Explains the constructor and destructor functions in C++ briefly

1. Computer Science
Constructors and Destructors…
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Saharsh Anand

2. </>
Constructor Function
giving the class object some sense…

3. Ever thought that what kind of
problems can an object create if it’s
not initialised.
It’s similar to giving your vehicle the
final gear just during the startup.

4. The Object
What if user
didn’t assign me
any value initially
(by chance he
forget) and finally
ask me my value?
I’ll be helpless.
So to make a
program
robust the
object should
be initialised
every time.
And in class we do this by constructors...

5. • Well! It is a special member function
that is used for initialization of objects
(data members).
• A constructor function is called
whenever an object is created.
• Constructors are also called when an
object is created as a part of another
object.
What is “ Constructor ” !!!

6. </>
Initialising a Constructor:
/* Outside the Class */
class twelfth{
int total; //Total twelfth of student
char sec; //Section of the class
public:
twelfth(void); //Constructor Function Declared
---
---
};
//Constructor Function Defined
twelfth :: twelfth( ){
total=50;
sec=‘A’;
}

7. Initialising a Constructor:
</> /* Outside the Class */
class twelfth{
int total; //Total twelfth of student
char sec; //Section of the class
public:
twelfth( ){ //Function Declared
total=50;
sec=‘A’;
}

8. Constructor Functions have “same
name” as that of class name.
They do not have return types, not even
void.
May or may not have parameters.
C++ has default constructors that are
called whenever a class is declared even
if no function with the same name exists
Some of it’s characteristics
Mind It!

9. They should be declared in “public
section”.
They are invoked “automatically” when
objects are created.
We cannot call it for the already
constructed object.

10. Default Constructor
• Because in C++ default constructor is there.
• This constructor has no arguments in it.
• Default Constructor is also called as no
argument constructor.
So you often didn’t
provide any argument to
constructor, but still
compiler didn’t bother to
show any error! How?

11. </>
Example for default constructor:
class person {
int DOB;
public:
person(){
cout<<“Contructor called";
}
};
void main(){
person obj;
getch();
}

12. Parameterised Constructor
• A parameterized constructor is just one that
has parameters specified in it.
• We can pass the arguments to constructor
function when object are created.
• A constructor that can take arguments are
called parameterized constructors.

13. </>
Example of a Parameterised Constructor:
class person{
int DOB;
public:
Creature(int year){ //Parameterized Constructor
DOB = year;
}
};

14. Copy Constructor
• Copy Constructor is used to declare and
initialize an object from another object.
• For example the statement:
abc c2(c1);
would define the object c2 and at the same
time initialize it to the value of c1.
• The process of initializing through a copy
constructor is known as copy initialization.

15. </>
Example of a Copy Constructor:
class abc{
int a, b;
public:
abc(int x, int y){
a = x;
b = y;
}
abc::abc(abc &p){
a = p.a;
b = p.b;
}
void showdata(){
cout << a << " " << b << endl;
}
};
continued…

16. void main(){
abc c1(10, 20);
abc c2(c1);
c1.showdata();
c2.showdata();
getch();
}
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continued…

17. Default Constructor
• Default argument is an argument to a
function that a programmer is not required
to specify.
• C++ allow the programmer to specify
default arguments that always have a value,
even if one is not specified when calling the
function.
• For example, in the following function
declaration:
int MyFunc(int a,int b,int c=12);

18. • The programmer may call this function in
two ways:
result = MyFunc(1, 2, 3);
result = MyFunc(1, 2);
• In the first case the value for the argument
called c is specified as normal. In the second
one, the argument is omitted, and the
default value of 12 will be used instead.
• It is possible to define constructors with
default arguments.

19. Something Important
• Automatically called when an object is
created.
• We can define our own constructors
• A constructor takes the same name as the
class name.
• We can’t define a constructor in the
private section.

20. • No return type is specified for a
constructor.
• Constructor must be defined in the
public.
• Overloading of constructors is possible.
• If an object is copied from another
object then the copy constructor is
called.

21. </>
Destructor Function
ending up the class data…

22. You won’t ever
want that your
program's
variable taking all
your hard disk
space.
In case if you want
that the program
should end up after
destroying the data
then you’ll have to
use destructors

23. •Destructors are special member
functions.
• Release dynamic allocated memory.
• Destructors are automatically named.
• Takes the same name of class name.
What is “ Destructor ” !!!

24. Syntax of Destructor
Well there is nothing to think about
the syntax of destructor if you are
familiar with the constructors.
Just add a tilde (~).
~class-name();

25. Something Important
• Destructors take the same name as
class name.
• As in constructors, destructors are also
defined in the public.
• Destructors cannot be overloaded.
• No return type is specified.

26. </>
Example of a Destructors:
class person{
int DOB;
public:
person(){
DOB=1998;
cout<<"constructor called"<<endl;
}
~person(){
cout<<"destructor called"<<endl;
}
};
continued…

27. void main(){
cout<<“n Main startn"<<endl;
{
person obj;
}
cout<<“n Main End"<<endl;
getch();
}
continued…
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28. </>
Another example of destructor:
#include<iostream.h>
#include<stdio.h>
class Line{
public:
void setLength(double len);
double getLength(void);
Line(); // This is the constructor declaration
~Line(); // This is the destructor: declaration
private:
double length;
};
continued…

29. // Member functions definitions including constructor
Line::Line(void){
cout << "Object is being created" << endl;
}
Line::~Line(void){
cout << "Object is being deleted" << endl;
}
void Line::setLength(double len){
length = len;
}
double Line::getLength(void){
return length;
}
// Main function for the program
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continued…

30. void main(){
Line line;
//Set line’s length
line.setLength(6.0);
cout<<"Length of line:"<<line.getLength()<<endl;
getch();
}
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continued…

31. Constructor
Destructor
Constructs (or
create) the data in
the objects of the
class.
Destroy (or delete)
the data in the
objects of the class.