How to clone an object in VB6

Question

I am trying to automatically clone an object without having to instantiate a new one and manually copy every single variable.

I remember back in the day (when I did VB6 everyday) I came up with a method of cloning objects using the PropertyBag, which was pretty cool. But I've lost the code and don't remember how to do it anymore.

Does anyone remember or have another method?

Answer 1

Every object in my application's framework has a Read and Store method. What I do is pass them a stream that writes to a bytearray and store the byte array. While this doesn't eliminate having to handle every property you only have to deal with this once for reading and once for writing.

An alternative is to use a property bag like Darrel Miller says but you still have to deal with each property separately. If you already have read and store then my suggestion should save some time.

Answer 2

Method I have used in the past is putting all instance variables on a UDT. As long as you keep the UDT up to date you can copy a class' data with a single method/statement.

Given a "Person" class here's a simple example:

Private Type tPerson
    ID As Long
    FirstName As String
    LastName As String
End Type  

Private m_Person As tPerson  

Public Sub InitPerson(ID As Long, FirstName As String, LastName As String)
    m_Person.ID = ID
    m_Person.FirstName = FirstName
    m_Person.LastName = LastName
End Sub  

Friend Sub SetData(PersonData As tPerson)
    m_Person = PersonData

End Sub  

Public Function GetClone() As Person
    Dim p As New Person
    p.SetData m_Person

    Set GetClone = p

End Function  

Public Property Get FirstName() As String
    FirstName = m_Person.FirstName

End Property  

To try the code:

Dim p As New Person
p.InitPerson 1, "MyName", "MyLastName"

Dim p2 As Person
Set p2 = p.GetClone

MsgBox p2.FirstName

If you maintain all instance varianbles inside the UDT instead of declaring them seperately you can have simple Clone method that needs very little maintenance.

Another advantage is you can put a UDT to a file handle for quick serialization to disk.

Public Sub Save(filePathName As String)
    Dim f As Integer
    f = FreeFile()

    Open filePathName For Binary Access Write Lock Read Write As #f
        Put #f, , m_Person
    Close #f

End Sub

A poor men's serialization solution really :-)

Answer 3

Is this what you were looking for? Article is copied below for posterity.

Serialize Data Using a PropertyBag

You can serialize your data quickly by placing it into a PropertyBag object, then reading the PropertyBags Contents property. This property is really a Byte array that is a serial representation of the data in your PropertyBag object. You can use this byte array for many purposes, including an efficient means of data transmission over DCOM:

Private Function PackData() As String
    Dim pbTemp  As PropertyBag

    'Create a new PropertyBag object
    Set pbTemp = New PropertyBag
    With pbTemp
        'Add your data to the PB giving each item a 
        'unique string key
        Call .WriteProperty("FirstName", "John")
        Call .WriteProperty("MiddleInitial", "J")
        Call .WriteProperty("LastName", "Doe")

        'Place the serialized data into a string 
        'variable.
        Let PackData = .Contents
    End With

    Set pbTemp = Nothing
End Function

To retrieve the serialized data, simply create a new PropertyBag object and set the serialized string to its Contents property. Convert the string into a byte array before assigning it to the Contents property:

Private Sub UnPackData(sData As String)
    Dim pbTemp  As PropertyBag
    Dim arData()    As Byte

    'Convert the string representation of the data to 
    'a Byte array
    Let arData() = sData

    'Create a new PropertyBag object
    Set pbTemp = New PropertyBag
    With pbTemp
        'Load the PropertyBag with data
        Let .Contents = arData()

        'Retrieve your data using the unique key
        Let m_sFirstName = .ReadProperty("FirstName")
        Let m_sMiddleInitial = _
            .ReadProperty("MiddleInitial")
        Let m_sLastName = .ReadProperty("LastName")
    End With

    Set pbTemp = Nothing
      End Sub

Mike Kurtz, McKees Rocks, Pa.

Answer 4

Also read: Persisting a Component's Data.