Marco van de Voort wrote:
Peter Vreman wrote:
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?
For pure shared libraries yes, for packages no. For packages, Peter is
right.
[...]
Assume everything on the 1st and 2nd level above is in a package A.
Progam B and C both use package A. What do we do about lastinsubtree?
Before I could answer that question, I needed to know a little bit more about
packages and of how VMTs are handled in packages. ... Found
http://wiki.freepascal.org/packages. So I assume we are talking about 'library
packages' here.
If a package is loaded dynamically, the preorder indices in the VMTs would need
to be recalculated. If it is loaded statically, the compiler can do the
calculation on beforehand. Of course the loading of packages must be done
synchronized (in a critical section), but that might already have been the
case. Also, it should be made possible to enumerate the VMTs that do reside in
a package.
I think this outweighs the performance gain for InheritsFrom (and 'is' and
'as'), and it is most beneficial for applications that do not use packages.
BTW does the compiler optimize
if (obj is TMyObject) then
SomeCodeWith(obj as TMyObject)
to
if (obj is TMyObject)
SomeCodeWith(TMyObject(obj))
(under the condition that SomeCodeWith does not change obj.)
Regards,
Bram
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