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A1880

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  • "A1880" is male
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Posts: 500

Date of registration: Jan 1st 2009

Location: Hanover, Germany

Occupation: Software Engineer

Hobbies: Hilbert Curves

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1

Monday, July 27th 2009, 1:19pm

Len for Variables

Hi,
in VB6 the Len() function returns the length of a string and the number of bytes used for a variable.
That is useful if you want to estimate memory consumption or have to pass a structure of dynamic size to an external API.

The following yields "0 14" in VB6:

Source code

 
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Private Type myType
    a As Integer
    s As String
    d As Double
End Type

Private Sub Command1_Click()
    Dim mt As myType
        
    MsgBox Len(myType) & " " & Len(mt)
End Sub


Jabaco does not compile this code. It expects only string arguments and thus complaints that conversion to String is not supported.

Basically, Len() acts similar to "sizeof()" in C/C++.
In Java, it takes some fiddling around to actually get the size of an object.
Jabaco's compiler might be able to know the size of some variables at compile time.

Greetings!

A1880

OlimilO

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Date of registration: Jan 18th 2009

Location: Germany

Occupation: software engineer

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2

Monday, July 27th 2009, 2:38pm

sizeof = LenB

Hi,

first: due to VB6, when working with UD-Types and API-functions, never use Len, but use LenB instead.

Jabaco Source

 
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Private Type myType
    a As Integer
    s As String
    d As Double
End Type
Private Sub Command1_Click()
    Dim mt As myType
     
    'MsgBox Len(myType) & " " & Len(mt)
    MsgBox Len(mt) & " " & LenB(mt)
    
End Sub


As you may know, the reason for that is: padding bytes

but you got to keep calm, normally all UD-Types concerning the win-API have a 4-byte alignment.

by the way: additional tipp: VB is able to deal with 1,2 and 4-byte-alignment, so packed records are also possible in VB, just use a typelibrary for your udtypes. In VB-modules only 4-byte aligned udtype-records will be produced by VB itself. Google search: "ActiveVB UDTypeAlignment.zip"



second: it's only for completeness reasons: there was a similar thread to this questions started by StefanSchnell :
Size of a structure
he also posted a solution for that kind of problems, togehter with his VarPtr.dll



third: All of this is not the answer to the JNA-questions. But we got to ask Manuel how the native-keyword is meant to be.

@Manuel, ho is the new native-keyword in conjunction with the new class-property ment to be used?
Is it maybe the way or even half the way to JNA?


regards
OlimilO

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