Constructors and Destructors

Constructors and Destructors

·         A constructor is a member function of a class whose function name is same as the name of the class.

·         It is used to initialize the values of data members.

·         It does not return a value nor is it void.

·         It may contain parameters.

·         It will be invoked whenever the object of the class is created.

·         The memory for the class will be allocated only when the object is created.

·         When no constructors are used explicitly by the programmer, the contructor constructs garbage value into its data members.

Eg of defining a constructor:

class sample

{

int x;

public:

sample()

{

cout<<"constructing..";

x=5;

}

...

};

void main()

{

y=s.y;

}

};

void main()

{

sample obj1,obj2(150,100); // statement to invoke the default and parameterized constructors

sample obj3=obj2; // statement to invoke the copy constructor

}

Function 1,2 and 3 together are constitute constructor overloading.

In Object oriented Programming structure, this concept is called as polymorphism.

Calls to the constructor may be implicit or explicit.

Implicit - sample s(13,14);

Explicit - sample s=sample(13,14);

The explicit call to a constructor allows you to create a temporary instance

Temporary instance - A temporary instance is an instance that is instantiated by a class to invoke a member function or functions and is immediately discarded.

eg:

class sample

{int i,j;

public:

sample(int a, int b)

{

i=a;j=b;}

void print()

{cout<<i<<" "<<j;}

};

void test()

{

sample s1(2,5);

s1.print();

sample(100,200).print();

}

In order to use temporary instance, a parameterized constructor should have been present.

What is destructor?
Destructor is a member function which destructs or deletes an object.

When is destructor called?
A destructor function is called automatically when the object goes out of scope:
(1) the function ends
(2) the program ends
(3) a block containing local variables ends
(4) a delete operator is called 

How destructors are different from a normal member function?
Destructors have same name as the class preceded by a tilde (~)
Destructors don’t take any argument and don’t return anything.

Exercises

Answer the questions(i) and (ii) after going through the following
class:
class planet
{
 char name[20];char distance[20];
public:
 planet() //Function 1
 {
 strcpy(name, "Venus");
 strcpy(distance,"38 million km");
 }
 void display(char na[],char d[]) //Function 2
 {
 cout<<na<<"has "<<d<<" distance from Earth"<<endl;
 }
 planet(char na[], char d[]) //Function 3
 {
 strcpy(name,na);
 strcpy(distance,d);
 }
 ~planet() //Function 4
 {
 cout<<"Planetarium time over!!!"<<endl;
 }
};
I. What is Function 1 referred as? When will it be executed?
Default constructor. It will be executed when the object is created.

II. Write suitable C++ statement to invoke Function 2. 

planet p;
p.display("Mars","20 million km");Observe the following C++ code and answer the questions (i) and (ii).
Note : Assume all necessary files are included.
class TEST
{
long TCode;
char TTitle[20];
float Score;
public:
TEST() //Member Function 1
{
TCode=100;strcpy(TTitle,"FIRST Test");Score=0;
}
TEST(TEST &T) //Member Function 2
{
TCode=E.TCode+1;
strcpy(TTitle,T.TTitle);
Score=T.Score;
}
};

void main()
{
_______________ //Statement 1
_______________ //Statement 2
}
(i) Which Object Oriented Programming feature is illustrated by
the Member Function 1 and the Member Function 2 together
in the class TEST ?  
(ii) Write Statement 1 and Statement 2 to execute Member
Function 1 and Member Function 2 respectively.

Observe the following C++ code and answer the questions (i) and (ii).
 Note : Assume all necessary files are included.
 class EXAM
 {
 long Code;
 char EName[20];
 float Marks;
 public:
 EXAM() //Member Function 1
 {
 Code=100;strcpy(EName,"Noname");Marks=0;
 }
 EXAM(EXAM &E) //Member Function 2
 {
 Code=E.Code+1;
 strcpy(EName,E.EName);
 Marks=E.Marks;
 }
 };
 void main()
 {
 _____________ //Statement 1
 _____________ //Statement 2
 }
 (i) Which Object Oriented Programming feature is illustrated by the Member
Function 1 and Member Function 2 together in the class EXAM ? 1
 (ii) Write Statement 1 and Statement 2 to execute Member Function 1 and
Member Function 2 respectively. 

Observe the following C++ code and answer the questions (i) and (ii). Assume all
necessary files are included :
 class FICTION
 {
 long FCode;
 char FTitle[20];
 float FPrice;
 Public: 
FICTION() //Member Function 1
 {
 cout<<”Bought”<<endl;
 FCode=100;strcpy(FTitle,”Noname”);FPrice=50;
 }
 FICTION(int C,char T[],float P) // Member Function 2
 {
 FCode=C;
 strcpy(FTitle,T);
 FPrice=P;
 }
 void Increase(float P) // Member Function 3
 {
 FPrice+=P;
 }
 void Show() // Member Function 4
 {
 cout<<FCode<<”:”<<FTitle<<”:”<<FPrice<<endl;
 }
 ~FICTION() // Member Function 5
 {
 cout<<”Fiction removed!” <<endl;
 }
 };
 void main() //Line 1
 { //Line 2
 FICTION F1,F2(101,”Dare”,75); //Line 3
 for (int I=0;I<4;I++) //Line 4
 { //Line 5
 F1.Increase(20);F2.Increase(15); //Line 6
 F1.Show();F2.Show(); //Line 7
 } //Line 8
 } //Line 9
 (i) Which specific concept of object oriented programming out of the following
is illustrated by Member Function 1 and Member Function 2 combined
together ? 1
 • Data Encapsulation
 • Data Hiding
 • Polymorphism
 • Inheritance
 (ii) How many times the message “Fiction removed!” will be displayed
after executing the above C++ code ? Out of Line 1 to Line 9, which line is

responsible to display the message “Fiction removed!” ?