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How does child class access parent's variablesDoes anyone know how to declare a variable in a class to be accessible ONLY from a classes instantiated within that class? For example: ************* CODE ***************** Public Class Parent '*** HOW TO DECLARE IT *** Dim Age As String = "1/1/2000" Public Sub New () Dim child As New Child() End Sub End Class Public Class Child Public Sub GetParentAge() '*** WHAT TO PUT HERE instead of MyParent*** MsgBox(MyParent.Age) End Sub End Class ***************************************** TIA Goran Djuranovic Goran Djuranovic wrote:
Show quoteHide quote > Hi all, Variables declared as Private are just that, private.> Does anyone know how to declare a variable in a class to be accessible > ONLY from a classes instantiated within that class? > > For example: > > ************* CODE ***************** > Public Class Parent > > '*** HOW TO DECLARE IT *** > Dim Age As String = "1/1/2000" > > Public Sub New () > Dim child As New Child() > End Sub > > End Class > > > Public Class Child > > Public Sub GetParentAge() > '*** WHAT TO PUT HERE instead of MyParent*** > MsgBox(MyParent.Age) > End Sub > > End Class > ***************************************** > > TIA > Goran Djuranovic Variables declared as Friend can be referenced by a sub class. Variables declared as Public are available to everyone. Unless Child is a sub-class of Parent, the Friend variables are not available. One thing I could suggest, if the Child really needs to see the Private or Friend variables of the Parent, keep them in a Private collection, with the key value being the name of the variable, and pass that collection byRef to the constructor of the Child class, which can then hold a reference to the collection. Tom "ByRef" was exactly what I used. It works like a charm. But thanks for your
response, anyway. Goran Djuranovic Show quoteHide quote "tomb" <t***@technetcenter.com> wrote in message news:zFdZf.185$Mf.31@bignews8.bellsouth.net... > Goran Djuranovic wrote: > >> Hi all, >> Does anyone know how to declare a variable in a class to be accessible >> ONLY from a classes instantiated within that class? >> For example: >> ************* CODE ***************** >> Public Class Parent >> '*** HOW TO DECLARE IT *** >> Dim Age As String = "1/1/2000" >> Public Sub New () >> Dim child As New Child() >> End Sub >> End Class >> Public Class Child >> Public Sub GetParentAge() >> '*** WHAT TO PUT HERE instead of MyParent*** >> MsgBox(MyParent.Age) End Sub >> End Class >> ***************************************** >> TIA >> Goran Djuranovic > > Variables declared as Private are just that, private. > Variables declared as Friend can be referenced by a sub class. > Variables declared as Public are available to everyone. > > Unless Child is a sub-class of Parent, the Friend variables are not > available. > > One thing I could suggest, if the Child really needs to see the Private or > Friend variables of the Parent, keep them in a Private collection, with > the key value being the name of the variable, and pass that collection > byRef to the constructor of the Child class, which can then hold a > reference to the collection. > > Tom Hello, Goran,
"Private" variables are only accessible to the class. "Protected" variables are accessible to the class and to any derived (i.e. Inherited) classes. Cheers, Randy Goran Djuranovic wrote: Show quoteHide quote > Hi all, > Does anyone know how to declare a variable in a class to be accessible > ONLY from a classes instantiated within that class? > > For example: > > ************* CODE ***************** > Public Class Parent > > '*** HOW TO DECLARE IT *** > Dim Age As String = "1/1/2000" > > Public Sub New () > Dim child As New Child() > End Sub > > End Class > > > Public Class Child > > Public Sub GetParentAge() > '*** WHAT TO PUT HERE instead of MyParent*** > MsgBox(MyParent.Age) > End Sub > > End Class > ***************************************** > > TIA > Goran Djuranovic Hi Randy,
Can't use "Protected" because a child class is not derived. I decided to go with ByRef parameter for child constructor. Thanks for your response Goran Djuranovic Show quoteHide quote "R. MacDonald" <sci***@NO-SP-AMcips.ca> wrote in message news:44356e20$0$10494$dbd43001@news.wanadoo.nl... > Hello, Goran, > > "Private" variables are only accessible to the class. "Protected" > variables are accessible to the class and to any derived (i.e. Inherited) > classes. > > Cheers, > Randy > > > Goran Djuranovic wrote: > >> Hi all, >> Does anyone know how to declare a variable in a class to be accessible >> ONLY from a classes instantiated within that class? >> For example: >> ************* CODE ***************** >> Public Class Parent >> '*** HOW TO DECLARE IT *** >> Dim Age As String = "1/1/2000" >> Public Sub New () >> Dim child As New Child() >> End Sub >> End Class >> Public Class Child >> Public Sub GetParentAge() >> '*** WHAT TO PUT HERE instead of MyParent*** >> MsgBox(MyParent.Age) End Sub >> End Class >> ***************************************** >> TIA >> Goran Djuranovic You've got a number of problems here.
