CONSTRUCTORS

AND

DESTRUCTORS

Harinder Singh

Sukhpal Singh

Thapar University, Patiala

Batch: 2011-2013

WHAT IS A CONSTRUCTOR ?

In c++ , a constructor is a ‘special’ member function whose task is to initialize the objects of its class.

It is special because its name is the same as the class name.

It is called constructor because it constructs the value of data members of the class.

The constructor is invoked whenever an object of its associated class is created.

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CLASS WITH A CONSTRUCTOR Class integer {

int m, n;

public:

integer(void); // constructor declared

};

Integer : : integer (void) // constructor defined

{

m=0; n=0;

}

Object declartion:

Integer int1; // object int1 created

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CHARACTERISTICS OF THE CONSTRUCTOR

They should be declared in public section.

They are invoked automatically when the objects are

created.

They don't have return types, cannot return values.

A constructor that accepts no parameters is called

default constructor. The default constructor for class

A is A:: A() . The statement A a; invokes the default

constructor of the compiler to create the object a.

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PARAMETERIZED CONSTRUCTOR The constructor that can take arguments.

Class integer {

int m, n;

public:

integer(int x, int y); //parameterized constructor

};

Integer : : integer (int x, int y)

{

m=x; n=y;

}

Object declaration statement “integer int1 ; “ do not work

here.

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CALL IN PARAMETERIZED CONSTRUCTOR

We must pass the initial values as arguments to

the constructor function when object is created.

This can be done in two ways:

By calling the constructor explicitly.

integer int1 = integer(0, 100);

By calling the constructor implicitly.

integer int1(0, 100);

This method is also called shorthand method.

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EXPLICIT/IMPLICIT CALL

int main()

{

integer int1(0, 100); //implicit call

integer int2 = integer(25, 75); //explicit call

cout<<“object 1:”

int1.function1();

cout<<“object 2:”

int2.function1();

return 0;

}

Output

object1: m=0 and n= 100

object2: m=25 and n= 75

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MULTIPLE CONSTRUCTOR IN A CLASS

Class integer

{

int m, n;

public:

integer() //default constructor

{m=0; n=0;}

integer(int a, int b) //parameterized constructor

{m=a; n=b;}

integer(integer & i) //copy constructor

{m= i.m; n= i.n;}

};

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CONSTRUCTORS WITH DEFAULT ARGUMENTS

complex(float real , float imag=0)

{

}

complex c1(5.0);

complex c2(2.0,3.0);

Difference b/w A::A() and A::A(int a=0).

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DYNAMIC INITIALIZATION OF OBJECTS

Provide various initialization formats using

overloaded constructors.

Fixed_deposit() { }

Fixed_deposit(long int p, int y ,float r=0.12);

Fixed_deposit( long int p, int y, int r);

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COPY CONSTRUCTOR

It is used to declare and initialize an object from another object.

e.g • code() { } • code( int a) {id = a;} • code( code &x){ id = x.id; } //copy constructor.

And various calls for them are:

• code A(100); // object A is Created and initialized.

• code B(A); // copy constructor called.

• code C = A; // copy constructor called again.

• code D; // object D is created , not initialized

D=A; // copy constructor is not called.

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DYNAMIC CONSTRUCTORS

It is used to allocate memory while creating

objects.

• Enable the system to allocate right amount of memory for each object when objects are not of same size.

• Thus resulting in saving of memory.

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DYNAMIC CONSTRUCTORS

class String

{

char *name;

int length;

public:

String ()

{ length = 0;

name = new char[length+1];

}

String (char *s)

{ length = strlen(s);

name = new char[length+1];

strcpy(name, s);

}

};

main()

{

Char *first = “ME_SE_CR” ;

String name1( first);

String name2(“ Rupinder”);

.

.

.

}

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DESTRUCTORS

~ integer() { }

As new is used to allocate memory in dynamic constructors, here we use delete to free that memory.

Never take any argument nor does it return any value.

It will be invoked implicitly by compiler upon exit from program or a block or a function.

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DESTRUCTORS

Int count = 0;

class test1

{

public :

~test1()

{

count ++;

cout <<“\n no. of object created “

<< count ;

} ~test1 ()

{ cout <<“\n no. of object destroyed”

<< count ; Count - - ; } };

main()

{

cout << “\n Enter Main \n”;

test1 t1, t2, t3, t4;

{ cout <<“\n Entered in block1” ;

test1 t5;

}

{ cout <<“\n Entered in block2” ;

test1 t6;

}

cout << “\n Re-Enter Main \n”;

return 0;

}

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OUT PUT

Enter Main

No. of object created 1

No. of object created 2

No. of object created 3

No. of object created 4

Enter block 1

No. of object created 5

No. of object destroyed 5

Enter block 2

No. of object created 5

No. of object destroyed 5

Re - Enter Main

No. of object destroyed 4

No. of object destroyed 3

No. of object destroyed 2

No. of object destroyed 1

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THANK YOU

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