Array of Pointers” approach is used to define the 10 objects of CATin the heap. Make sure your program will not introduce memory leak.

141

String - char Arrays • A string is an array of characters

char Greeting[] =

{'H','e','l','l','o',' ','W','o','r','l','d', '\0' };

• A string can be simply initialized as follows:

The null characterrepresents the end of string; it must be added.

• However, this method can easily introduce error

• C++ provides a shorthand that makes use of the double quote " ".

char Greeting[] = {"Hello World"};Totally 12bytes are allocated

for Greeting .Null character is

automatically generated.


Size of

array is 12

The braces can be removed

142

C strings � char arrays

� char array will contain an extra null character at the end of the string

Example:char codes[] = “sample”;

115 97 109 112 108 101 0


608_01

144

• Characters and ASCII codes:char x;

x = ‘a’; /* x = 97*/

Notes:– ‘a’ and “a” are different; why?

‘a’ is the 97“a” is an array of characters, { ‘a’, ‘\0’} or {97, 0}

–“a” + “b” +”c” is invalid but ‘a’+’b’+’c’ = ? (hint: ‘a’ = 97 in ASCII)

Character and C String

1 38

‘a’ + ‘b’ + ‘c’ = 97 + 98 + 99 = 294 = 256 + 38 -> 38

in the memory


145


Copying String • We often need to copy string from one character array to

another character array

char Greeting[12] = "Hello World";

char Greeting2[12];

• Common errors:

Greeting2 = Greeting;

Wrong. Greeting2 is a constantpointer; we cannot assign anything to it -See explanation in the next page.Wrong. Greeting2 is a constantpointer; we cannot assign anything to it -See explanation in the next page.

146


0100 0101 0102 0103 010a 010b 010c 010dAddress

The Stack

Greeting[12]

'H' 'e' 'l' 'd' '\0''l'…

0200 0201 0202 0203 020a 020b 020c 010dAddress

The Stack ? ? ? ? ??…

Greeting2[12]

Greeting2 = Greeting;Wrong. We try to make Greeting2 = 0101. However, Greeting2 must = 0201 as it is assigned by the OS, and is constant

Wrong. We try to make Greeting2 = 0101. However, Greeting2 must = 0201 as it is assigned by the OS, and is constant

147


Very wrong. Greeting2[12] means only the 13th

element of Greeting2 , not the whole string. Besides, there is no 13th element in Greeting or Greeting2

Very wrong. Greeting2[12] means only the 13th

element of Greeting2 , not the whole string. Besides, there is no 13th element in Greeting or Greeting2

Greeting2[12] = Greeting[12];

Note: Greeting2[11] = Greeting[11]; is valid, which means assigning the 12th element of Greeting2 with the value of the 12th element of Greeting .

148

strcpy() and strncpy()• C++ inherits from C a library of functions for tackling

strings

• Two most common ones are strcpy() and strncpy()

#include <iostream>

#include <cstring>

using namespace std;

int main()

{ char String1[] = {"Copy String1 to String2"};

char String2[ 80];

strcpy(String2,String1);

cout << "String1: " << String1 << endl;


return 0;

}

Copy String1 to String2

State the definition of strcpy()You may omit this line because it is already included by iostream


Result

Warning: unsafe

149

• strcpy() will overwrite past the end of the destination if the source were larger than the destination, damaging other data

• To solve the problem, strncpy() can be used#include <iostream>

#include <cstring>


int main()

{ const int MaxLength = 80; //MaxLength > strlen(String1) = 23

char String1[] = "Copy String1 to String2";

char String2[ MaxLength+1 ]; // Initialize String2 if MaxLength<=23

strncpy(String2,String1,MaxLength); //String2 big enough



return 0;

}Strncpy() needs another parameter

MaxLength which specifies the maximum number of data (not including null ) to be copied

State the definition of strncpy()

Computer Programming6. Pointers and Arrays Warning: unsafe

150

Exercise 6.2ca. Write a program that

creates an array of 3 objects of the class ACCOUNTin the free store.

b.Ask the user to input three names and save to each element of the array

c. Read the content of each element in the array and display on the screen.

class ACCOUNT{public:

void writename(char nm[] );char * readname();

private:char name[80];

};

void ACCOUNT::writename(char nm[]){

strncpy(name,nm,79);}

char * ACCOUNT::readname(){

return name;}


Pointer of array

151

String Conversion Functions� Conversion functions

– In <cstdlib> (general utilities library)

� Convert strings of digits to integer and floating-point values

Prototype Description

double atof( const char *nPtr ) Converts the string nPtr to double .

int atoi( const char *nPtr ) Converts the string nPtr to int .

long atol( const char *nPtr ) Converts the string nPtr to long int .

double strtod( const char *nPtr, char **endPtr )

Converts the string nPtr to double .

long strtol( const char *nPtr, char **endPtr, int base )

Converts the string nPtr to long .

unsigned long strtoul( const char *nPtr, char **endPtr, int base )

Converts the string nPtr to unsigned long .


