On 21.09.2014 15:09, Marco van de Voort wrote:
In our previous episode, Sven Barth said:
No, not multiple roots. If a class is declared as refcounted (and it's
parent class is not) then a hidden field is inserted into its list of
fields, just as a normal one would, only that it isn't accesible using
the "."-operator (because it won't have a valid Pascal identifier). The
compiler of course knows the offset at compile time and at runtime the
field can be accessed using the class' RTTI which would have a
"RefCountOffset" field added (if that offset is < 0 then the object does
not support reference counting).
So in summary: the compiler must pass the fieldoffset to the runtime helper,
instead of having one refcounted root with known offset.
Yes. That avoids having a single rooted refcounted object and thus
allowing the usage of "refcounted" further down the inheritance tree.
Considering that the helper would only consist of a
"InterlockedIncrement(AddressOfRefCount)" or
"InterlocedDecrement(AddressOfRecCount)" one could optimize this by
declaring the helper compilerproc as "inline", thus avoiding one call.
Did anyone till now really complain about the performance impact of
reference counting on interfaces? Especially since they are used *so*
much in today's OOP designs?
I never really benchmarked it. I never use interfaces in my own abstract
data structure designs, mostly because the existing classes don't, and I
don't like the refcount. (since i usually also need to access the classes as
normal instances)
I did benchmark interfaces in combination with variants as part of a decal
evaluation. That was, euh, painful.
I can imagine that benchmarking variants was painful. ;)
But considering that interfaces are so "en vogue" today in Delphi
communities the performance hit might not be that killer criteria...
function Test: TObject;
begin
Result := (TTestClass.Create as IInterface) as TTestClass;
end;
Hmm. This looks like a (Delphi) faq item to me, how to convert interface back
to a
class. Usually the answer is IComponentreference (or what it is called), if
this is legal, I wonder why it didn't come up before.
The intf-to-class-as-operator was added with Delphi 2007 AFAIK and FPC
supports it as well in 2.6.0. If you want specifics, then look at the
helper functions fpc_intf_is_class (for the is operator) and
fpc_intf_cast_class (for the as operator) in $fpcdir/rtl/inc/objpas.inc.
Regards,
Sven
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