On 21 Jul 2007, at 12:42, Bram Kuijvenhoven wrote:
If the vmts are stored in memory in depth-first order [of the class
hierarchy], then an O(1) implementation could be made by adding
only one field to the vmt: vmtNextSibling; then you'd simply get
something like
InheritsFrom := (AClass <= Self) and (Self < PClass(pointer
(AClass) + vmtNextSibling)^);
(In this implementation vmtNextSibling should not be nil for those
classes that have no NextSibling in the hierarchy, but have a value
larger than all vmts. An alternative is to have an vmtLastChild,
which is always properly defined; it points to the last one of:
itself, its children, its grandchildren, etc.)
I don't think this can work, since a class hierarchy is a tree and
not a list. So in case of
TObject
|
+-- TA
|
+-- TB
How would you linearise that in memory? As TObject - TA - TB ?
In that case, TB inherirtsFrom TA would be (afaics)
(TA <= TB) and (TB < TA+(inf))
This would return true, while it's not true. You cannot solve this by
duplicating vmt's for different inheritance paths (in fact,
duplicating vmt's would complicate the is/as problems a lot).
Are vmts stored in depth first order?
They are probably stored per unit in the order they are declared (not
sure about forward-defined classes).
If not, an alternative is to have another vmt field with a number
that indicates its index in depth-first order of the class hierarchy.
You would still have to determine that two classes are part of the
same hierarchy (same branch of the inheritance tree). I also don't
know whether changing the vmt layout is feasible at this point.
Jonas
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