Constructors and destructors

Constructors & Destructors
Prof. K. Adisesha

Learning Outcomes
 Introduction
 Defining Constructors
 Characteristics of Constructor
 Types of constructor
 Constructor Overloading
 Destructors
 Characteristics of destructor
2

Introduction
Introduction
 It is sometimes convenient if an object can initialize itself
when it is first created, without the need to make a
separate call to member functions.
 Automatic initialization is carried out using special
member functions called constructors.
 Automatic de-allocating of memory when an object of
that class is destroyed is carried out using special member
functions called destructor.
3

Constructors
Constructors:
 A Constructor is a special member function that is
called automatically when an object is created.
 The purpose of a constructor is to mainly initialize the
member variables of a class.
 The general syntax of a the constructor in C++ is:
class MyClass // The class
{ public: // Access specifier
MyClass() // Constructor
{ cout << "Hello World!";
}
}; 4

Constructors
Characteristics of Constructor:
 The name of the constructor is the same as the name of the
class.
 A Constructor, even though it is a function, has no return type
function nor a void function.
 The constructor should be declared in the public section.
 Constructors are executed automatically.
 They are executed when a class object is created.
 A class can have more than one constructor.
 It is not possible to refer to the address of the constructors.
 The constructors make implicit calls to the operator new and
delete when memory allocation is required.
5

Constructors
Need for a Constructor:
 Constructors are named as constructors because they are
getting called when an object is constructed.
 The use of a constructor can be cleverly done especially
in those problems where it is necessary to initialize certain
data members compulsorily.
 Instead of having separate member functions for
initializing we can perform those operations inside the
constructor itself.
6

Constructors
Types of constructor:
 Constructors are normally classified as follows:
 Default Constructors.
 Parameterized Constructors.
 Copy Constructors.
7

Constructors
Default Constructors:
 A default constructor is a special member function which is
invoked by the C++ compiler without any argument for
initializing the object of a class.
 It is also called as zero argument constructors.
 Some of the features of the default constructors are:
 A default constructor function initializes the data member with no
argument.
 It can be explicitly written in the public section of the class.
 In case, default constructor is not defined in a program, the C++
compiler automatically generates it in a program.
 The purpose of the default constructor is to construct a default object of
the class type.
8

Constructors
Default Constructors:
 The general format of default constructor is as follows.
9
Syntax:
class Class_Name
{
public:
Class_Name( )
{
…….
}
};
Example:
class Number
{
public:
Number( )
{
n = 0;
}
};

Constructors
Disadvantages of default constructors are:
 When many objects of the same class are created, all
objects are initialized to same set of values by default
constructors.
 It is not possible to initialize different objects with
different initial values using default constructors.
10

Constructors
Parameterized Constructors:
 A constructor that takes one or more arguments is called
parameterized constructor.
 Using this constructor, it is possible to initialize different
objects with different values.
 Parameterized constructors are also invoked automatically,
whenever objects with arguments are created.
 Some of the features of the parameterized constructors are:
 The parameterized constructors can be overloaded.
 For an object created with one argument, constructor with only one
argument is invoked and executed.
 The parameterized constructor can have default arguments and default
values.
11

Constructors
Parameterized Constructors:
 The general format of Parameterized constructor is as follows.
12
Syntax:
class Class_Name
{
public:
Class_Name(arg1, arg2)
{
…….
}
};
Example:
class Number
{
public:
Number(int a, int b )
{ if (a > b)
big = a;
else
big = b; }
};

Constructors
Copy Constructors:
 Copy constructor is a parameterized constructor using one
object can be copied to another object.
 Copy Constructors are used in the following situations:
 To initialize an object with the values of already existing
objects.
 When objects must be returned as function values.
 To state objects as by value parameters of a function.
 Copy Constructor can accept a single argument of reference to
same class type.
 The argument must be passed as a constant reference type.
13

Constructors
Copy Constructors:
 The general format of copy constructor is as follows.
14
Syntax:
class Class_Name
{
public:
Class_Name( Class_Name &ptr )
{
…….
}
};
Example:
class Number
{
public:
Number(int n)
{ a = n; }
Number(Number &X)
{ a = X.a;
cout<<”Copy Constructor
invoked”;
} };

Constructors
Invoking Constructors
 A Constructor is automatically invoked by C++ compiler
with an object declaration.
 The constructor can be invoked through the following
methods.
 Implicit Call
 Explicit Call
 Initialization with “ = ” Assignment operator
15

Implicit Call:
 An Implicit call means the declaration of the object is followed
by argument list enclosed in parenthesis.
 Syntax:
Class_name Object(Arg1, Arg2,…)
 Example:
num obj1(10, 20); //Implicit Call
16

Explicit Call:
 In explicit call, declaration of an object is followed by
assignment operator, constructor name and argument list
enclosed in parenthesis.
 Syntax:
Class_name Object=Constructor_name(Arg1, Arg2,…)
 Example:
num obj1 = num(10, 20); //Explicit Call
num obj2 = num(40, 50); //Explicit Call
17

Initialization with “ = ” Assignment operator:
 This method is used for the constructor with exactly one
argument.
 In this method declaration is followed by assignment operator
and value to be initialized..
 Syntax:
Class_name Object=Arg1
 Example:
num obj1 = 100
num obj2 = 200
18

#include<iostream.h>
#include<conio.h>
class num
{
private:
int a, b;
public:
num ( int m) // Constructor
{ a = m; b=m; }
num (int m, int n) // Constructor
{
a = m;
b = n;
}
void display( )
{
cout<<” a = “ << a <<” b = “ << b;
}
};
void main( )
{
num obj2=num(30, 40); //Explicit Call
Num obj3=50; //Assignment operator
obj1.display( );
obj2.display( );
Obj3.display( );
}
19
Program to initialize objects using Invoking Constructors

Constructor
Constructor Overloading:
 A class has two or more constructor functions with the same
name but different signatures are called Constructors
Overloading.
 Depending upon the type of argument, the constructors will be
invoked automatically by the compiler to initialize the objects.
 Example: class num
{
public:
num ( ); // Constructor with no agruments
num ( int m); // Constructor with one agrument
num (int m, int n); // Constructor with two agruments
}; 20

Destructors
Destructors:
 A destructor is special member function that is executed
when an object of that class is destroyed.
 Destroying an object means, de-allocating all the
resources such as memory that was allocated for the
object by the constructor.
 It will have like constructor, the name same as that of the
class but preceded by a tilde (~).
21

Destructors
Destructors:
 The general format of destructor is as follows.
22
Syntax:
class Class_Name
{
public:
Class_Name( );
~ Class_Name( );
};
Example:
class Counter
{
public:
Counter( ) //Constructor
{
n = 0;
}
~Counter ( ) //Destructor
{ }
};

Destructors
Characteristics of destructor:
 The destructor name must have the same name as the
class preceded by a tilde (~).
 The destructor cannot take arguments therefore cannot be
overloaded.
 The destructor has no return type.
 There can be only one destructor in each class.
 In should have public access in the class declaration.
 The destructor cannot be inherited.
23

Discussion
Questions:
 What is Constructor? Give the rules for writing a constructor
function.
 What is default constructor? Write a program to illustrate it.
 Explain parameterized constructor with syntax and example.
 Mention the different methods by which constructor are
invoked. Explain anyone with an illustrative example.
 Explain the features of copy constructor.
 Explain destructor with syntax and example.
24

Constructors and destructors

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