Object Oriented Programming (OOP) - CS304 Power Point Slides Lecture 12

8/13/2019 Object Oriented Programming (OOP) - CS304 Power Point Slides Lecture 12

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Object Oriented Programming

(OOP)Lecture No. 12


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Review

Constant data members

Constant objects

Static data members


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Static Data Member

Definition

A variable that is part of a class, yet is notpart of an object of that class, is calledstatic data member


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Static Data Member

They are shared by allinstances of the class

They do not belong to anyparticular instance of a class


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Class vs. Instance Variable

Student s1, s2, s3;

Class Space

s1(rollNo,)

s2(rollNo,)

s3(rollNo,)

Instance VariableClassVariable


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Static Data Member (Syntax)

Keyword static is used to make a datamember static

class ClassName{

static DataType VariableName;

};


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Defining Static Data Member

Static data member is declared inside theclass

But they are defined outside the class


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Defining Static Data Member

class ClassName{

static DataType VariableName;

};

DataType ClassName::VariableName;


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Initializing Static Data Member

Static data members should beinitialized once at file scope

They are initialized at the timeof definition


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Example

class Student{

private:

static int noOfStudents;

public:

};

int Student::noOfStudents= 0;

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


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Initializing Static Data Member

If static data members are notexplicitly initialized at the time

of definition then they areinitialized to 0


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Example

int Student::noOfStudents;

is equivalent to

int Student::noOfStudents=0;


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Accessing Static Data Member

To access a static data member there aretwo ways

Access like a normal data memberAccess using a scope resolution operator ::


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Example

class Student{

public:


};


int main(){

Student aStudent;

aStudent.noOfStudents = 1;

Student::noOfStudents = 1;

}


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Life of Static Data Member

They are created even when there is noobject of a class

They remain in memory even when allobjects of a class are destroyed


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Example

class Student{

public:


};int Student::noOfStudents;

int main(){


}


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Example

class Student{

public:


};


int main(){

{

Student aStudent;

aStudent.noOfStudents = 1;

}


}


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Uses

They can be used to storeinformation that is required by

all objects, like global variables


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Example

Modify the class Student suchthat one can know the number

of student created in a system


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Example

class Student{

public:


Student();~Student();

};

int Student::noOfStudents = 0;


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Example

Student::Student(){

noOfStudents++;

}Student::~Student(){

noOfStudents--;

}


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Exampleint Student::noOfStudents = 0;

int main(){cout


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Problem

noOfStudents is accessible outside the class

Bad design as the local data member is keptpublic


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Static Member Function

Definition:

The function that needs access to the

members of a class, yet does not needto be invoked by a particular object, iscalled static member function


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Static Member Function

They are used to access static datamembers

Access mechanism for static member

functions is same as that of static datamembers

They cannot access any non-static members


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Example

class Student{


int rollNo;

public:

static int getTotalStudent(){

return noOfStudents;}

};

int main(){

int i = Student::getTotalStudents();

}


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Accessing non static data members

int Student::getTotalStudents(){

return rollNo;

}

int main(){

int i = Student::getTotalStudents();

/*Error: There is no instance of Student,

rollNo cannot be accessed*/

}


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this Pointer

thispointer is passed implicitly to memberfunctions

thispointer is not passed to static memberfunctions

Reason is static member functions cannotaccess non static data members


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Global Variable vs. Static Members

Alternative to static member is to useglobal variable

Global variables are accessible to allentities of the program

Against information hiding


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Array of Objects

Array of objects can only be created if anobject can be created without supplying anexplicit initializer

There must always be a default constructorif we want to create array of objects


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Example

class Test{

public:

};

int main(){

Test array[2]; // OK

}


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Example

class Test{

public:

Test();

};int main(){

Test array[2]; // OK

}


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Example

class Test{

public:

Test(int i);

};int main(){

Test array[2]; // Error

}


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Example

class Test{public:

Test(int i);

}

int main(){

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

}


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Documents

Object Oriented Programming (OOP) - CS304 Power Point Slides Lecture 12