chapter 5 classes and objects §5.1 specifying a class §5.2 defining member functions §5.3 memory...

Chapter 5 Classes and Objects

§5.1 Specifying a class§5.2 Defining Member Functions§5.3 Memory Allocation and Static Members§5.4 Passing Objects to/from Functions§5.5 Friendship

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§ 5.1 Specifying a Class

Object（对象） Abstraction of entities State + Behavior

Class （类） A user-defined data type that declare objects Variables + Functions

radius,getArea()

Declaring a Class

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class class_name {private: variable declarations; function declarations;public: variable declarations; function declarations;};

class class_name {private: variable declarations; function declarations;public: variable declarations; function declarations;};

class item{ int number; float cost; public: void getdata(int a, float b); //function declaration void putdata(void);};

No initial value in declaration!

The following members are accessible outside the class

The following members are NOT accessible outside the class

private:

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Creating Objects

The syntax to create an object

ClassName variableName;

For example,

item x;

item x, y, z;

Memory for x is createdMemory for x is created

Data type vs. VariableClass vs. Object

Data type vs. VariableClass vs. Object

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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.

Data hiding in class

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class xyz{ int x; int y; public: int z;};

int main(){ xyz p; p.x = 0; //error, x is private p.z =10; //OK, z is public}

Private Member Functions

Normally: data in private; functions in public Some functions in private sections

Can only be called by member function of its class To protect data from modifications by outsiders

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class sample {

int m; void read(void);

public: void update(void); void write(void);

};

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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()returnType getPropertyName()

bool isPropertyName()bool isPropertyName()

void setPropertyName(dataType propertyValue)void setPropertyName(dataType propertyValue)

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A Simple Circle Class

To demonstrate creating objects, accessing data, and using functions

TestCircleTestCircle RunRun

Pointers to Members

A class member pointer can be declared using the operator ::* with the class name.

For example:

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class A { private:

int m; public:

void show();};

int A::* ip = &A::m;

§ 5.2 Defining Member Functions

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return-type class-name :: function-name (parameters){ Function body}

return-type class-name :: function-name (parameters){ Function body}

Identity labelScope resolution operation

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Inline Declaration

A member function can be implemented inside a class declaration automatically an inline function

class A {

public:

A() {

// do something;

}

double f1() {

// return a number

}

double f2();

};

Inline functionInline functionInline functionInline function

Regular functionRegular function

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

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

inline double A::f2()

Inline functionInline function

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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.hCircle.h RunRunCircle.cppCircle.cpp

#include <iostream> using namespace std; class set{

int m, n; public:

void display(void);int largest(void);

}; int set::largest(void){

if(m<=n) return m; else return n;

} void set::display(){

cout<<largest()<<"\n"; }

Nesting of Member Functions

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A member function can be called by another member function of the same class

•Via function name directly (no object name)

§ 5.3 Memory Allocation and Static Members

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Memory of member functions -- Common for all objects -- Created/Allocated when defined

Memory of member functions -- Common for all objects -- Created/Allocated when defined

Memory of member variables -- specific to an object -- Created/Allocated when object created

Memory of member variables -- specific to an object -- Created/Allocated when object created

Member function1Member function1

Member function2Member function2

Object1: Member variable1Object1: Member variable1




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Array of Objects

employee manager[3];

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

RunRunTotalAreaTotalArea

namename

ageageManager[0]Manager[0]

namename


namename


Static/Class Variables (静态 /类变量） Data fields shared by all the objects of the same class Stored in static memory region To declare:

static type var_name;//In declaration To initialize:

type ClassName::var_name = 0;

//re-declare it with initial value in implementation To access/reference:

(ClassName::)var_name…

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Static Functions

Member function that can be called without creating an instance of the class

To declare:

static type functionName(parameters){} “static” appears in declaration file

To call:

ClassName::functionName(arguments);

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Example of Static Members

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Circle -radius: double

-numberOfObjects: int

+getNumberOfObjects(): int

+getArea(): double

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radius

circle1 radius = 1 numberOfObjects = 2

Instantiate （实例化）

Memory

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5

radius

numberOfObjects

UML Notation: underline: static variables or functions

circle2 radius = 5 numberOfObjects = 2

RunRun

TestCircle5.cppTestCircle5.cppCircle5.hCircle5.h Circle5.cppCircle5.cpp

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§ 5.4 Passing Objects to/from Functions

You can pass objects by value or by reference/address.

RunRun

RunRunPassObjectByValuePassObjectByValue

PassObjectByReferencePassObjectByReference

RunRunPassObjectToPointerPassObjectToPointer

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

Returning Objects

Objects can be returned to caller Example:

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class complex{ //x + iy form float x; float y;public: void input(float real, float imag) { x = real; y = imag;} friend complex sum(complex, complex); void show(){ cout << c.x << " + i" << c.y <<"\n"; }};

complex sum(complex c1, complex c2){ complex c3; c3.x = c1.x + c2.x; c3.y = c1.y + c2.y; return (c3);}

int main(){ complex A, B, C; A.input(3.1, 5,65); B.input(2.75, 1.2); C = sum(A, B); cout << "C = "; C.show();}

Friendship!Friendship!

§5.5 Friendly Function/Class

Friends(友元 ): some special classes/functions that

can directly access the private members of a class.

Syntax:

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class Name{ … friend class FriendName; friend type funcName(); …};

class Name{ … friend class FriendName; friend type funcName(); …};

Notes

Friends can be declared in either “public” or “private” part No difference.

All the member functions of a friend class are friend functions

The implementation of friend function can be put inside OR outside the definition of the class

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Example of Friend Function

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RunRunTestFriendFunctionTestFriendFunction

class Date { friend void p(); private: int year; int month; int day; }; void p() { Date date; date.year = 2000; cout << date.year; }

class Date { friend void p(); private: int year; int month; int day; }; void p() { Date date; date.year = 2000; cout << date.year; }

Example of Friend Class

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RunRunTestFriendClassTestFriendClass

Date2 .hDate2 .h

class AccessDate { public: static void p() { Date birthDate; birthDate.year = 2000; cout << birthDate.year; } };

class AccessDate { public: static void p() { Date birthDate; birthDate.year = 2000; cout << birthDate.year; } };

class Date {

public:

friend class AccessDate;

private:

int year; int month; int day;

};

class Date {

public:

friend class AccessDate;

private:

int year; int month; int day;

};

Summary

Declaration of class Access control: public vs. private

Data hiding

Class member pointer Memory allocation Static members Object as parameter Object as returning value Friendly function/class

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Review Questions

Q5.1 Q5.7 Q5.8

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chapter 5 classes and objects §5.1 specifying a class §5.2 defining member functions §5.3 memory...

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