Will this object get destroyed?
in [Visual Basic]
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From: Larry Serflaten on 13 Oct 2009 08:20 "Simon Woods" <simonjwoods(a)hotmail.com> wrote > If I have a factory method like this in a module (or class) > > Public Function CreateMyObject() as MyObject > > dim myLocalObject as MyObject > > Set myLocalObject = new MyObject > Set CreateMyObject = myLocalObject > End Function > > and in another class I instantiate it > > Private Sub Test > > Dim myRemoteObject as MyObject > > Set myRemoteObject = Factory.CreateMyObject > ... > > End Sub > > Will myLocalObject get destroyed when Sub Test finishes? The two variables myLocalObject and myRemoteObject reference the same object, as you may have surmised. When CreateMyObject exits, myLocalObject falls out of scope, releasing its reference to the created object. So to answer the question; Does myLocalObject get destroyed when Sub Test finishes, that would be a no, correct? When Sub Test 'finishes', myLocalObject is not referencing anything. When Sub Test ends, the variable myRemoteObject is actually the only (remaining) reference to the created object, and upon exit, it too falls out of scope. It is at that time the created object gets destroyed. (there was only one created object throughout....) FYI, This also works, and may be less confusing? Public Function CreateMyObject() as MyObject Set CreateMyObject = New MyObject End Function HTH LFS
From: Ralph on 13 Oct 2009 08:48 Simon Woods wrote: > On 13/10/2009 12:45, Ralph wrote: > For completeness (is that a word? <g>) since this is another area that can also be confusing ... VB performs reference counting on objects passed ByVal, but doesn't on object references passed ByRef. Example: ' class module CJunk Private mJunk As Long Public Property Let Junk(ByVal vData As Long) mJunk = vData End Property Public Property Get Junk() As Long Junk = mJunk End Property ' form module FMain with one button Private Sub Command1_Click() On Error GoTo Command1_Click_Error Dim cls As CJunk Set cls = New CJunk: cls.Junk = 5 DestroyJunkByValue cls Debug.Print cls.Junk DestroyJunkByReference cls Debug.Print cls.Junk ' errors Exit Sub Command1_Click_Error: Debug.Print "Error: " & Err.Description & " " & CStr(Err.Number) Debug.Assert False End Sub Private Sub DestroyJunkByValue(ByVal cs As CJunk) cs.Junk = 10 Set cs = Nothing End Sub Private Sub DestroyJunkByReference(ByRef cs As CJunk) cs.Junk = 10 Set cs = Nothing End Sub
From: Bob Butler on 13 Oct 2009 09:11 "Ralph" <nt_consulting64(a)yahoo.com> wrote in message news:eqPd7NATKHA.4020(a)TK2MSFTNGP05.phx.gbl... > Simon Woods wrote: >> On 13/10/2009 12:45, Ralph wrote: >> > > For completeness (is that a word? <g>) since this is another area that can > also be confusing ... VB performs reference counting on objects passed > ByVal, but doesn't on object references passed ByRef. I think that's misleading; VB always does reference counting but when passing 'ByRef' no additional reference is created so the count is not incremented. Passing 'ByVal' does create an additional reference and that does increment the count.
From: Ralph on 13 Oct 2009 09:39 Bob Butler wrote: > "Ralph" <nt_consulting64(a)yahoo.com> wrote in message > news:eqPd7NATKHA.4020(a)TK2MSFTNGP05.phx.gbl... >> Simon Woods wrote: >>> On 13/10/2009 12:45, Ralph wrote: >>> >> >> For completeness (is that a word? <g>) since this is another area >> that can also be confusing ... VB performs reference counting on >> objects passed ByVal, but doesn't on object references passed ByRef. > > I think that's misleading; VB always does reference counting but when > passing 'ByRef' no additional reference is created so the count is not > incremented. Passing 'ByVal' does create an additional reference and > that does increment the count. I've heard it explained that way too, but felt that was equally "misleading". But however, you say it, the result is the same. -ralph
From: Bob Butler on 13 Oct 2009 09:46
"Ralph" <nt_consulting64(a)yahoo.com> wrote in message news:OWom1qATKHA.504(a)TK2MSFTNGP06.phx.gbl... > Bob Butler wrote: >> "Ralph" <nt_consulting64(a)yahoo.com> wrote in message >> news:eqPd7NATKHA.4020(a)TK2MSFTNGP05.phx.gbl... >>> Simon Woods wrote: >>>> On 13/10/2009 12:45, Ralph wrote: >>>> >>> >>> For completeness (is that a word? <g>) since this is another area >>> that can also be confusing ... VB performs reference counting on >>> objects passed ByVal, but doesn't on object references passed ByRef. >> >> I think that's misleading; VB always does reference counting but when >> passing 'ByRef' no additional reference is created so the count is not >> incremented. Passing 'ByVal' does create an additional reference and >> that does increment the count. > > I've heard it explained that way too, but felt that was equally > "misleading". Using your sample code as a basis: DestroyJunkByValue cls Debug.Print cls.Junk DestroyJunkByReference cls Debug.Print cls.Junk ' errors Private Sub DestroyJunkByValue(ByVal cs As CJunk) cs.Junk = 10 Set cs = Nothing End Sub Private Sub DestroyJunkByReference(ByRef cs As CJunk) cs.Junk = 10 Set cs = Nothing End Sub If you took the subs and rewrote them in-line you'd have this: 'DestroyJunkByValue cls set cs=cls ' passing ByVal causes VB to create the new ref cs.junk=10 set cs=nothing Debug.Print cls.Junk 'DestroyJunkByReference cls cls.junk=10 ' passing ByRef causes VB to use the same reference set cls=nothing Debug.Print cls.Junk ' errors In the ByRef case only the names are changed to confuse the innocent |