Constructor,destructors cpp

ObjectivesIn this chapter you will learn:
About default constructors
How to create parameterized constructors
How to work with initialization lists
How to create copy constructors
1

Advanced Constructors
Constructors can do more than initialize data
members
They can execute member functions and
perform other types of initialization routines
that a class may require when it first starts
2

Default Constructors
The default constructor does not include
parameters, and is called for any declared objects
of its class to which you do not pass arguments
Remember that you define and declare
constructor functions the same way you define
other functions, although you do not include a
return type because constructor functions do not
return values
3

Constructor Function
4
A Constructor is a special member function
whose task is to initialize the object of its
class. It is special because it name is same as
class name.
The Constructor is invoked whenever an
object of its associated class is created. It is
called constructor because it constructs the
values of data members of the class.

Default Constructors
The constructor function in Figure is an example
of a default constructor
In addition to initializing data members and
executing member functions, default constructors
perform various types of behind-the-scenes class
maintenance
5

Simple Program
6
// default constructor
#include<iostream>
Using namespace std;
Class integer
{
Int m,n;
Public:
Integer(void); // constructor declared
Void getinfo( );
};
Integer :: integer void( ) // constructor defined
{
m= 0;
n=0;
}

Parameterized Constructors
The constructor integer( ) define above initialize the
data members of all the objects to zero. However is
practice it may be necessary to initialize the various
data elements of different objects with different values,
when they are created.
A parameterized constructor allows a client to pass
initialization values to your class during object
instantiation
Instead of assigning default values, you can allow a
client to pass in the value for these data members by
designing the constructor function with parameters.
7

8
Class integer
{
Int m, n;
Public :
Integer (int x, int y);
};
Integer :: integer(int x,int y)
{
m= x;
n=y;
}
By Calling the Constructor explicitly
By Calling the consructor implicitly

 integer int1 = integer(0,100);
Explicitly called
Integer int1(0,100)
Implicitly called.
Shorthand method.
9

Copy Constructor
There will be times when you want to instantiate a
new object based on an existing object
C++ uses a copy constructor to exactly copy each of
the first object’s data members into the second
object’s data members in an operation known as
member wise copying
A copy constructor is a special constructor that is
called when a new object is instantiated from an old
object
10

Copy Constructor
The syntax for a copy constructor declaration is
class_name :: class_name (class_name &ptr)
The Copy Constructor may be used in the following format
also using a const keyword.
class_name :: class_name (const class_name & ptr)
Copy Constructor are always used when the compiler has to
create a temporary object of a class object.The copy
constructor are used in the following situations:-
The Initialization of an object by another object of the same
class.
Return of object as a function value.
Stating the object as by value parameters of a function.
11

Copy Constructor Example with Program
//fibonacci series using copy constructor
#include<iostream>
Using namespaces std ;
Class fibonacci {
Private :
Unsigned long int f0,f1,fib;
Public :
Fiboancci () // constructor
{
F0=0;
F1=1;
Fib=f0+f1;
}
Fibonacci (fibonacci &ptr) // copy construtor
{
F0=ptr.f0;
F1=ptr.f1;
Fib=prt.fib;
}
12
Void increment ( )
{
F0=f1;
F1=fib;
Fib=f0+f1;
}
Void display ( )
{
Cout<<fib<<‘/t’;
}
};
Void main( )
{
Fibonacci number ;
For(int i=0;i<=15;++i)
{
Number.display();
Number.increment();
}
}

#include<iostream> void display (void)
Using namespace std ; { cout<<id;
Class code // class name }
{ };
Int id; int main()
Public : {
Code() code A(100);//object A is created
// constructor name same as class name code B(A); // copy const. called
{ code C= A; // CC called again
} code D; // D is created, not intilized
Code(int a ) { // constructor again D=A; // C C not called
Id=a; cout<<“n” id of A:=A.display();
} cout<<“n” id of B:=B.display();
Code (code &x) // copy constuctor cout<<“n” id of C:=C.display();
{ id=x.id; // copy in the value cout<<“n” id of D:=D.display();
} return 0;
}
13

Destructors
Just as a default constructor is called when a class
object is first instantiated, a default destructor is
called when the object is destroyed
A default destructor cleans up any resources
allocated to an object once the object is destroyed
The default destructor is sufficient for most
classes, except when you have allocated memory
on the heap
14

Destructors
You create a destructor function using the name of
the class, the same as a constructor
A destructor is commonly called in two ways:
When a stack object loses scope because the function in
which it is declared ends
When a heap object is destroyed with the delete operator
15

Syntax rules for writing a dectructor function :
A destructor function name is the same as that of the
class it belongs except that the first character of the
name must be a tilde ( ~).
It is declared with no return types ( not even void)
since it cannot even return a value.
It cannot de declared static ,const or volatile.
It takes no arguments and therefore cannot be
overloaded.
It should have public access in the class declaration.