1. In the constructor of Parent you create an instance of a Child object and it immediately goes out of scope. 2. Every time you create an instance of a parent object, you automatically create an instance of a Child object. This never takes care of the situation where the parent has no children. If you rewrite it thus, then you will be able to achieve what you are asking for: Public Class Parent Private m_age As String Private m_children As ArrayList Public Sub New() m_age = String.Empty m_children = New ArrayList End Sub Public Sub New(age As String) m_age = age End Sub Public Property Age() As String Get Return m_age End Get Set(value As String) m_age = value End Set End Property Public Sub AddChild() m_children.Add(New Child(Me)) End Sub Public ReadOnly Property Child(index As Integer) As Child Get Return CType(m_children(index), Child) End Get End Property End Class Public Class Child Private m_parent as Parent Public Sub New(parent As Parent) m_parent = parent End Sub Public ReadOnly Property ParentAge() As String Get Return m_parent.Age End Get End Property End Class Then you can use the objects thus: Dim _parent As New Parent _parent.Age = "1/1/2000" _parent.AddChild() Console.WriteLine(_parent.Child(0).ParentAge) or: Dim _parent As New Parent("1/1/2000") _parent.AddChild() Console.WriteLine(_parent.Child(0).ParentAge) "Goran Djuranovic" <goran.djuranovic@newsgroups.nospam> wrote in message Does anyone know how to declare a variable in a class to be accessible ONLY news:OL5hmwZWGHA.1900@TK2MSFTNGP04.phx.gbl... Hi all, from a classes instantiated within that class? For example: ************* CODE ***************** Public Class Parent '*** HOW TO DECLARE IT *** Dim Age As String = "1/1/2000" Public Sub New () Dim child As New Child() End Sub End Class Public Class Child Public Sub GetParentAge() '*** WHAT TO PUT HERE instead of MyParent*** MsgBox(MyParent.Age) End Sub End Class ***************************************** TIA Goran Djuranovic Hi Stephany,
Thanks for your response. I don't want to get into 1. & 2. discussion, because I just wrote the code so people can understand what I want, easily. From the code you sent, it looks like every child will create a new instance of its parent, which hold the refs for all the children created up to that point, no? Seems like a waste of memory, because I want to access ONLY ONE property of a parent. Also, I might be wrong, but in your code you are passing a parent parameter by value, a not reference, which means if that ONE property changes for the parent, I am not going to be able to see it in my child object, no? Anyway, I decided to go with ByRef parameter for the child object constructor. Do you see any problems with that? I appreciate your response. Goran Djuranovic Show quoteHide quote "Stephany Young" <noone@localhost> wrote in message news:uwDPw4iWGHA.4652@TK2MSFTNGP04.phx.gbl... > You've got a number of problems here. > > 1. In the constructor of Parent you create an instance of a Child object > and it > immediately goes out of scope. > > 2. Every time you create an instance of a parent object, you automatically > create > an instance of a Child object. This never takes care of the situation > where > the parent has no children. > > If you rewrite it thus, then you will be able to achieve what you are > asking for: > > Public Class Parent > > Private m_age As String > Private m_children As ArrayList > > Public Sub New() > > m_age = String.Empty > > m_children = New ArrayList > > End Sub > > Public Sub New(age As String) > > m_age = age > > End Sub > > Public Property Age() As String > > Get > Return m_age > End Get > > Set(value As String) > m_age = value > End Set > > End Property > > Public Sub AddChild() > > m_children.Add(New Child(Me)) > > End Sub > > Public ReadOnly Property Child(index As Integer) As Child > > Get > Return CType(m_children(index), Child) > End Get > > End Property > > End Class > > Public Class Child > > Private m_parent as Parent > > Public Sub New(parent As Parent) > > m_parent = parent > > End Sub > > Public ReadOnly Property ParentAge() As String > > Get > Return m_parent.Age > End Get > > End Property > > End Class > > Then you can use the objects thus: > > Dim _parent As New Parent > > _parent.Age = "1/1/2000" > > _parent.AddChild() > > Console.WriteLine(_parent.Child(0).ParentAge) > > or: > > Dim _parent As New Parent("1/1/2000") > > _parent.AddChild() > > Console.WriteLine(_parent.Child(0).ParentAge) > > > "Goran Djuranovic" <goran.djuranovic@newsgroups.nospam> wrote in message > news:OL5hmwZWGHA.1900@TK2MSFTNGP04.phx.gbl... > Hi all, > Does anyone know how to declare a variable in a class to be accessible > ONLY from a classes instantiated within that class? > > For example: > > ************* CODE ***************** > Public Class Parent > > '*** HOW TO DECLARE IT *** > Dim Age As String = "1/1/2000" > > Public Sub New () > Dim child As New Child() > End Sub > > End Class > > > Public Class Child > > Public Sub GetParentAge() > '*** WHAT TO PUT HERE instead of MyParent*** > MsgBox(MyParent.Age) > End Sub > > End Class > ***************************************** > > TIA > Goran Djuranovic > Goran,
Stephany has showed you the standard solution for your problem and AFAIK the only right possible one (although you can as well use collection base instead of arraylist or any other collection, but that is a detail, she wanted to show it you probably as short as possible). Why don't you accept it? It is so stupid to have to read that you want to save memory and than tells that you use the by ref, which uses forever more than the by value. Although this is absolute not related to your question and even not to mention because it are AFAIK only 8 bytes. With that for me and probably for most showing your knowledge, don't try to hide that, we all had to start once. Just my 2 eurocents. Cr Read inline...
"Cor Ligthert [MVP]" <notmyfirstn***@planet.nl> wrote in message You know not far, and please take that [MVP] out of your name news:%23IO6qsZXGHA.3908@TK2MSFTNGP02.phx.gbl... > Goran, > > Stephany has showed you the standard solution for your problem and AFAIK > the only right possible one (although you can as well use collection base > instead of arraylist or any other collection, but that is a detail, she > wanted to show it you probably as short as possible). representation. People may think you are a Most Valuable Programmer. > Why don't you accept it? Do I have to?> It is so stupid to have to read that you want to save memory and than Ouhh. I am sorry. Some people were not born as MVPs.> tells that you use the by ref, which uses forever more than the by value. Anyway, ByVal & ByRef are VERY confusing keywords. There are hundreds of discussions on the web about them, and people still can't have it straightened out. Logically, one would think the ByVal passes a new copy of a variable, and ByRef passes a pointer. A copy of an object (rather than a pointer) WOULD mean more memory requirements. Not in VB.NET's case. > Although this is absolute not related to your question and even not to Again, you know not far. It is actually 4 bytes. You are embarrasing the > mention because it are AFAIK only 8 bytes. real MVPs. :-) > With that for me and probably for most showing your knowledge, don't try If you don't have anything better to say, at least don't insult people in > to hide that, we all had to start once. this group. Look at Brian's response for a "responding template". > Just my 2 eurocents. And last, keep your stinking 2 cents for yourself. I will survive without them. Cheers Goran Djuranovic wrote:
> Hi Stephany, Only the New keyword can create object instances. Each child will have> Thanks for your response. I don't want to get into 1. & 2. discussion, > because I just wrote the code so people can understand what I want, easily. > > From the code you sent, it looks like every child will create a new instance > of its parent, which hold the refs for all the children created up to that > point, no? Seems like a waste of memory, because I want to access ONLY ONE > property of a parent. > a reference to it's parent, but it will not actually create a new instance of the parent. There is very little memory usage associated with using this technique. > Also, I might be wrong, but in your code you are passing a parent parameter You will see the change. Since the variable is passed by value that> by value, a not reference, which means if that ONE property changes for the > parent, I am not going to be able to see it in my child object, no? > means a new reference is created on the stack. However, that new reference just happens to point to the same parent object. Changes