Pointer / reference clarification

W

Workgroups

I need some clarification with the whole "no pointers in VB.NET" thing,
because it seems like there are "quazi-pointers" happening but I want to
make sure I'm interpreting all this correctly.

Here's a generic class declaration for the sake of example:

Public clsClass
'properties, methods
End Class

And now some declarative snippits, followed by abstracts of what I think is
happening:

Dim objClass As clsClass

That sets up something like this in memory:

{objClass = Nothing} <- Can hold a memory address, but currently holds
Nothing so far

So as I understand it, this essentially creates something akin to a
"pointer". It cannot be treated like a C++ pointer, but it is something of
a pointer in the sense that it will "point" to memory.

objClass = New clsClass

Now it's set to an instance of an object, and so here's what I assume is now
going on in memory:

{@ address 1, clsClass instance} <- is now physically stored in the managed
memory area
{objClass = @1} <- now stores the memory address for lookup of the clsClass
instance

Dim objClass2 As clsClass = objClass

Because they are references, this is what I assume is happening after this
declaration:

{@ address 1, clsClass instance} <- still physically stored in the managed
memory area
{objClass = @1} <- still storing the memory address for lookup
{objClass2 = @1} <- now storing the same memory address for lookup as
objClass

Is this essentially correct? I still have only one actual instance, and two
smaller pointers (well, references) available for lookups against the same
instance. Let's add one more:

Dim objClass3 as New clsClass

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}

So now two instances, three references (two of which point to the same
instance). Now I start wandering out into stuff I'm not sure I understand,
for example a HashTable. Here's my guess:

Dim objHash as New HashTable:

{@ memory address 1, clsClass instance}
{objClass = 1}
{objClass2 = 1}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{objHash = 3}

And then...

objHash.Add("A", objClass2)

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{HashTable "A" = @1} <- ...something like that?
{objHash = @3}

Is the Hash table storing data in this fashion? Do I still have just one
instance of clsClass?

And finally does the garbage collector kill the instance for me once all the
references are gone?

objClass = Nothing
objClass2 = Nothing

{@ memory address 1, clsClass instance}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{HashTable "A" = 1}
{objHash = 3}

Is the HashTable item "A" maintaining my instance @1?

objHash.Remove(objHash("A"))

[...GC runs]

{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{objHash = @3}

I'm starting a project using lots and lots of classes in multiple 2D arrays
of hashtables and I want to make sure I have this stuff more or less correct
in my mind before I start allocating memory like gangbusters.
 
G

Guest

Workgroups,

It looks to me like you are interpreting this correctly.

Kerry Moorman


Workgroups said:
I need some clarification with the whole "no pointers in VB.NET" thing,
because it seems like there are "quazi-pointers" happening but I want to
make sure I'm interpreting all this correctly.

Here's a generic class declaration for the sake of example:

Public clsClass
'properties, methods
End Class

And now some declarative snippits, followed by abstracts of what I think is
happening:

Dim objClass As clsClass

That sets up something like this in memory:

{objClass = Nothing} <- Can hold a memory address, but currently holds
Nothing so far

So as I understand it, this essentially creates something akin to a
"pointer". It cannot be treated like a C++ pointer, but it is something of
a pointer in the sense that it will "point" to memory.

objClass = New clsClass

Now it's set to an instance of an object, and so here's what I assume is now
going on in memory:

{@ address 1, clsClass instance} <- is now physically stored in the managed
memory area
{objClass = @1} <- now stores the memory address for lookup of the clsClass
instance

Dim objClass2 As clsClass = objClass

Because they are references, this is what I assume is happening after this
declaration:

{@ address 1, clsClass instance} <- still physically stored in the managed
memory area
{objClass = @1} <- still storing the memory address for lookup
{objClass2 = @1} <- now storing the same memory address for lookup as
objClass

Is this essentially correct? I still have only one actual instance, and two
smaller pointers (well, references) available for lookups against the same
instance. Let's add one more:

Dim objClass3 as New clsClass

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}

So now two instances, three references (two of which point to the same
instance). Now I start wandering out into stuff I'm not sure I understand,
for example a HashTable. Here's my guess:

Dim objHash as New HashTable:

{@ memory address 1, clsClass instance}
{objClass = 1}
{objClass2 = 1}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{objHash = 3}

And then...

objHash.Add("A", objClass2)

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{HashTable "A" = @1} <- ...something like that?
{objHash = @3}

Is the Hash table storing data in this fashion? Do I still have just one
instance of clsClass?