16

Class employee
{
Private :
Char name[20];
Int ecode ;
Char address[30];
Public :
Employee ( ); // constructor
~ Employee ( ); // destructor
Void getdata( );
Void display( );
};
17

18
#include<iostream>
#include<stdio>
Class account {
Private :
Float balance;
Float rate;
Public:
Account( ); // constructor name
~ account ( );// destructor name
Void deposit( );
Void withdraw( );
Void compound( );
Void getbalance( );
Void menu( );
}; //end of class definition
Account :: account( ) // constructor
{
Cout<<“enter the initial balancen”;
Cin>>balance;
}
Account :: ~account( ) // destructor

19
{
Cout<<“data base has been deletedn”
}
// to deposit
Void account ::deposit( )
{
Float amount;
Cout<<“how much amount want to deposit”;
Cin>>amount;
Balance=balace+amount ;
}
// the program is not complete , the purpose is to clear the function
of constructor and destructor

#include<iostream>
Using namespace std;
Int count =0;
Class alpha
{
Public :
Alpha()
{
Count ++;
Cout <<“n No. of object created “<<count;
}
~alpha()
{
20

Cout<<“n No. of object destroyed “<<count;
Count --;
}
};
Int main()
{ cout<<“n n Enter mainn”;
Alpha A1 A2 A3 A4;
{
Cout<<“nn Enter block 1n”;
Alpha A5;
}
{ cout<<“nn Enter Block 2 n”;
Alpha A6;
} 21

Cout<<“n n RE-enter Mainn”;
Return 0; Enter Block 2
No. of object created :5
} Re-Enter Main
No. of object destroyed
Output : Enter Main 4,3,2,1
No. of object created 1
Enter Block 1
No. ofobject created 5
No. of object Destroyed 5
22

Static Class Members
You can use the static keyword when declaring
class members
Static class members are somewhat different
from static variables
When you declare a class member to be static,
only one copy of that class member is created
during a program’s execution, regardless of how
many objects of the class you instantiate
Figure 7-38 illustrates the concept of the static
and non-static data members with the Payroll
class
23

Multiple Class Objects with
Static and Non-Static Data
Members
24

Static Data Members
You declare a static data member in your
implementation file using the syntax static
typename;, similar to the way you define a
static variable
static data members are bound by access
specifiers, the same as non-static data members
This means that public static data members
are accessible by anyone, whereas private
static data members are available only to other
functions in the class or to friend functions
25

Static Data Members
You could use a statement in the class constructor,
although doing so will reset a static variable’s value
to its initial value each time a new object of the class
is instantiated
Instead, to assign an initial value to a static data
member, you add a global statement to the
implementation file using the syntax type
class::variable=value;
Initializing a static data member at the global level
ensures that it is only initialized when a program first
executes—not each time you instantiate a new object
26

Static Data MembersFigure 7-39 contains an example of the Stocks class,
with the iStockCount static data member
Statements in the constructor and destructor
increment and decrement the iStockCount variable
each time a new Stocks object is created or destroyed
Figure 7-40 shows the program’s output
27

Static Data MembersYou can refer to a static data member by appending
its name and the member selection operator to any
instantiated object of the same class, using syntax such
as stockPick.iStockCount
By using the class name and the scope resolution
operator instead of any instantiated object of the class,
you clearly identify the data member as static
Figure 7-41 in the text shows an example of the Stocks
class program with the dPortfolioValue static data
member
The dPortfolioValue static data member is assigned
an initial value of 0 (zero) in the implementation file
28

Static Data Members
To add to the Estimator class a static data
member that stores the combined total of each
customer’s estimate, follow the instructions listed
on pages 403 and 404 of the textbook
29

Static Member FunctionsStatic member functions are useful for accessing
static data members
Like static data members, they can be accessed
independently of any individual class objects
This is useful when you need to retrieve the
contents of a static data member that is
declared as private
Static member functions are somewhat limited
because they can only access other static class
members or functions and variables located
outside of the class
30

Static Member Functions
You declare a static member function similar to
the way you declare a static data member-- by
preceding the function declaration in the interface
file with the static keyword
You do not include the static keyword in the
function definition in the implementation file
Like static data members, you need to call a
static member function’s name only from inside
another member function of the same class
31

Static Member Functions
You can execute static member functions even
if no object of a class is instantiated
One use of static member functions is to access
private static data members
To add to the Estimator class a static member
function that returns the value of the static
lCombinedCost data member, use the steps on
pages 405 and 406 of the textbook
32

Constant Objects
If you have any type of variable in a program that
does not change, you should always use the const
keyword to declare the variable as a constant
Constants are an important aspect of good
programming technique because they prevent
clients (or you) from modifying data that should
not change
Because objects are also variables, they too can be
declared as constants
33

Constant Objects
As with other types of data, you only declare an
object as constant if it does not change
Constant data members in a class cannot be
assigned values using a standard assignment
statement within the body of a member function
You must use an initialization list to assign initial
values to these types of data members
There is an example of this code shown on page
407 of the textbook
34

Constant Objects
Another good programming technique is to always
use the const keyword to declare get functions
that do not modify data members as constant
functions
The const keyword makes your programs more
reliable by ensuring that functions that are not
supposed to modify data cannot modify data
To define the Estimator class’s get functions as
constant, perform the procedures listed on page
408 of the textbook
35

Summary
The default constructor is the constructor that
does not include any parameters and that is called
for any declared objects of its class to which you
do not pass arguments
Initialization lists, or member initialization lists,
are another way of assigning initial values to a
class’s data members
A copy constructor is a special constructor that is
called when a new object is instantiated from an
old object
36

Summary
Operator overloading refers to the creation of
multiple versions of C++ operators that perform
special tasks required by the class in which an
overloaded operator function is defined
When you declare a class member to be static,
only one copy of that class member is created
during a program’s execution, regardless of how
many objects of the class you instantiate
37

Constructor,destructors cpp

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Constructor,destructors cpp