optional argument - basic.visual

On Mon, 1 Dec 2003 11:15:53 +0100, Frank wrote:

>I have a function with an optional argument (formobject type). When I call
>the function without the optional arg, but DO FILL IN the optional argument
>in the function. I do not get an error! Is that coincidance? The returned
>argument can be used without problem.
>
>Public Function isFormActive(psFormname As String, Optional pFrm As Form) As
>Boolean
>set pFrm=someform
>.....
>isFormActive=true
>end function
>
>if isFormActive("formname") then
>...

This is normal, and has nothing to do with it being optional. Arguments
are normally passed by giving the called sub or function the memory
address of the arg (aka by reference, which can be done explicitly (and
unnecessarily) with ByRef - ByRef x As String, for example). Therefore,
any changes made to those arguments are also made to any variables that
were passed.

If you don't want this to happen, then pass the args by value using
ByVal, like this:
Sub foo (ByVal x As String, ByVal y as Byte)

This forces VB to make a copy of those args marked as ByVal, and then
any changes happen to the copies, which are discarded when the sub or
function terminates.
--
auric "underscore" "underscore" "at" hotmail "dot" com
*****
People who live in glass houses shouldn't.

"Auric__" <not.my.real@email.address> wrote in message
news:jp5msvght679n6edqdd6gr691utnuc3bji@4ax.com...
> On Mon, 1 Dec 2003 11:15:53 +0100, Frank wrote:
>
> >I have a function with an optional argument (formobject type). When I
call
> >the function without the optional arg, but DO FILL IN the optional
argument
> >in the function. I do not get an error! Is that coincidance? The returned
> >argument can be used without problem.
> >
> >Public Function isFormActive(psFormname As String, Optional pFrm As Form)
As
> >Boolean
> >set pFrm=someform
> >.....
> >isFormActive=true
> >end function
> >
> >if isFormActive("formname") then
> >...
>
> This is normal, and has nothing to do with it being optional. Arguments
> are normally passed by giving the called sub or function the memory
> address of the arg (aka by reference, which can be done explicitly (and
> unnecessarily) with ByRef - ByRef x As String, for example). Therefore,
> any changes made to those arguments are also made to any variables that
> were passed.
>
> If you don't want this to happen, then pass the args by value using
> ByVal, like this:
> Sub foo (ByVal x As String, ByVal y as Byte)
>
> This forces VB to make a copy of those args marked as ByVal, and then
> any changes happen to the copies, which are discarded when the sub or
> function terminates.
> --
> auric "underscore" "underscore" "at" hotmail "dot" com
> *****
> People who live in glass houses shouldn't.

Thats all very nice Auric, but I DON'T pass an argument and can still use
the parameter in the function. And that is a bit strange, isn't it?
Frank

> > >I have a function with an optional argument (formobject type). When I
> call
> > >the function without the optional arg, but DO FILL IN the optional
> argument
> > >in the function. I do not get an error! Is that coincidance? The
returned
> > >argument can be used without problem.
> > >
> > >Public Function isFormActive(psFormname As String, Optional pFrm As
Form)
> As
> > >Boolean
> > >set pFrm=someform
> > >.....
> > >isFormActive=true
> > >end function
> > >
> > >if isFormActive("formname") then
> > >...
> >
> > This is normal, and has nothing to do with it being optional. Arguments
> > are normally passed by giving the called sub or function the memory
> > address of the arg (aka by reference, which can be done explicitly (and
> > unnecessarily) with ByRef - ByRef x As String, for example). Therefore,
> > any changes made to those arguments are also made to any variables that
> > were passed.
> >
> > If you don't want this to happen, then pass the args by value using
> > ByVal, like this:
> > Sub foo (ByVal x As String, ByVal y as Byte)
> >
> > This forces VB to make a copy of those args marked as ByVal, and then
> > any changes happen to the copies, which are discarded when the sub or
> > function terminates.
> > --
> > auric "underscore" "underscore" "at" hotmail "dot" com
> > *****
> > People who live in glass houses shouldn't.
>
> Thats all very nice Auric, but I DON'T pass an argument and can still use
> the parameter in the function. And that is a bit strange, isn't it?
> Frank

Optional does **not** mean "it does not **exist** if no value is assigned to
it"; it simply means that you have the choice of giving a value to it when
calling the function or not. Think of the argument list as *specialized* set
of Dim statements for the function (notice the similarities between the two,
especially when using the As <Type> qualifiers). The variables listed in the
argument list are "declared into existence" as local variables and are
available for use within the function.

Actually, I'm having trouble envisioning how you could possibly segment your
function code in a way that didn't rely on the Optional argument as
"existing", even if it was as simple a case as

Function SomeFunc(Optional Z As Variant) As <Whatever>
If IsMissing(Z) Then
Z= SomeInitializingValue
End If

for a Variant or, for VB6,

Function SomeFunc(Optional Z As Long = -1) As <Whatever>
If Z = -1 Then Z = SomeInitialValue
.......

You use the Optional variables in code all the time. The fact that it is an
Object for your case does not change the concept any. Functionally (no pun
intended), the code you posted, assuming no argument was passed in, is no
different than the following...

Public Function isFormActive(psFormname As String) As Boolean
Dim pFrm As Form
Set pFrm = someform
.....
isFormActive = True
End Function

if isFormActive("formname") then
....

Rick - MVP

"Frank" <frank@frank.com> wrote in message news:<bqf4cs$b44$1@news2.tilbu1.nb.home.nl>...
> Hi,
>
> I have a function with an optional argument (formobject type). When I call
> the function without the optional arg, but DO FILL IN the optional argument
> in the function. I do not get an error! Is that coincidance? The returned
> argument can be used without problem.
>
> Public Function isFormActive(psFormname As String, Optional pFrm As Form) As
> Boolean
> set pFrm=someform
> .....
> isFormActive=true
> end function
>
> if isFormActive("formname") then
> ...

Optional parameters exist inside the procedure whether the caller
passed them or not. If they are omitted then VB creates temporary
variables to use during the procedure so in this case 'pFrm' will have
a value of Nothing (because it is an object reference) if it was not
passed.

: Thats all very nice Auric, but I DON'T pass an argument and can still use
: the parameter in the function. And that is a bit strange, isn't it?

No ... it's just you didn't pass a value a declared variable (what the
optional part means), not that the existence of the variable was optional
(ie if you don't use it, it does not exist). Thus you can reference or use
the variable however you want in the routine -- it's just that the variable
may or may not have initially contained data passed to the routine.

--

Randy Birch
MVP Visual Basic
http://www.mvps.org/vbnet/
Please respond only to the newsgroups so all can benefit.