152

atoi and atol

� atoi converts alphanumeric to i nt � atol converts alphanumeric to l ong

int atoi(char *numericStr)

long atol(char *numericStr)

� Examples:int number; long lnumber;number = atoi("57");

lnumber = atol("50000");


153

atof

� atof converts a numeric string to a floating point number, actually a doubledouble atof(char *numericStr)

� Example:double dnumber;

dnumber = atof("3.14159");


String Conversion Functions

154


A few more library functions• To measure the

length of a string, the function strlen() is useful

• To combine two strings into one, we can use the function strcat() .

#include <iostream>

#include <cstring>


int main()

{ char String1[100];

cout << "Please enter a word: ";

cin >> String1 ;

char String2[] = " has ";

char String3[5];

char String4[] = " characters.";

itoa ( strlen(String1) ,String3,10);

strcat (String1,String2); //ret String1

strcat(String1,String3);

strcat(String1,String4);

cout << String1 << endl;

return 0;

}

Convert an integer into a string

Convert an integer into a string

Decimal radix

Append

a string

5 warnings

Return an int

155

itoa

� itoa converts an int to an alphanumeric string� Allows user to specify the base of conversion

itoa(int num, char *numStr, int base)

� Example: To convert the number 1200 to a hexadecimal string

char numStr[10];

itoa(1200, numStr, 16);

� The function performs no bounds-checking on the array numStr


A few more library functions

156

strcat

strcat(char *dest, char *source)

� Takes two C-strings as input. It adds the contents of the second string to the end of the first string:

char str1[15] = "Good ";

char str2[30] = "Morning!";

strcat(str1, str2);

cout << str1; // prints: Good Morning!

� No automatic bounds checking: programmer must ensure that str1 has enough room for result



157


• We can compare if two strings are the same using the function strcmp().

#include <iostream>

#include <cstring>


int main()

{ char String1[100];

cout << "Please enter your name: ";

cin.getline (String1,100);

char String2[] = "Dr F Leung";

if (strcmp(String1, String2)==0)

cout << "Welcome Dr Leung.\n";

else

cout << "Login incorrect.\n";

return 0;

}

Syntax:int strcmp(string1, string2)Return 0 if string1 is the same as string2 ; otherwise returns a +ve or −ve number.

Syntax:int strcmp(string1, string2)Return 0 if string1 is the same as string2 ; otherwise returns a +ve or −ve number.

Get one line of text (including the spaces in between)

No warning

158

strcmpint strcmp(char *str1, char*str2)

� Compares strings stored at two addresses to determine their relative alphabetic order:

� Returns a value:less than 0 if str1 precedes str2

equal to 0 if str1 equals str2

greater than 0 if str1 succeeds str2

if(strcmp(str1, str2) == 0)cout << "equal";

else cout << "not equal";



159

Character PointersString constant acts like a character pointerchar *pc = “ABCDE”; // declare a character pointer variable

Variable Address Valueconstant 731 ‘A’constant 732 ‘B’constant 733 ‘C’constant 734 ‘D’constant 735 ‘E’constant 736 ‘\0’pc 800 731

char s1[] = “abc”;

Variable Address Values1[0] 900 ‘a’s1[1] 901 ‘b’s1[2] 902 ‘c’s1[3] 903 ‘\0’

‘A’

‘B’

‘C’

‘D’

‘E’

‘\0’

731

732

733

734

735

736

731800


160

� Arrays can contain pointers� For example: an array of strings

char *suit[4] = {"Hearts", "Diamonds",

"Clubs", "Spades"};– Strings are pointers to the first character– char * – each element of suit is a pointer to a char

– The strings are not actually stored in the array suit , only pointers to the strings are stored

– suit array has a fixed size, but strings can be of any size

Arrays of Pointers

suit[3]

suit[2]

suit[1]

suit[0] ’H’ ’e’ ’a’ ’r’ ’t’ ’s’ ’\0’

’D’ ’i’ ’a’ ’m’ ’o’ ’n’ ’d’ ’s’ ’\0’

’C’ ’l’ ’u’ ’b’ ’s’ ’\0’

’S’ ’p’ ’a’ ’d’ ’e’ ’s’ ’\0’


