by Kenneth Buckmaster (4 Submissions)
Category: Miscellaneous
Difficulty: Intermediate
Date Added: Wed 3rd February 2021
Rating: 
(4 Votes)
People often think they can only store one item in a tag and that that item must be a string. Not so. A tag is a string limited in length only by memory available - we can treat it as an array of bytes and store any data in it including multiple items. The best way to illustrate this is by an example.
API DeclarationsCopymemory
VBcoders
'in a form
'set autoredraw to true
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (pDst As Any, pSrc As Any, ByVal ByteLen As Long)
'in our example we'll store
'a date + a long
'+ a string
'(as per a normal tag)
'at the end
'the date is 8 bytes,
'the long 4 so we need
'the tag to be 12 bytes
'= 6 characters
'to be able store these
Sub initialiseTag(obj As Object)
obj.Tag = String(6, Chr(0)) & obj.Tag
End Sub
'the first item in the tag
'is going to be a date
Sub setTagDate(obj As Object, dte As Date)
'we can't copymemory directly to the tag
'we must use a temporary string
Dim tmpstr As String
tmpstr = obj.Tag
'make sure the tag has been initialised
If Len(tmpstr) > 5 Then
'copy date to tmpstr
CopyMemory ByVal StrPtr(tmpstr), ByVal VarPtr(dte), 8 'date = 8 bytes
End If
'set tag to altered tmpstr
obj.Tag = tmpstr
End Sub
Function getTagDate(obj As Object) As Date
If Len(obj.Tag) > 5 Then
'we can use copymemory to copy from the tag
CopyMemory ByVal VarPtr(getTagDate), ByVal StrPtr(obj.Tag), 8
End If
End Function
'same for the second item we'll store - a long
Sub setTagLong(obj As Object, ll As Long)
Dim tmpstr As String
tmpstr = obj.Tag
If Len(tmpstr) > 5 Then
'only this time we need to add
'the sum of the previous items
'to strptr
'previous was one date = 8 bytes
CopyMemory ByVal StrPtr(tmpstr) + 8, ByVal VarPtr(ll), 4 'long = 4 bytes
End If
obj.Tag = tmpstr
End Sub
Function getTagLong(obj As Object) As Long
If Len(obj.Tag) > 5 Then
CopyMemory ByVal VarPtr(getTagLong), ByVal StrPtr(obj.Tag) + 8, 4
End If
End Function
'last item well use like a normal tag
Sub setTagString(obj As Object, tagstring As String)
If Len(obj.Tag) > 5 Then
'6 character = 12 bytes = length of the date and long we're storing
'before this string
obj.Tag = Left(obj.Tag, 6) & tagstring
End If
End Sub
Function getTagString(obj As Object) As String
If Len(obj.Tag) > 6 Then
getTagString = Right(obj.Tag, Len(obj.Tag) - 6)
End If
End Function
Private Sub Form_Load()
'allocate space for the fixed length data
initialiseTag Me
'set values
setTagString Me, "String"
setTagDate Me, Date
setTagLong Me, -87454
'recall Values
Me.Print getTagDate(Me)
Me.Print getTagLong(Me)
Me.Print getTagString(Me)
End Sub