Can one Class know the object of another Class?

[jmsrickland]

Let's say I have two class modules one called Apple and the other called Orange.

Now in a .bas file I have the following declarations:

Public appl As Apple
Public oran As Oranges

Now in the main Form I have some code like this:

'
'
'
Set appl = New Apple
Set oran = New Orange
'
'
'
So my question is; is there a way in class Apple, for example, it has a way of knowing that oran was set as the object of class Orange?

[LaVolpe]

Yes....
If TypeOf Oran Is Orange Then ' oran is an orange class

Additionally. You can use TypeName(object) to check its name as a string.

Edited: And if you really want to prevent any potential of error

  
   If Not Oran Is Nothing ' will error if Oran not set to new Orange
      If TypeOf Oran Is Orange Then ' retun true?
   End If

FYI: If Oran hasn't been set to New Orange, then TypeName(Oran)="Nothing" & (Oran Is Nothing)=True

[jmsrickland]

Not sure I understand.

In your first statement you say If TypeOf oran Is Orange Then but here the name oran has to be already known where my question is how can class Apple know what the object name is so I couldn't very well use that since it is asking if typeof oran is orange and class Apple doesn't know that name.

Now in your second statement you say Additionally. You can use TypeName(object) to check its name as a string so what exactly is (object) in this statement and exactly where and how would I use that?

To make it more clear as to what I am trying to do I will try to put it this way.

In the main Form I have

Set oran = New Orange

now later in class Apple I need to do this:

If oran.OrangeType = "Naval" Then

where OrangeType is a Public variable in class Orange

but class Apple does not know that it needs to say oran.OrangeType = "Naval" because class Apple doesn't know the object name is oran. So I am asking how can class Apple find out what object name was set to class Orange.

Does that make any sense?

[LaVolpe]

How can your class possibly know all the variables in your project? You may want to give more details of why this is needed. If you are going to pass to Apple through one of Apple's functions, an object and you want Apple to know what type of object that is, here is one way.

' apple class
Public Sub DoSomething(theObject As Object)
   Select Case TypeName(theObject)
   Case "Nothing"  ' whatever was passed is not instantiated
   Case "Orange" ' this is an orange object/class

       Debug.Print "Orange type is "; theObject.OrangeType

   Case "Apple" ' this is an apple object/class
   Case "PictureBox" ' it is a picture box
   Case "Image" ' it is an image control
   etc etc etc
   End Select
  
   ' do whatever to theObject
   
End Sub

Edited: Gave a more specific example. Note that when passing classes/controls/etc to a routine as Object, only public properties, functions, subs are available. So in the above example, only if Orange's OrangeType is a public property/function can you call it; otherwise, error.

[LaVolpe]

And once you master the above, should you really want to get froggy, you can do a little research on Implementations. That is fun and is kind of what I think you are attempting to do. However, it can be very confusing to most & is a bit more difficult, but at the same time, more flexible than the method shown above.

[jmsrickland]

How can your class possibly know all the variables in your project?

Well I think this because when you write a program and your program contains a Form module and a .bas module and in the .bas module you put a Public variable then you know that the Form module knows that this variable is in the .bas module, right?

So, Class Apple knows there is a Public variable in Class Orange called OrangeType just in the same sense that the main Form knows that there is a Public variable in the .bas module

I could, since I am making the program, just hard code the object name because I already know that oran was set to class Orange so in class Apple I could do just that: If oran.OrangeType = "Naval" Then which would definitely satisfy the results of the question. However, what if I gave class Apple to another programmer to use in his code and class Apple had it hard coded then I would have to tell the other programmer that he must use oran as the object name if he was to instantiate class Orange.

Maybe what I am asking is not possible at all. I was just wanting to see if there was a way to do this without having to hard code the oran in class Apple and use whatever object name was set to class Orange.

I'm trying to do something in VB that is already done in Java. I have a Java program that has two classes and if I write the main code I can instantiate one of the classes by any name I want and if I instantiate the other class then the other class knows the name I used for the first class. I don't know how it knows that but I know it does. For all I know it works by maybe one class is a subclass of the other but I am not sure of this.

Just out of curriosity is it possible to subclass in VB in the same sense that you subclass in Java?

[jmsrickland]

And once you master the above, should you really want to get froggy, you can do a little research on Implementations. That is fun and is kind of what I think you are attempting to do. However, it can be very confusing to most & is a bit more difficult, but at the same time, more flexible than the method shown above.

Now I'm beggining to think that's how it's done in Java. I instaniate one class but implement the other. Although I don't see that in the Java program that does kind of make sense.

[LaVolpe]

What you are describing is inheritance if I am not mistaken. VB doesn't do that; though .Net does. The closest we have with VB is known as Implementation. And implementation kinda works like this.

ICommonObject is a class. And it has these functions/properites
Shape, Width, Height, Draw, etc

cCircle is a class, cRectangle is a class, cTriangle is a class. Those classes implement ICommonObject. And because they implement ICommonObject, they must also implement ICommonObject's functions/properties. Example follows. For sake of brevity, only Shape is coded.

' class name: ICommonObject
Option Explicit
' impelmentations contain no code (generally), their functions are placeholders
' though they may contain public enumerations, declarations
Public Property Get Shape() As String

End Property

' class name: cCircle
Option Explicit
Implements ICommonObject
Private Property Get ICommonObject_Shape() As String
    ICommonObject_Shape = "Circle"
End Property

' Class name: cRectangle
Option Explicit
Implements ICommonObject
Private Property Get ICommonObject_Shape() As String
    ICommonObject_Shape = "Rectangle"
End Property

' on a form....
Private Sub Command1_Click()
    Dim myShape As ICommonObject
    
    Set myShape = New cCircle
    Debug.Print myShape.Shape
    
    Set myShape = New cRectangle
    Debug.Print myShape.Shape
End Sub