Chapter 9 Objects and Classes

§9.1 Introduction§9.2 Defining Classes and Objects§9.3 Access Objects§9.4 Separating Declaration from Implementation§9.5 Data Encapsulation and Class Scope§9.6 Class vs. Data Type§9.7 Class Abstraction and Encapsulation§9.8 The C++ string Class

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§9.1 Introduction

Object-Oriented Programming (OOP)（面向对象编程） Programming using objects

Object（对象） Represents an entity in the real world that can be

distinctly identified. For example, a student, a desk, a circle, and even a loan.

Has a unique identity, state, and behaviors. State: a set of data fields (or properties) with values Behavior: a set of functions

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Real World vs. Cyber World

Cyber WorldReal worldSolution

Problem Computer Program

materialization

Abstraction

(Functions,Classes/Objects)

Entity

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Objects and Class（类与对象）

Circle objects

radius,getArea()

Circle class

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Class

The structure that defines objects of the same type. Variables:

to define data fields (properties) Functions:

to define behaviors

radius,getArea()

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§9.2 Defining Classes and Objects

class Name {public: data fields functions Name()private: data fields functions};

class Circle { public: // The radius of this circle double radius; // Construct a circle object Circle() { radius = 1; } // Construct a circle object Circle(double newRadius) { radius = newRadius; } // Return the area of this circle double getArea() { return radius * radius * 3.14159; } };

Data field

Function

Constructors

1. Capitalize the initials of class name

2. No initial value when declaring data fields

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UML Class Diagram

Circle radius: double Circle() Circle(newRadius: double) getArea(): double

circle1: Circle radius: 10

Class name Data fields Constructors and Functions

circle2: Circle radius: 25

circle3: Circle radius: 125

UML Class Diagram

UML notation for objects

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Constructor（构造函数）

A special type of function to construct objects from the class Plays the role of initializing objects Exactly the same name as the class No return type—not even void Executed automatically by the system, NOT YOU!

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Constructor

Can be overloaded To construct objects with different initial data values

No-arg constructor A constructor with no parameters

Default constructor No parameters, empty body Provided automatically only if no explicitly declared

constructors class Circle { public: double radius; double getArea() { return radius * radius * 3.14159; } };

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Creating Objects

The syntax to create an object ClassName variableName; For example, Circle circle1; Circle a;

A constructor is invoked when an object is created

no-arg constructor

Data type vs. VariableClass vs. Object

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Constructing Objects with Arguments

ClassName variableName(arguments); For example,

Circle circle2(5.5); Circle b(11);

Invoking the Circle class’s constructor with a specified radius 5.5

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§9.3 Accessing Objects

To access the members of a class/object Data fields and functions

objectName.dataField --references a data field in the object.

objectName.function(arguments) --invokes a function on the object.

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A Simple Circle Class

Objective: demonstrate creating objects, accessing data, and using functions

TestCircle Run

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Memberwise Copy

“=”: to copy the contents from one object to the other. By default, each data field of one object is copied to its

counterpart in the other object. For example,

circle2 = circle1;

radius:10getArea()

radius:15getArea()

radius:15getArea()

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Anonymous Object（匿名对象） Objects without name

Occasionally, you may create an object and use it only once.

ClassName(); ClassName(arguements);

Circle circle1;Circle circle2(5.0);cout<<circle1.getArea();cout<<circle2.getArea();cout<<Circle().getArea();cout<<Circle(100).getArea();

No “()”!

“()”!

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Accessing Object Members via Pointers

Pointing to existing objects

Creating dynamic objects

Access using “.” or “->”

Circle circle1;Circle *pCircle = &circle1;

ClassName *pObject = new ClassName();ClassName *pObject = new ClassName;

cout << "The radius is " << (*pCircle).radius << endl;cout << "The area is " << (*pCircle).getArea() << endl;(*pCircle).radius = 5.5;cout << "The radius is " << pCircle->radius << endl;cout << "The area is " << pCircle->getArea() << endl;

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§9.4 Separating Declaration from Implementation

Similar to the definition of function Declaration（声明） :

In classname.h file, conventionally Simply lists all the data fields, constructor prototypes,

and the function prototypes. Implementation（实现） :

In classname .cpp file, conventionally Implements the constructors and functions.

Circle.h RunCircle.cpp

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Inline Declaration（声明） Inline Declaration:

A member function automatically becomes an inline function if it is implemented inside a class declaration.

class A {public: A() { // do something; } double f1() { // return a number } double f2();};

Inline functionInline function

Regular function

double A::f2(){ // return a number}

inline double A::f2()

Inline function

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§9.5 Data Encapsulation and Class Scope

Public（公有） A visibility/accessibility keyword All the following data

fields/functions are accessible outside the class

Private（私有） All the following data

fields/functions are NOT accessible outside the class

class Circle {public:

double radius;Circle();Circle(double);double getArea();

};

Not good!-Data may be tampered!-Class is difficult to maintain!

class Circle {public:

Circle();Circle(double);double getArea();

private:double radius;

