Peter Vreman wrote:
On 21 Jul 2007, at 15:06, Jonas Maebe wrote:
Sorry, no, this would be (TA <= TB) and (TB < TB), so it would be ok.
Still, I don't think laying out classes in memory like this is easy
to do without complex linker scripts (which are not supported on all
platforms).
There is no garauntee either within a single unit. With forward defined classes
you can make any
order you want.
And relying on order in memory has also problems with shared libraries. afaik
there is no rule
that new libaries are always loaded at larger adresses
Thank you for this information, Peter. I hadn't thought of shared libraries yet.
Anyway, as far as I understood shared libraries each have their own VMTs, so
you can't compare classes that come from different dll's or the main
executable. Is that correct?
Can we then assume that at some moment in time the compiler has access to the
whole class hierarchy of the whole application? At such a time could it then
either re-order the VMTs, or, more simply, fill in a VMT field that is reserved
to hold an index, as well as the NextSubtree/LastInSubtree field [see below]?
BTW I now realize that I was using a little bit confusing terminology; with
'depth-first order traversal' I meant 'preorder traversal'. The 'NextSibling'
and 'LastChild' names are also confusing. What I actually meant, I'd rather
call 'NextSubtree' and 'LastInSubtree'.
Consider the following example:
TObject
|
+- TPersistent
| |
| +- TMyPersistent
| |
+ +- TComponent
| |
| +- TDataset
|
+- TList
Here:
- TMyPersistent.NextSubtree = TComponent, .LastInSubtree = TMyPersistent
- TComponent .NextSubtree = TList, .LastInSubtree = TDataset
- TDataSet .NextSubtree = TList, .LastInSubtree = TDataset
- TList .NextSubtree = <inf>, .LastInSubtree = TList
An every-day example is a directory treeview, such as e.g. the left pane in
Windows Explorer. When all nodes are expanded, iterating the rows from top to
bottom corresponds exactly with preorder traversal. With this picture in mind,
it is relatively easy to see the correctness of the proposed InheritsFrom
implementation.
Regards,
Bram
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