Computer Notes - Friend Functions

8/3/2019 Computer Notes - Friend Functions

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Friend FunctionsFriend Functions

http://ecomputernotes.com


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CompositionCompositionConceptual notation:Conceptual notation:

String()

SetString(char *) : void

GetString() const : const char *

~String()

gpa : float

rollNo : int

name : String

Student(char * = NULL, int = 0,

float = 0.0);

Student(const Student &)

GetName() const : String

GetNamePtr() const : const char *

SetName(char *) : void~Student()

Student

string : char *

String



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CompositionComposition

Main Function:Main Function:

intintmain(){main(){StudentStudent aStudent("FakhiraStudent("Fakhir", 899,", 899,

3.1);3.1);coutcout


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CompositionCompositionOutput:Output:

Constructor::StringConstructor::String....

Constructor::StudentConstructor::Student....

Name:Name: FakhirFakhir

Destructor::StudentDestructor::Student....Destructor::StringDestructor::String....



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CompositionCompositionStudent::Student(charStudent::Student(char * n,* n,

intint roll, float g){roll, float g){

coutcout


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CompositionCompositionTo assign meaningful values to the object,To assign meaningful values to the object,

the functionthe function SetStringSetString is called explicitly inis called explicitly inthe constructorthe constructor

This is an overheadThis is an overhead

SubSub--objectobject namename in thein the studentstudent class canclass can

be initialized using the constructorbe initialized using the constructor

Member initialization listMember initialization listsyntax is usedsyntax is used



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CompositionCompositionAdd an overloaded constructor to theAdd an overloaded constructor to the

StringString class defined above:class defined above:

class String{class String{char *char *ptrptr;;

public:public:String();String();

String(charString(char *);*);String(constString(const String &);String &);

voidvoidSetName(charSetName(char *);*);~String();~String();

};};

//String(char * = NULL);



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CompositionCompositionString::String(charString::String(char ** strstr){){

if(strif(str != NULL){!= NULL){

ptrptr = new char[strlen(str)+1];= new char[strlen(str)+1];

strcpy(ptrstrcpy(ptr,, strstr););}}

elseelseptrptr = NULL;= NULL;

coutcout


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CompositionCompositionStudent class is modified asStudent class is modified asfollows:follows:

class Student{class Student{

private:private:

floatfloat gpagpa;;intint rollNumberrollNumber;;

String name;String name;

public:public:

Student(charStudent(char *=NULL,*=NULL, intint=0, float=0.0);=0, float=0.0);

};};



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Student class continued:Student class continued:Student::Student(charStudent::Student(char ** n,intn,int roll,roll,

float g):float g): name(nname(n)){{coutcout


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Main Function:Main Function:

intintmain(){main(){StudentStudent aStudent("FakhiraStudent("Fakhir", 899,", 899,

3.1);3.1);coutcout


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OverloadedOverloadedConstructor::StringConstructor::String....

Constructor::StudentConstructor::Student....

Name:Name: FakhirFakhir

Destructor::StudentDestructor::Student....Destructor::StringDestructor::String....



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CompositionCompositionNow consider the following case:Now consider the following case:

String

name: char*

String()

String(char *)

~String()

Date()

Date(int,int,int)

Date(const Date &)

Student()

Student( char *,

const Date &, int, float)

SetName(char *) : voidGetName() : char *

~Student()

name : StringbirthDate : Date

Student

day: int

Month: intyear: int

Date


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CompositionCompositionStudent class is modified asStudent class is modified asfollows:follows:

class Student{class Student{

private:private:

DateDatebirthDatebirthDate;;

String name;String name;

public:public:

Student(charStudent(char *, const Date &,*, const Date &, intint,,float);float);

~Student();~Student();

};};



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Student class continued:Student class continued:Student::Student(charStudent::Student(char * n, const Date & d,* n, const Date & d, intint

roll,roll, flaotflaot g):g): name(n),birthDate(dname(n),birthDate(d)){{

coutcout


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Main function:Main function:

intintmain(){main(){Date _date(31, 12, 1982);Date _date(31, 12, 1982);

StudentStudent aStudent("FakhiraStudent("Fakhir",",

_date,899,3.5);_date,899,3.5);

return 0;return 0;

}}http://ecomputernotes.com


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OverloadedOverloadedConstructor::DateConstructor::Date....

CopyCopy Constructor::DateConstructor::Date....

OverloadedOverloadedConstructor::StringConstructor::String....Constructor::StudentConstructor::Student....

Destructor::StudentDestructor::Student....

Destructor::StringDestructor::String....Destructor::DateDestructor::Date....

Destructor::DateDestructor::Date....