And finally does the garbage collector kill the instance for me once all the
references are gone?

objClass = Nothing
objClass2 = Nothing

{@ memory address 1, clsClass instance}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{HashTable "A" = 1}
{objHash = 3}

Is the HashTable item "A" maintaining my instance @1?

objHash.Remove(objHash("A"))

[...GC runs]

{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{objHash = @3}

I'm starting a project using lots and lots of classes in multiple 2D arrays
of hashtables and I want to make sure I have this stuff more or less correct
in my mind before I start allocating memory like gangbusters.
Click to expand...
 
M

Michael C#

Sounds about right. In VB, we deal in "Value types" and "Reference types".
Value types are the basic types that hold their data within their own memory
allocation. Numeric types, booleans, chars, dates are all value types.
Reference types contain a pointer to another memory location that holds the
data. Arrays, strings, class types, etc. are all reference types. Here's
MSKB discussion on this:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vbcn7/html/vbconValueRefTypes.asp



Workgroups said:
I need some clarification with the whole "no pointers in VB.NET" thing,
because it seems like there are "quazi-pointers" happening but I want to
make sure I'm interpreting all this correctly.

Here's a generic class declaration for the sake of example:

Public clsClass
'properties, methods
End Class

And now some declarative snippits, followed by abstracts of what I think
is happening:

Dim objClass As clsClass

That sets up something like this in memory:

{objClass = Nothing} <- Can hold a memory address, but currently holds
Nothing so far

So as I understand it, this essentially creates something akin to a
"pointer". It cannot be treated like a C++ pointer, but it is something
of a pointer in the sense that it will "point" to memory.

objClass = New clsClass

Now it's set to an instance of an object, and so here's what I assume is
now going on in memory:

{@ address 1, clsClass instance} <- is now physically stored in the
managed memory area
{objClass = @1} <- now stores the memory address for lookup of the
clsClass instance

Dim objClass2 As clsClass = objClass

Because they are references, this is what I assume is happening after this
declaration:

{@ address 1, clsClass instance} <- still physically stored in the managed
memory area
{objClass = @1} <- still storing the memory address for lookup
{objClass2 = @1} <- now storing the same memory address for lookup as
objClass

Is this essentially correct? I still have only one actual instance, and
two smaller pointers (well, references) available for lookups against the
same instance. Let's add one more:

Dim objClass3 as New clsClass

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}

So now two instances, three references (two of which point to the same
instance). Now I start wandering out into stuff I'm not sure I
understand, for example a HashTable. Here's my guess:

Dim objHash as New HashTable:

{@ memory address 1, clsClass instance}
{objClass = 1}
{objClass2 = 1}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{objHash = 3}

And then...

objHash.Add("A", objClass2)

{@ address 1, clsClass instance}
{objClass = @1}
{objClass2 = @1}
{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{HashTable "A" = @1} <- ...something like that?
{objHash = @3}

Is the Hash table storing data in this fashion? Do I still have just one
instance of clsClass?

And finally does the garbage collector kill the instance for me once all
the references are gone?

objClass = Nothing
objClass2 = Nothing

{@ memory address 1, clsClass instance}
{@ memory address 2, clsClass instance}
{objClass3 = 2}
{@ memory address 3, HashTable instance}
{HashTable "A" = 1}
{objHash = 3}

Is the HashTable item "A" maintaining my instance @1?

objHash.Remove(objHash("A"))

[...GC runs]

{@ address 2, clsClass instance}
{objClass3 = @2}
{@ address 3, HashTable instance}
{objHash = @3}

I'm starting a project using lots and lots of classes in multiple 2D
arrays of hashtables and I want to make sure I have this stuff more or
less correct in my mind before I start allocating memory like gangbusters.
Click to expand...
 

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