};

int main(){ Circle circle1; circle1.radius=0; cout << circle1.radius; cout<<circle1.getArea()<<endl;}

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Getter and Setter

For users/clients of a class to retrieve and modify a data field encapsulated by “private”

Getter/Accessor A function usually named getXxx:

Setter/Mutator A function usually named setXxx, e.g.:

returnType getPropertyName()bool isPropertyName()

void setPropertyName(dataType propertyValue)

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Example: New Circle Class

Circle

-radius: double +Circle() +Circle(radius: double) +getRadius(): double +setRadius(radius: double): void +getArea(): double

The radius of this circle (default: 1.0). Constructs a default circle object. Constructs a circle object with the specified radius. Returns the radius of this circle. Sets a new radius for this circle. Returns the area of this circle.

The - sign indicates private modifier

Circle2.h RunCircle2.cpp

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Note “public” and “private” can appear any times in any

order in a class Data fields and functions can be declared in any order

in a classFor example, all the following declarations are equivalent

class Circle { public: Circle(); Circle(double); private: double radius; public: double getArea(); double getRadius(); void setRadius(double); }; (a) (b)

class Circle { private: double radius; public: double getArea(); double getRadius(); void setRadius(double); public: Circle(); Circle(double); };

class Circle { public: Circle(); Circle(double); double getArea(); double getRadius(); void setRadius(double); private: double radius; };

(c)

Preferred: one public and one private, public first

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The Scope of Variables

Global variables: declared outside all functions Scope: from declaration to the end of the program.

Local variables: defined inside functions Scope: from declaration to the end of its block (may not

the end of the function!)

int max(int x, int y){ … int i=0; …}

for (int i=0; i<10;i++){ … int j=0; …}

…{ … int j=0; …}…

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The Scope of Data Fields

Class scope: The data fields are accessible to all constructors and

functions in the class. Hidden data field:

The data field with the same name as a local variable inside a function

The local variable takes precedence

RunHideDataField void p() { int x = 20; // local variable cout << "x is " << x << endl; cout << "x of class is " << Foo::x << endl; cout << "y is " << y << endl; }

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The “this” Pointer A special built-in pointer that points to the current object Used to cope with hidden data fields

// Construct a circle object Circle::Circle(double radius) {

this->radius = radius; (*this).radius = radius;

}

// Set a new radius void Circle::setRadius(double radius) {

this->radius = (radius >= 0) ? radius : 0; }

Circle::getThis() { return *this;

}

A Special Scenario

Accessing the data fields of the objects of the same class

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class MyPoint{ private: double x, y; public: MyPoint(); MyPoint(double x, double y; double getX(); double getY(); double distance(const MyPoint point){ sqrt((x-point.x)*(x-point.x)+ (y-point.y)* (y-point.y)); }};

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§9.6 Class vs. Data Type

Class is a Type You can also use class names to declare object names,

like a primitive data type. Object name is a constant

Once an object name is declared, it references to an object. It cannot be reassigned to reference another object.

Class replaces “struct”struct Student{ int id; char firstName[30]; char mi; char lastName[30];};

class Student{public: int id; char firstName[30]; char mi; char lastName[30];};

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Passing Objects to Functions You can pass objects by value or by reference.

Run

RunPassObjectByValue

PassObjectByReference

RunPassObjectToPointer

c: Circle

radius: 5.0

myCircle: Circle radius: 5.0

Copy myCircle to c myCircle: Circle

radius: 5.0

c is an alias for myCircle

myCircle: Circle radius: 5.0

c:

address of myCircle

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Array of Objects

Circle circleArray[3] = {Circle(3), Circle(4), Circle(5)};

RunTotalArea

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Object Data Field Class as data field type

An object is a member of another object/class

Construction of member object A member object is not created/constructed on its

declaration It is created when its owner object is created

class Time{private:

int hour; int minute; int second;

};

class Action{private: Time time;};

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Default Constructor of Object Data Field

The default constructor is automatically invoked Compiling error, if default constructor is not

available -> Due to the declaration of constructor with arg

DefaultConstructor1

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Constructor Initializer Used by a constructor To initialize data fields of the class

Circle::Circle() : radius(1) { }

Circle::Circle() { radius = 1; }

Same as

(a)

(b) class Time{

public: Time(int hr, int min, int sec) { } private:

int hour; int minute; int second;

};

class Action{public: Action(int hr, int min, int sec) :time(hr, min, sec) { } private: Time time;};

NoDefaultConstructor2

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§9.7 Class Abstraction and Encapsulation

Class Abstraction To separate class implementation from the use of the class The creator of the class provides a description of the class and let

the user know how the class can be used. The detail of implementation is encapsulated and hidden

from the user. The user of the class does not need to know how the class is

implemented.