to the parent will be seen from within the child. I know...the ByVal and ByRef keywords are confusing. > Anyway, I decided to go with ByRef parameter for the child object Yes. When passing ByRef you are allowing the possibility that code> constructor. Do you see any problems with that? could change the caller's reference variables and what they are actually referencing. It's rare that you would need to pass an object reference ByRef. Let me explain it another way. ByVal and ByRef do not determine whether the object is a reference type or a value type. They control how variables are passed to functions. Show quoteHide quote > > I appreciate your response. > Goran Djuranovic > > Thanks Brian. Your explanation was very helpfull. I was very suprised by how
ByVal and ByRef behave. But, it looks like behavior is different for .NET v1.0 & .NET v1.1. From DotNetExtreme.com: "Passing a parameter by Reference means that if changes are made to the value of a variable passed, these changes are reflected back in the calling routine. Objects are Reference type data. They maintain a reference to where the actual data is stored on the stack. Normally what occurs when an object is passed by reference is that this memory pointer from the stack is passed to the function or procedure. As a result of only passing a pointer to the referenced data, the function or procedure has access to the original data. However, VB.NET does not follow this model. When an object's property is passed by reference to a function or procedure, the data is copied. Not only is the data copied, but since by definition variables passed by reference allow for the return of changes, the data is copied back. Thus, while most implementation avoid copying the data when a variable is passed by reference, VB.NET actually passes it twice -- a large performance hit on a function call that passes data by reference." The problem here is that they don't say this is valid for .NET v1.0, ONLY. In .NET v1.1, there is no copying, and overhead. Follow the links: http://www.codeproject.com/useritems/VBnet_Methods.asp http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vbls7/html/vblrfvbspec7_1_6_2.asp Thanks everyone who responded: tomb, R. MacDonald, Steph, and Brian. Consider this thread CLOSED. Goran Djuranovic Show quoteHide quote "Brian Gideon" <briangid***@yahoo.com> wrote in message news:1144786246.349783.54840@u72g2000cwu.googlegroups.com... > Goran Djuranovic wrote: >> Hi Stephany, >> Thanks for your response. I don't want to get into 1. & 2. discussion, >> because I just wrote the code so people can understand what I want, >> easily. >> >> From the code you sent, it looks like every child will create a new >> instance >> of its parent, which hold the refs for all the children created up to >> that >> point, no? Seems like a waste of memory, because I want to access ONLY >> ONE >> property of a parent. >> > > Only the New keyword can create object instances. Each child will have > a reference to it's parent, but it will not actually create a new > instance of the parent. There is very little memory usage associated > with using this technique. > >> Also, I might be wrong, but in your code you are passing a parent >> parameter >> by value, a not reference, which means if that ONE property changes for >> the >> parent, I am not going to be able to see it in my child object, no? >> > > You will see the change. Since the variable is passed by value that > means a new reference is created on the stack. However, that new > reference just happens to point to the same parent object. Changes to > the parent will be seen from within the child. I know...the ByVal and > ByRef keywords are confusing. > >> Anyway, I decided to go with ByRef parameter for the child object >> constructor. Do you see any problems with that? > > Yes. When passing ByRef you are allowing the possibility that code > could change the caller's reference variables and what they are > actually referencing. It's rare that you would need to pass an object > reference ByRef. Let me explain it another way. ByVal and ByRef do > not determine whether the object is a reference type or a value type. > They control how variables are passed to functions. > >> >> I appreciate your response. >> Goran Djuranovic >> >> > 
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