I am PGT(Comp. Sc) in KV(AFS) Utarlai publishing Constructor and Destructor for Class XII

1. Constructor & Destructor

2. Constructors
• Def- A constructor is a special member
function that is a member of a class and
has same name as that class.
• Use- It is used to initialize the object of the
class type with a legal initial value.

3. Special Characteristics of Constructors
1. These are called automatically when the objects are
created.
2. All the objects of the class having a constructor are
initialized before some use.
3. These should be declared in the public section for
availability to all the functions.
4. Constructor has no return type even void.
5. They can not be inherited.
6. These cannot be static.
7. Default and copy constructor are generated by the
compiler whenever required. Generated constructors
are public.
8. Constructors can have default argument as other C++
functions.

4. Declaration and Definition
• It can be defined either inside the class
definition or outside the class definition
class X { int i ;
public:
int j,k ;
X() { i = j = k =0;}
};

5. class X { int i ;
public:
int j,k ;
X() { i = j = k =0;}
};
X::X()
{
I = j = k =0;
}

6. Note
• Generally a constructor should be defined
under the public section of a class, so that
its object can be created in any function

7. class X { int i ;
X() { i = j = k =0;}
public:
int j,k ;
void check(void); // member function
};
void X :: check (void)
{
X obj1; //valid
}
int main()
{
X obj2; //invalid
}

8. Default constructor
• A constructor that accept no parameter is
called the default constructor.

9. Copy Constructor
• A copy Constructor is used to initialize an object
from another object.
• If you have not defined a copy constructor, the
complier automatically creates it and it is public
• Copy constructor always takes argument as a
reference object.
Consider the following class definition

10. class sample {
int i, j;
public:
sample ( int a, int b)
{
i = a;
j = b;
}
sample ( sample & s)
{
i = s.i ;
j = s.j ;
}
void print(void)
{
cout<<i <<“ “<<j <<“n”;
}
Above constructors may be used as
follows
sample s1 (10,12); //1st const. called
sample s2 (s1) // copy const. called
sample s3 = s1; copy const. called

11. Home work
• In a class does not define a constructor,
what will be the initial value of its object.
• What are the significance of default
constructor.
• How many situations when a copy
constructor is automatically called.

12. Default Arguments
• Just like any other function a constructor
can also have default arguments
• This constructor is equivalent to default
constructor. Because its also allows us to
create objects without any value provided.
• For example

13. class A
{
int i,j;
public:
X(int x=10,int y=20);
};
A::X(int x,int y)
{
i=x;
j = y;
}
int main()
{
A obj1;
A obj2(250);
A obj3(2,4);
getch();
return 0;
}
i=10
j=20
i=250
j=20
i=2
j=4
obj1 obj2 obj3

14. Order of Constructor Invocation
• The objects are constructed in the order they are
defined. Consider the following statement
• Sample s1,s2,s3; // the order of construction is
s1,s2,s3.
• But when a class containing objects of another
class as its members. In such a case, the
constructor for member objects are invoked first
and then only, the constructor for containing
class is invoked.
• For example

15. class A
{
public:
A()
{
cout<<“Constructing A”
<<endl;
}};
class B
{
public:
B()
{
cout<<“Constructing B”
<<endl;
}};
class C
{
private:
A objA;
B objB;
public:
C()
{
cout<<“Constructing C”
<<endl;
}};
int main()
{
clrscr();
C objC;
getch();
}