Collections without methods!

[S@NSIS]

found an example!!

Take the Drives collection of the FileSystemObject. It is a collection of the Drive object and yet this Drives collection has no add,remove methods only Count and Item properties. In other words it is read-only and you can go:

For each Drive in Drives
 msgbox Drive.AvailableSpace
Next

This is how I want to able to make my collection. (no add or remove method)

Thanks in advance

Shaun

[PaulLewis]

As far as I am aware there is no implementation in VB for Class Static Methods. The example you gave is invalid because Drives is an instance of a collection object.

Otherwise the For..Each loop would not work on it.

When you are in a class module programming, you are really programming for an instance of the class. In otherwords, the only way to make any of your code function is to create an object of your class.

In C++, Java and others, you can have static methods of a class which means you do not have to have an instance of the class in order to execute some of your code. In VB I think you are going to have to put up with the limitation.

It's not such a big problem though as long as your class name is something like "clsContact" and you simply have a global variable in your code called "Contacts". Then you will get the exact duplicate of how the Drives collection in your example works. Don't forget to create the instanciate your global variable of course.

' in a module or if it is only to be used in the form, in the form declaration area
Public Contacts as clsContact

...

' in the Form_Load or Sub_Main
Set Contacts =  New clsContact

One more hint: When naming classes, try to use the singular form of the noun. Save plurals like "Contacts" for naming collections and so on.

Regards

P.S. I have just re-read everything and realise I have waffled on about something you no doubt know and more importantly, I didn't help towards your question. But I will leave the post in case it helps future readers...
More to follow


[Edited by PaulLewis on 09-18-2000 at 08:06 PM]

[PaulLewis]

Code for the class and a test form to partially emulate a Collection. The fact that the For..Each loop can be used on an array or a collection (not UDT's of course) means you can do this.

Hopefully it gives you a new direction to look at.

' Class1 Class
Option Explicit

Dim mCol As Collection
Dim mCol2() As String
Dim mCount As Integer

Private Sub Class_Initialize()
  Set mCol = New Collection
End Sub

Public Static Sub AddItem(anItem As String)
  ' adding an item adds to both forms of collection
  mCol.Add anItem
  mCount = mCount + 1
  ReDim Preserve mCol2(1 To mCount)
  mCol2(mCount) = anItem
  
End Sub

Public Static Function GetCol() As Collection
  ' example of getting the collection
  ' problem is that you get the add/remove methods too
  ' which is fair enough as it is a collection
  Set GetCol = mCol
End Function

Public Static Function GetCol2() As String()
  ' example of getting to the data but it is not a collection
  ' you will have to implement your own equivalent functions
  ' in your Class to provide what a collection does
  GetCol2 = mCol2()
End Function

Option Explicit
Dim myobj As Class1


Private Sub Form_Load()
  Dim item As Variant
  
  Set myobj = New Class1
  myobj.AddItem "1"
  myobj.AddItem "2"
  Debug.Print
  Debug.Print
  
  ' looks similar
  With myobj
    For Each item In GetCol2
      Debug.Print item
    Next
  End With
End Sub

[S@NSIS]

Cheers Paul,
Funny actually, my classes are named clsContact (for an individuals) and my collection class is called (funnily enough) clsContacts.

I guess that there isn't a way of doing it through VB as you said yourself

' example of getting the collection
' problem is that you get the add/remove methods too
' which is fair enough as it is a collection

which is what I am trying to avoid!

I don't think the Drives example is an invalid example because
A: It is a collection (or so the object browser tells me)
B: It doesn't have Add or Remove methods which collections are supposed to have.

I did consider the array way of doing it (more work) as you suggested and it does seem that this is the way that I will have to go!

Oh well, thanks for the pointers

Cheers

Shaun

[S@NSIS]

Ps. I am no expert on this class OOP stuff yet although I hope to be getting there!

So..If it helps, here is the code from the clsContacts object.

Option Explicit
Private mContact As New clsContact 'Set an instance of the clsContact object
Private mcol_Contacts As Collection
Private mcolPointer As Long

Private Sub Class_Initialize()
    Set mcol_Contacts = New Collection
    'Populate mcol_Contacts collection with clsContact objects whose properties are set
    'by a call to the database
    Set mcol_Contacts = PopulateContacts
    If mcol_Contacts.Count > 0 Then
        'Set mContact to be equal to the first item in our collection
        Set mContact = mcol_Contacts.Item(1)
        mcolPointer = 1 'Points to first item
    Else
        mcolPointer = 0 'No items to point to
    End If
End Sub

Public Function Add(ByVal NewContact As clsContact) As clsContact
    'Add the new contact to the mcol_Contacts collection
    mcol_Contacts.Add NewContact
    Set Add = NewContact
    'Set the pointer to point at the latest record added
    mcolPointer = mcol_Contacts.Count
    'Set the mContact object to have the properties of the latest object added
    Call ReloadMembers(NewContact)
End Function

Public Property Get Contact() As clsContact
    'Set the property equal to the current mContact object in use
    Set Contact = mContact
End Property

Private Sub ReloadMembers(aContact As clsContact)
'Set the mContact object equal to aContact
Set mContact = aContact
End Sub

