Computer Notes - Static Data Member

8/3/2019 Computer Notes - Static Data Member

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Static Data MemberStatic Data Member

http://ecomputernotes.com


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ReviewReview

Constant data membersConstant data members

Constant objectsConstant objects

Static data membersStatic data members



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DefinitionDefinition

A variable that is part of a class, yet is not partA variable that is part of a class, yet is not part

of an object of that class, is called static dataof an object of that class, is called static data

membermember



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They are shared by all instances ofThey are shared by all instances ofthe classthe class

They do not belong to anyThey do not belong to anyparticular instance of a classparticular instance of a class



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Class vs. Instance VariableClass vs. Instance Variable

Student sStudent s11,, s2s2,, s3s3;;

Class Space

s1(rollNo,)

s2(rollNo,)

s3(rollNo,)

Instance VariableClass

Variable


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Static Data Member (Syntax)Static Data Member (Syntax)

Keyword static is used to make a dataKeyword static is used to make a datamember staticmember static

classclass ClassNameClassName{{

staticstatic DataTypeDataTypeVariableNameVariableName;;

};};



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Defining Static Data MemberDefining Static Data Member

Static data member is declared inside theStatic data member is declared inside theclassclass

But they are defined outside the classBut they are defined outside the class



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Defining Static Data MemberDefining Static Data Member

classclass ClassNameClassName{{

staticstatic DataTypeDataTypeVariableNameVariableName;;

};};

DataTypeDataType ClassName::VariableNameClassName::VariableName;;



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Initializing Static Data MemberInitializing Static Data Member

Static data members should beStatic data members should beinitialized once at file scopeinitialized once at file scope

They are initialized at the time ofThey are initialized at the time ofdefinitiondefinition



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ExampleExampleclass Student{class Student{

private:private:

static intstatic int noOfStudentsnoOfStudents;;

public:public:

};};

intint Student::noOfStudentsStudent::noOfStudents = 0;= 0;

/*private static member cannot be accessed outside the/*private static member cannot be accessed outside theclass except for initialization*/class except for initialization*/



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Initializing Static Data MemberInitializing Static Data Member

If static data members are notIf static data members are notexplicitly initialized at the time ofexplicitly initialized at the time of

definition then they are initializeddefinition then they are initialized

to 0to 0



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ExampleExampleintint Student::noOfStudentsStudent::noOfStudents;;

is equivalent tois equivalent to

intint Student::noOfStudentsStudent::noOfStudents=0;=0;



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Accessing Static Data MemberAccessing Static Data Member

To access a static data member there are twoTo access a static data member there are two

waysways

Access like a normal data memberAccess like a normal data member

Access using a scope resolution operatorAccess using a scope resolution operator::::



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public:public:


};};

intint Student::noOfStudentsStudent::noOfStudents;;

int main(){int main(){

StudentStudent aStudentaStudent;;

aStudent.noOfStudentsaStudent.noOfStudents = 1;= 1;

Student::noOfStudentsStudent::noOfStudents = 1;= 1;

}}



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Life of Static Data MemberLife of Static Data Member

They are created even when there is no objectThey are created even when there is no object

of a classof a class

They remain in memory even when all objectsThey remain in memory even when all objects

of a class are destroyedof a class are destroyed



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public:public:static intstatic int noOfStudentsnoOfStudents;;

};};

intint Student::noOfStudentsStudent::noOfStudents;;int main(){int main(){


}}



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public:public:

static intstatic int noOfStudentsnoOfStudents;;};};

intint Student::noOfStudentsStudent::noOfStudents;;


{{

StudentStudent aStudentaStudent;;aStudent.noOfStudentsaStudent.noOfStudents = 1;= 1;

}}


}}



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UsesUses

They can be used to storeThey can be used to store

information that is required by allinformation that is required by all

objects, like global variablesobjects, like global variables



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ExampleExample

Modify the class Student such thatModify the class Student such thatone can know the number ofone can know the number ofstudent created in a systemstudent created in a system



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public:public:


Student();Student();

~Student();~Student();

};};

intint Student::noOfStudentsStudent::noOfStudents = 0;= 0;



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ExampleExampleStudent::Student(){Student::Student(){

noOfStudentsnoOfStudents++;++;

}}

Student::~Student(){Student::~Student(){noOfStudentsnoOfStudents----;;

}}



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ExampleExampleintint Student::noOfStudentsStudent::noOfStudents = 0;= 0;

intintmain(){main(){

cout


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ProblemProblemnoOfStudentsnoOfStudents is accessible outside the classis accessible outside the class

Bad design as the local data member is keptBad design as the local data member is keptpublicpublic



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Static Member FunctionStatic Member Function

Definition:Definition:

The function that needs access to theThe function that needs access to the

members of a class, yet does not need to bemembers of a class, yet does not need to be

invoked by a particular object, is called staticinvoked by a particular object, is called staticmember functionmember function



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Static Member FunctionStatic Member FunctionThey are used to access static data membersThey are used to access static data members

Access mechanism for static member functionsAccess mechanism for static member functionsis same as that of static data membersis same as that of static data members

They cannot access any nonThey cannot access any non--static membersstatic members



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intint rollNorollNo;;public:public:

static int getTotalStudent(){static int getTotalStudent(){

returnreturn noOfStudentsnoOfStudents;;

}}

};};


int i = Student::getTotalStudents();int i = Student::getTotalStudents();

}}



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Accessing non static data membersAccessing non static data membersint Student::getTotalStudents(){int Student::getTotalStudents(){

returnreturn

rollNorollNo

;;

}}


intint i = Student::getTotalStudents();i = Student::getTotalStudents();

/*Error: There is no instance of Student,/*Error: There is no instance of Student,rollNorollNo cannot be accessed*/cannot be accessed*/

}}



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this Pointerthis Pointerthisthis pointer is passed implicitly to memberpointer is passed implicitly to member

functionsfunctionsthisthis pointer is not passed to static memberpointer is not passed to static member

functionsfunctions

Reason is static member functions cannotReason is static member functions cannot

access non static data membersaccess non static data members



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Global Variable vs. Static MembersGlobal Variable vs. Static Members

Alternative to static member is to useAlternative to static member is to use

global variableglobal variable

Global variables are accessible to allGlobal variables are accessible to all

entities of the programentities of the programAgainst information hidingAgainst information hiding



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Array of ObjectsArray of ObjectsArray of objects can only be created if an objectArray of objects can only be created if an object

can be created without supplying an explicitcan be created without supplying an explicitinitializerinitializer

There must always be a default constructor ifThere must always be a default constructor if

we want to create array of objectswe want to create array of objects



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ExampleExampleclass Test{class Test{

public:public:

};};

intintmain(){main(){Test array[2]; // OKTest array[2]; // OK

}}



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public:public:Test();Test();

};};

intintmain(){main(){Test array[2]; // OKTest array[2]; // OK

}}



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public:public:Test(intTest(int i);i);

};};

intintmain(){main(){Test array[2]; // ErrorTest array[2]; // Error

}}



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}}


Test array[2] =Test array[2] ={Test(0),Test(0)};{Test(0),Test(0)};

}}



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}}


Test a(1),b(2);Test a(1),b(2);

Test array[2] = {Test array[2] = {a,ba,b};};

}}


Computer Notes - Static Data Member

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