161

Syntax:int *pi[3]; /* pi[0], pi[1], pi[2] */

float *pf[3]; /* pf[0], pf[1], pf[2] */

Example 1:

int i=1, j=2, k=3;

int *pi[3] = {&i, &j, &k};Variable Address Value

constant 90 ‘A’constant 91 ‘B’constant 92 ‘C’constant 93 ‘\0’constant 94 ‘D’constant 95 ‘E’constant 96 ‘F’constant 97 ‘\0’constant 98 ‘G’constant 99 ‘H’Constant 100 ‘\0’pc[0] 200 90pc[1] 202 94pc[2] 204 98

Pointer Arrays

Constcan not bechanged

Variable Address Value

i 80 1

j 82 2

k 84 3

pi[0] 100 80

pi[1] 101 82

pi[2] 102 84

Example 2:

char *pc[3]={“ABC”, “DEF”, “GH”};


162

Dangers in using Functions from <cstring>� There is a very real danger associated with the fun ctions strcpy and

strcat. � Both these functions copy characters until a null c haracter is found in

the source string, without regard to whether space is available in the target .

� If there is no space in the target, strcpy and strc at will happily overwrite any variables in memory beyond the target array.

� This may be some of your variables, or it could be something that your system depends on to run correctly.

� There could be a segmentation violation or illegal operation error, with your program crashing, and no further problems.

� The operating system could crash and burn.� Nothing apparent may happen. But the next applicati on started could

crash and burn on loading. Be careful.



163

The C++ Standard string class � The Standard Library supplied class string provides far more utility

than the cstrings C++ gets by way of its C heritage .� Class strings behave very much like built-in data t ypes and are far

safer than cstrings.� Let s1, s2, and s3 be objects of class string, and suppose s1 and s2

have string values. Then + may be used for concaten ation:s3 = s1 + s2;

� Additional space needed is allocated for s3 automat ically.� The default constructor generates an empty string� There is a constructor that takes a cstring argumen t:

string phrase, word1(“Hello “), word2(“World”);phrase = word1 + word2;cout << phrase << endl;

� The output will be Hello World



164

� Characteristic use of the getline function follow:

#include <iostream>

#include <string>using namespace std;//. . .string str1;getline(cin, str1);//insert into str12 all input up to ‘\n’//getline discards the ‘\n’

� NOTE THAT class string objects do not range check index values .� If you want range checked indexing into strings, us e the string member

function at(int_index) .str1.at(9); //Checks index value 9 for legality in str1.

//If legal,returns the character at index value 9.

The C++ Standard string class 6. Pointers and Arrays


165

650

166

� argc and argv // int main( int argc, char* argv[ ])

– In environments those support C++, there is a way to pass command-line arguments or parameters to a program when it begin executing.

– When main is called to begin execution, it is called with two arguments –argc and argv

� argc : The first (conventionally called argc) is the number of command-line arguments the program was invoked with

� argv : The second (conventionally called argv) is a pointer to an array of character strings that contain the arguments, one per string.

Example:

if echo is a program and executed on command prompt, such asC:\> echo hello world

Command-Line Arguments

pointer arrayargv

argc

e c h o \0

h e l l o \0

w o r l d \0null3


167

Command-Line Processing

• In many operating systems, command-line optionsare allowed to input parametersto the program

SomeProgram Param1 Param2 Param3

• To achieve this, main() allows two input parameters to hold the command-line options.

int main(int argc, char *argv[]){ return 0;}

argc stores the number of parameters

argv is an array. Each element of the array is a

character pointer


e.g.ipconfig /all

You can also write *argv[]as**argv

The values of argc and argv[] are provided indirectly

168


ExampleSomeProgram Param1 Param2 Param3

int main(int argc, char *argv[]){

// function statements herereturn 0;

}

argc = 4argc = 4

argv[0] = "SomeProgram"argv[1] = "Param1"argv[2] = "Param2"argv[3] = "Param3"

argv[0] = "SomeProgram"argv[1] = "Param1"argv[2] = "Param2"argv[3] = "Param3"

main() is inside the projectSomeProgram

i.e. argv[0][0] is 'S'

169

Command-Line Processing

#include <iostream>using namespace std;int main( int argc, char *argv[] ){

cout << "Received " << argc << " arguments." << endl;for (int i = 0; i<argc; i++)

cout << "argument " << i << ": " << argv[i] << endl; return 0;

}


To add command-line options, this program must be run in command-line, not Start Without Debugging .

Every command line options will be printed out

The program can flow according to the command line options.

170

Exercise 6.2dBuild the program in the last page with the project name Ex6p2d . Try to locate the executable file of the built program using the Windows Explorer . Open a Command Prompt to execute this program with the following command line:

Ex6p2d aa bb param3 param4

What are shown on the screen?

Try to input different number of command-line options to see the result.