Class Contract (Signatures of

public methods and public constants)

Class

Class implementation is like a black box hidden from the clients

Clients use the

class through the contract of the class

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Example: The Loan Class

TestLoanClass RunLoan.cpp

Loan

-annualInterestRate: double -numberOfYears: int -loanAmount: double +Loan() +Loan(annualInterestRate: double,

numberOfYears: int, loanAmount: double)

+getAnnualInterestRate(): double +getNumberOfYears(): int +getLoanAmount(): double +setAnnualInterestRate( annualInterestRate: double): void +setNumberOfYears( numberOfYears: int): void +setLoanAmount( loanAmount: double): void +getMonthlyPayment(): double +getTotalPayment(): double

The annual interest rate of the loan (default: 2.5). The number of years for the loan (default: 1) The loan amount (default: 1000). Constructs a default Loan object. Constructs a loan with specified interest rate, years, and

loan amount. Returns the annual interest rate of this loan. Returns the number of the years of this loan. Returns the amount of this loan. Sets a new annual interest rate to this loan.

Sets a new number of years to this loan. Sets a new amount to this loan. Returns the monthly payment of this loan. Returns the total payment of this loan.

Loan.h

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§9.8 The C++ “string” Class A C++ library class defined in <string> With more features than C-string functions

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Constructing a String

A string object with empty string:

string newString;

A string object from a string literal or an char array: string newString(stringLiteral);

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Appending a String

string s1("Welcome");s1.append(" to C++"); // appends " to C++" to s1cout << s1 << endl; // s1 now becomes Welcome to C++string s2("Welcome");s2.append(" to C and C++", 0, 5); cout << s2 << endl; string s3("Welcome");s3.append(" to C and C++", 5); cout << s3 << endl; string s4("Welcome"); s4.append(4, 'G'); cout << s4 << endl;

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Assigning a String

You can use several overloaded functions to assign new contents to a string. For example, see the following code:

string s1("Welcome");s1.assign("Dallas"); // assigns "Dallas" to s1cout << s1 << endl; // s1 now becomes Dallasstring s2("Welcome");s2.assign("Dallas, Texas", 0, 5); // assigns "Dalla" to s2cout << s2 << endl; // s2 now becomes Dallastring s3("Welcome");s3.assign("Dallas, Texas", 5); // assigns "Dalla" to s3cout << s3 << endl; // s3 now becomes Dallastring s4("Welcome");s4.assign(4, 'G'); // assigns "GGGG" to s4cout << s4 << endl; // s4 now becomes GGGG

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Functions at, clear, erase, and empty

You can use the at(index) function to retrieve a character at a specified index, clear() to clear the string, erase(index, n) to delete part of the string, and empty() to test if a string is empty. For example, see the following code:

string s1("Welcome");cout << s1.at(3) << endl; // s1.at(3) returns ccout << s1.erase(2, 3) << endl; // s1 is now Wemes1.clear(); // s1 is now emptycout << s1.empty() << endl; // s1.empty returns 1 (means true)

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Comparing StringsOften, in a program, you need to compare the contents of two strings. You can use the compare function. This function works in the same way as the C-string strcmp function and returns a value greater than 0, 0, or less than 0. For example, see the following code:

string s1("Welcome");string s2("Welcomg");cout << s1.compare(s2) << endl; // returns -2cout << s2.compare(s1) << endl; // returns 2cout << s1.compare("Welcome") << endl; // returns 0

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Obtaining SubstringsYou can obtain a single character from a string using the at function. You can also obtain a substring from a string using the substr function. For example, see the following code:

string s1("Welcome");cout << s1.substr(0, 1) << endl; // returns Wcout << s1.substr(3) << endl; // returns comecout << s1.substr(3, 3) << endl; // returns com

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Searching in a String You can use the find function to search for a substring or a character in a string. For example, see the following code:

string s1("Welcome to HTML");cout << s1.find("co") << endl; // returns 3cout << s1.find("co", 6) << endl; // returns -1cout << s1.find('o') << endl; // returns 4cout << s1.find('o', 6) << endl; // returns 9

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Inserting and Replacing StringsHere are the examples to use the insert and replace functions:

string s1("Welcome to HTML");s1.insert(11, "C++ and ");cout << s1 << endl; // s1 becomes Welcome to C++ and HTMLstring s2("AA");s2.insert(1, 4, 'B');cout << s2 << endl; // s2 becomes to ABBBBAstring s3("Welcome to HTML");s3.replace(11, 4, "C++");cout << s3 << endl; // returns Welcome to C++

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String Operators

Operator Description [] Accesses characters using the array subscript operators.

= Copies the contents of one string to the other.

+ Concatenates two strings into a new string.

+= Appends the contents of one string to the other.

<< Inserts a string to a stream

>> Extracts characters from a stream to a string delimited by a

whitespace or the null terminator character.

==, !=, <, Six relational operators for comparing strings.

<=, >, >=

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Summary Concept of class/object Declaration of class

Class header file Construction of objects

Object member Object as parameter Object pointer Access control: public vs. private Scope of class member

Homework Questions

1. What are the difference between constructors and member functions?

2. Design and implement a simple class as you want, with constructors and member functions.

3. What is the purpose of using accessibility control?

4. What are the benefits of data field encapsulation?

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