chapter 4. class and object

8/17/2019 Chapter 4. Class and Object

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Chapter 4. CLASS and OBJECT


Minh Quang Nguyen

Hanoi National University of Education

September 2015


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Class and Class members

Encapsulating class components

Implementing functions in class

Static class members

this Pointer

Constructor

Q & A


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Introduction

Classes are the central feature of C++ that supportsobject-oriented programming and are often called user-definedtypes

A class is a category of objects; it is a new data type youcreate that is more complex than the basic data types

A class is used to specify the form of an object and itcombines data representation and methods for manipulating

that data into one neat package. The data and functions within a class are called members of

the class.


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Introduction





the class.


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Introduction





the class.


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Introduction





the class.


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Structure

class

{

data members

function members

};

Example

class Student

{

int id_num;

string last_name;

double grade_point_average;};


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Object declaration and use

To declare Student object, the Student name is used just as youuse scalar type names.

Student David;

To refer to the specific attributes of the object:

object_name.atttribute;

Example:

David.id_num = 7645;

cout


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Object declaration and use

To declare Student object, the Student name is used just as youuse scalar type names.

Student David;

To refer to the specific attributes of the object:

object_name.atttribute;

Example:

David.id_num = 7645;

cout


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Access modifier

By default, all members of a class are private, meaning theycannot be accessed using any statements in any functions that arenot also part of the class.Declare class data members to be public instead of private.

class Student

{

public:

int id_num;

string last_name;

double grade_point_average;

};

Using the keyword public means that the fields are now accessiblewhen they are used with a Student object in a main() function.

Ch 4 CLASS d OBJECT


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What is encapsulating

When you create a class name for a group of associated variables,you contain, or encapsulate, the individual components.Programmers sometimes refer to encapsulation as an example of using a ”black box”.A black box is a device that you can use, but cannot look inside tosee how it works.

In contrast, the interface intercedes between you and the morecomplicated inner workings of the radio

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Chapter 4 CLASS and OBJECT


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Designing a class: A&Q

Before designing a new class, some questions and its answers mustbe stated. For example, when designing class Student

Q: What shall we call it?

A: Student.

Q: What are its attributes? A: It has an integer ID number, a string last name, and a

double grade point average.

Q: What methods are needed by Student?

A: A method to assign values to a member of this class (forexample, one Student’s ID number is 3232, her last name is”Walters”, and her grade point average is 3.45).

Q: Any other methods?

A: A method to display data in a member of this class (for

example, display one Student’s data).



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Designing a class: an Example

For most object-oriented classes, then, you declare both fields andfunctions

You declare a field using a data type and an identifier.

You declare a function by writing its prototype, which serves

as the interface to the function

class Student

{

private:

int id_num;string last_name;

double grade_point_average;

public:

void display_student_data();

};



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Implementation section: what is it?

The first part is a declaration section, which contains the classname, variables (attributes), and function prototypes. The secondpart created is an implementation section, which contains thefunctions themselves. Implementation is in class interface Implementation is outside class interface

class Student

{

private:

int id_num;public:

int get_id_num()

{

return id_num;

}

};



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p


Implementation section: example

Example

// implementation section:

void Student::displayStudentData(){

cout


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Why is ”static”?

When objects are instantiated, each one gets its own block of memory for its data members. For example, if you create an arrayof 100 objects, 100 blocks of memory are set asideSometimes every instantiation of a class requires the same value.For example100 Student objects, all Students need their own ID and

grade point average, but not all Students need their own

copy of the athletic fee figure. If each Student object

contains a copy of the athletic fee, you repeat the sameinformation 100 times, wasting memory.

To avoid this, you can declare the athletic fee variable as static,meaning that only one memory location is allocated, no matterhow many objects of the class you instantiate.



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When objects are instantiated, each one gets its own block of memory for its data members. For example, if you create an arrayof 100 objects, 100 blocks of memory are set asideSometimes every instantiation of a class requires the same value.

For example100 Student objects, all Students need their own ID and







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Defining

Structure

static data_type variable_name;

A static data member is defined (given a value) in a single

statement outside the class definition.

class Student

{

private:

int idNum;static double athleticFee;

public:

void setIdNum(int);

int getIdNum();

double getAthleticFee();

};


S l b


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Some features of static data member

A static class member exists, even when you have not

instantiated any objects of the class. They belong to the class, and you can use them even if you

never instantiate an object.

For a non-class member function (such as main()) to use the

static field directly, it cannot be private.


S i l b


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St ti l ss e be s


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static function member

If static data member were private, and you wanted to access

it without using an object, you would have to use a publicfunction to access the value.

Additionally, the function would have to be a static function.

A static function is one you can use with or without a declared

object




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static function member

If static data member were private, and you wanted to access

it without using an object, you would have to use a publicfunction to access the value.

Additionally, the function would have to be a static function.

A static function is one you can use with or without a declared

object


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this Pointer


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It would waste space if you stored the code for the member

functions separately for each object. C++ does not store member functions separately for each

instance of a class. Instead, one copy of each memberfunction is stored, and each instance of a class uses the same

function code→ When calling a non-static member function, it needs to knowwhich object to use.Within a member function, the address of the calling object isstored in a special pointer called the ”this” pointer. The thispointer holds the memory address of the current object that isusing the function;

Example


this Pointer


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Example


this Pointer


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Example


this Pointer


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Example


this Pointer


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Example


this Pointer


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Example


Constructor


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Constructor function is a function which is automaticallycalled when you create a new object.

important Constructor name is identical to class name.

Constructor(s) do not return values. If not declaration for constructor, a default constructor is

create.

If there is a declaration of a constructor, there would be no

default constructor.


Constructor


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Constructor: Example

Exampleusing namespace std;

class Employee

{

private:

int idNum;double hourlyRate;

public:

Employee();

void setIdNum(const int);

void setHourlyRate(const double);

int getIdNum();

double getHourlyRate();

};

Employee::Employee()

{

idNum = 9999;


Constructor


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Constructor: overloading

A class can have multipleconstructors, with differentparameters.

This technique is calledoverloading the constructor.

Example

class Writer

{private:

string firstName;

string middleName;

string lastName;

// other data members can go hepublic:

Writer(string, string, string

Writer(string, string);

string toString();

// other functions can go here

};


Constructor


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Destructor

Only use when heapmemory is allocated for

objects. (what is heapmemory?)

Usually use for free thememory location allocated.

Example

class array

{private:

char* c;

public:

array()

{ c = new char[100];

}

~array()

{

delete c;

}


Q & A


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QUESTION and ANSWER

chapter 4. class and object

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