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AggregationAggregationComposition vs. AggregationComposition vs. Aggregation

Aggregation is aAggregation is a weak relationshipweak relationship

Room(char *, int)~Room()

FoldChair(int) : bool

Chair()

DoSomething() : voidFoldChair() : bool

UnFoldChair() : bool

~Chair()

area : float

chairs[50]:Chair *

Room

Chair


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AggregationAggregationIn aggregation, a pointer or referenceIn aggregation, a pointer or reference

to an object is created inside a classto an object is created inside a class

The subThe sub--object has a life that isobject has a life that is NOTNOT

dependant on the life of its master classdependant on the life of its master classe.ge.g::

Chairs can be moved inside or outside at anytimeChairs can be moved inside or outside at anytime

When Room is destroyed, the chairs may orWhen Room is destroyed, the chairs may or maymaynotnot be destroyedbe destroyed


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AggregationAggregationclass Room{class Room{private:private:

float area;float area;

Chair * chairs[50];Chair * chairs[50];

Public:Public:Room();Room();

voidvoidAddChair(ChairAddChair(Chair *,*, intint chairNochairNo););

Chair *Chair * GetChair(intGetChair(int chairNochairNo););

boolbool FoldChair(intFoldChair(int chairNochairNo););

};};



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AggregationAggregationChair *Chair * Room::GetChair(intRoom::GetChair(int chairNochairNo){){

if(chairNoif(chairNo >= 0 &&>= 0 && chairNochairNo< 50)< 50)returnreturn chairs[chairNochairs[chairNo];];

elseelse

return NULL;return NULL;

}}

boolbool Room::FoldChair(intRoom::FoldChair(int chairNochairNo){){

if(chairNoif(chairNo >= 0 &&>= 0 && chairNochairNo< 50)< 50)

returnreturn chairs[chairNochairs[chairNo]]-->>FoldChairFoldChair();();elseelse

return false;return false;

}}



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AggregationAggregationintintmain(){main(){

Chair ch1;Chair ch1;{{

Room r1;Room r1;

r1.AddChair(&ch1, 1);r1.AddChair(&ch1, 1);r1.FoldChair(1);r1.FoldChair(1);

}}

ch1.UnFoldChair(1);ch1.UnFoldChair(1);

return 0;return 0;

}}http://ecomputernotes.com


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Friend FunctionsFriend FunctionsConsider the following class:Consider the following class:

class X{class X{

private:private:

intint a, b;a, b;public:public:

voidvoidMemberFunctionMemberFunction();();

}}



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Friend FunctionsFriend FunctionsGlobal function:Global function:

voidvoidDoSomething(XDoSomething(X objobj){){

obj.aobj.a = 3;= 3; //Error//Error

obj.bobj.b = 4;= 4; //Error//Error

}}



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Friend FunctionsFriend FunctionsIn order to access the member variables of the class, functionIn order to access the member variables of the class, functiondefinition must be made a friend function:definition must be made a friend function:

class X{class X{

private:private:

intint a, b;a, b;

public:public:

friend voidfriend voidDoSomething(XDoSomething(X objobj););

}}

Now the functionNow the function DoSomethingDoSomething can access data members ofcan access data members ofclass Xclass X



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Friend FunctionsFriend Functions

Prototypes of friend functionsPrototypes of friend functionsappear in the class definitionappear in the class definition

But friend functions are NOTBut friend functions are NOTmember functionsmember functions



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Friend FunctionsFriend FunctionsFriend functions can be placed anywhere in the classFriend functions can be placed anywhere in the class

without any effectwithout any effectAccessAccess specifiersspecifiers dondont affect friend functions ort affect friend functions orclassesclasses

class X{class X{

......

private:private:

friend voidfriend voidDoSomething(XDoSomething(X););

public:public:friend voidfriend voidDoAnything(XDoAnything(X););

......

};}; http://ecomputernotes.com


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Friend FunctionsFriend FunctionsWhile the definition of the friend function is:While the definition of the friend function is:

voidvoidDoSomething(XDoSomething(X objobj){){

obj.aobj.a = 3;= 3; // No Error// No Error

obj.bobj.b = 4;= 4; // No Error// No Error

}}

friendfriendkeyword is not given in definitionkeyword is not given in definition


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Friend FunctionsFriend FunctionsIf keywordIf keyword friendfriendis used in the functionis used in the functiondefinition, itdefinition, its a syntax errors a syntax error

//Error//Error

friend voidfriend voidDoSomething(XDoSomething(X objobj){){

}}



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Friend ClassesFriend ClassesSimilarly, one class can also be made friend ofanother class:

class X{

friend class Y;

};

Member functions of class Y can accessprivate data members of class X



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Friend ClassesFriend ClassesExample:

class X{friend class Y;

private:int x_var1, x_var2;

...

};



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Friend ClassesFriend Classesclass Y{

private:int y_var1, y_var2;X objX;

public:void setX(){objX.x_var1 = 1;

}

};



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Friend ClassesFriend Classes

int main(){Y objY;objY.setX();return 0;

}


Computer Notes - Friend Functions

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