Public Sub MoveFirst()
    'Set mContact to be equal to the first object in the collection
    Set mContact = mcol_Contacts.Item(1)
End Sub
Public Sub MoveLast()
    'Set mContact to be equal to the last object in the collection
    Set mContact = mcol_Contacts.Item(mcol_Contacts.Count)
End Sub
Public Sub MovePrevious()
    'Check to see if we are not already at the start of the collection and if not then move one object back in
    'the collection
    If mcolPointer > 1 Then
        mcolPointer = mcolPointer - 1
        Set mContact = mcol_Contacts.Item(mcolPointer)
    End If
End Sub
Public Sub MoveNext()
    'check to see if we are not already at the last object in the collection and if not then move
    'one object forward in the collection
    If mcolPointer < mcol_Contacts.Count Then
        mcolPointer = mcolPointer + 1
        Set mContact = mcol_Contacts.Item(mcolPointer)
    End If
End Sub
Public Property Get Count() As Long
    Count = mcol_Contacts.Count
End Property

Public Property Get AllContacts() As Collection
    'Return the collection
    Set allContacts = mcol_Contacts
End Property

Any thoughts or comments wil be appreciated!!

Cheers

Shaun

[PaulLewis]

In tat case, I imagine that when writing for a dll in C++, you can mask certain inherited behaviour of a Collection such as in your example.

At least though we agree that your example "Drives" is a collection. Now all that remains is to see if what you want to do can be achieved...

If I discover anything I'll drop you a line.

regards

[PaulLewis]

Thanks for posting the question because it has encouraged me to research a topic I have not bothered too much with in the past.

I have produced the following code in my test application. This is ALL the code there is. Of course, there is an activeX DLL I wrote which provides the classes.

Option Explicit
Dim myWheel As clsWheel
Private Sub Command1_Click()
  Set myWheel = New clsWheel
  Dim spoke As clsSpoke
  
  With myWheel
    For Each spoke In .Spokes
      Debug.Print spoke.Name, spoke.Number
    Next
  End With
End Sub

I used examples from the Programmers manual (hence Wheel, Spokes, Spoke). Look for Creating Your Own Collection Classes.

The myWheel.Spokes collection displays ONLY the count and Item methods exactly as you want. Object Browser displays only these methods as well.

The code for the classes is below. There are a few things you need to do using the Tools | Procedure Attributes |Advanced menu but I can tell you those after the code if you need.

' note that these are the class files 
VERSION 1.0 CLASS
BEGIN
  MultiUse = -1  'True
  Persistable = 0  'NotPersistable
  DataBindingBehavior = 0  'vbNone
  DataSourceBehavior  = 0  'vbNone
  MTSTransactionMode  = 0  'NotAnMTSObject
END
Attribute VB_Name = "clsSpoke"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = True
Option Explicit

Dim mNum As Integer
Dim mName As String

Public Property Let Name(newName As String)
  mName = newName
End Property
Public Property Get Name() As String
Attribute Name.VB_UserMemId = 0
  Name = mName
End Property

Public Property Let Number(newNum As Integer)
  mNum = newNum
End Property
Public Property Get Number() As Integer
  Number = mNum
End Property


VERSION 1.0 CLASS
BEGIN
  MultiUse = -1  'True
  Persistable = 0  'NotPersistable
  DataBindingBehavior = 0  'vbNone
  DataSourceBehavior  = 0  'vbNone
  MTSTransactionMode  = 0  'NotAnMTSObject
END
Attribute VB_Name = "clsWheel"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = True
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = True
Option Explicit

Dim mSpokes As clsSpokes
Dim mShape As Integer

Public Property Get Spokes() As clsSpokes
  Set Spokes = mSpokes
End Property

Public Property Get Shape() As Integer
  Shape = mShape
End Property
Public Property Let Shape(newShape As Integer)
  mShape = newShape
End Property

Private Sub Class_Initialize()
  Set mSpokes = New clsSpokes
End Sub



VERSION 1.0 CLASS
BEGIN
  MultiUse = -1  'True
  Persistable = 0  'NotPersistable
  DataBindingBehavior = 0  'vbNone
  DataSourceBehavior  = 0  'vbNone
  MTSTransactionMode  = 0  'NotAnMTSObject
END
Attribute VB_Name = "clsSpokes"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = True
Option Explicit
Private mcol As Collection

Public Function NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4
Attribute NewEnum.VB_MemberFlags = "40"
   Set NewEnum = mcol.[_NewEnum]
End Function

Public Property Get Count() As Integer
  Count = mcol.Count
End Property

Private Sub Class_Initialize()
  Set mcol = New Collection
  Dim spoke As clsSpoke
  
  Dim c As Integer
  
  For c = 1 To 10
    Set spoke = New clsSpoke
    
    With spoke
      .Name = "Spoke" & c
      .Number = c
    End With
    
    mcol.Add spoke, "Item " & Str(c)
  Next
End Sub

Public Function Item(ByVal Index As Variant) As clsSpoke
Attribute Item.VB_UserMemId = 0
   Set Item = mcol.Item(Index)
End Function

Note that to use these classes you should copy them into files directly and not attempt to paste them into the VB IDE.

I hope it makes sense. The VB documentation on this topic is excellent so if you get stuck, ask me or see if your documentation helps too

Cheers