How to use Classes in QTP

Here i am giving an useful information about how to use classes in QTP.

Before going to that we will discuss about using functions in QTP.

If there is a function in our script like this

'*********** ********* ********* ********* ********* ********* **
Function Demo(a,b)
demo=a+b
End Function
'*********** ********* ********* ********* ********* ********* **

This is a function to add two numbers.
To call this function we write
'*********** ********* *****
val=demo(2,3)
msgbox val
'*********** ********* *****
We can store the functions in library files (.vbs) and to use this function, Library should be associated to the QTP Test or library file should be executed by QTP Test using ExecuteFile statement.
This means we can use the functions which are written in QTP Script or in Library files by associating it or by executing it.

But this is not possible when using classes in our QTP Test. If we want to use a class which is there in the libraryfile its mandotory that the library should be executed using Executefile statement.
Classes will not work which are there in associated libraries. Here is an example...

'*********** ********* ********* ********* ********* **
Class Maths
Function Add1(a,b)
add1=a+b
End Function

Function sub1(a,b)
sub1=a-b
End Function

End Class
'*********** ********* ********* ********* ********* **
If the above code is there with in the test script then the usage of this class will be like this
'*********** ********* ********* ********* ********* **
Set mat=New Maths
val=mat.add1( 2,3)
msgbox val
'*********** ********* ********* ********* ********* **
If the class code is there with in the library file then the usage of this class will be like this
'*********** ********* ********* ********* ********* **
ExecuteFile (Library Path)
Set mat=New Maths
val=mat.add1( 2,3)
msgbox val
'*********** ********* ********* ********* ********* **
This class will not work If you just associate the library with in the
File--> Settings --> Resources
But there is a way of using the classes even though if it is in associated library.

First Method:
Where ever the class is declared in the library it self create the class instance and use it in your qtp script.
'*********** ********* ********* ********* ********* **
Set mat=New Maths

Class Maths
Function Add1(a,b)
add1=a+b
End Function

Function sub1(a,b)
sub1=a-b
End Function

End Class
'*********** ********* ********* ********* ********* **
To Use this class
'*********** ********* *
val=mat.add1( 2,3)
msgbox val
'*********** ********* *

Second Method:
Create a function which is returning class object by creating instance of the class.
'*********** ********* ********* ********* ********* **
Function mat()
Set mat=New Maths
End Function

Class Maths
Function Add1(a,b)
add1=a+b
End Function

Function sub1(a,b)
sub1=a-b
End Function

End Class
'*********** ********* ********* ********* ********* **
To Use this class
'*********** ********* *
Set mth=mat ' mat is a function which will return the class instance
val=mth.add1( 2,3)
msgbox val
'*********** ********* *