171


Exercise 6.2e

For the program in Ex6.2d, modify it such that a warning message will be given if any two command-line options are the same.

172

#include <iostream>using namespace std;void swap(int x, int y){ int temp;

temp = x;x=y;y=temp;

} int main(){ int x = 5, y = 10;

cout << "Main. Before swap, x: "<< x << "y: " << y << "\n";

swap(x,y);cout << "Main. After swap, x: "

<< x << " y: " << y << "\n";return 0; }

Pass Parameters – Pass by Value• To pass

parameters to functions, we may passparameters by value, i.e. pass copies of the parameter values to functions.

x and y are swapped only in swap() but not

in main()


173

1. At main()1. At main()


0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The stack

Variables

5 10

x y

x and y of main()

4. When swap() returns4. When swap() returns

5 10

x y

x and y of swap()

2. When swap() is called2. When swap() is called

pass-by-value

3. When x and y is swapped3. When x and y is swapped

10 5

x y

x and y of swap()

temp = x;x = y;y = temp;

174

Pass by Pointers

• pointerallows passparameters by reference.

#include <iostream>using namespace std;void swap(int *px , int *py ){ int temp;

temp = *px;*px=*py;*py=temp;

}int main(){ int x = 5, y = 10;

cout << "Main. Before swap, x: "<< x << " y: " << y << "\n";

swap( &x, &y);cout << "Main. After swap, x: "

<< x << " y: " << y << "\n";return 0;

}

The addressesof x and y in main() are passed

to swap()


175

Parameter Passing - by Pointers� Pointer px refers to the memory cell of a.


176


0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The stack

Variables

5 10

x y


x and y of main()

0100 0104

px py

px and py of swap()

2. When swap() is called2. When swap() is called

pass-by-ref

10 5

temp = *px; //5*px = *py; //10*py = temp; //5

3. When *px and *py is swapped3. When *px and *py is swapped

0100 0104

px py

px and py of swap()

10 5

4. When swap() returns4. When swap() returns

177

Return Multiple Values• Normal function

can only return 1 value

• If more than 1 values are to be returned, it can be done by passing two or more parametersto a function by reference.

#include <iostream>using namespace std;void opt(int, int * , int * ); //prototypeint main(){ int num, sqr, cub;

cout << "Input a number: "; cin >> num;opt(num, &sqr, &cub);cout << "Square = " << sqr

<< endl;cout << "Cube = " << cub << endl;return 0;

}void opt(int n, int *pS, int *pC){

*pS = n*n;*pC = n*n*n;

}


sqr and cub of main()are changed by opt()

178

??

0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The stack

Variables

3 ??

num sqr

num, sqr and cub of main()



cub

pass-by-ref

0104 0108

pS pC

n, pS and pC of opt()

2. When opt() is called2. When opt() is called

3

n

pass-by-value

*pS = n*n;*pC = n*n*n; 3. When *pS and *pC are

computed in opt()3. When *pS and *pC are computed in opt()

0104 0108

pS pC

n, pS and pC of opt()

3

n

9 279 27

4. When opt() returns4. When opt() returns

179


Passing an array of parameters to a function

• To save effort from separately passing a number of parameters to a function, they may be passed as an array.

#include <iostream>#include <cstring>using namespace std;void opt(char * str);int main(){ char string[] =

{"This is a string"};cout << string << endl;opt(string);cout << string << endl;return 0;

}

void opt( char * str ){ strcpy(str,"New String");}

string[] is a character array, so string is a character pointer

string[] is a character array, so string is a character pointer

array name is a

constant pointer

function input

and output

180


• Note that in the previous example, string is modified inside the function opt()

• It is because passing the name of an array is pass-by-reference

• When copying the string "New String" to str in opt() , it is just the same as copying to the original string .

Result of executing the last programResult of executing the last program

stringand str are the same

181

Exercise 6.3

The following program defines a class CATthat contains a private variable name[80] . A function is also defined. The function will swap the name of two cats by using pointers.

Design the nameswap() function and the main() that will(i) create & initialize the name of two cats as Frisky & Felix in the stack.(ii) show the initial name of the two cats created(iii) swap the nameof the two cats using the pointer approach(iv) show the name again.


Only swap the name, not the object

182

#include <iostream>#include <cstring>using namespace std;class CAT{public:

CAT(char * firstname) {strncpy(name,firstname,79);}~CAT() {;}char * GetName() {return name;}void SetName(char *nameinput) {strncpy(name,nameinput,79);}

private:char name[80];

};

void nameswap(CAT *CatA, CAT *CatB);

Exercise 6.3 (Cont)


5. Basic Object Oriented Programming What is an Object?

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