Hello together!
I've decided to provide a little status report on the "class helper"
feature that I'm currently implementing.
I found the time to put up a first version of a wiki page:
http://wiki.freepascal.org/Class_helpers
Does someone know a good place to link this page from? Maybe something
where other language/compiler features are referenced from or can be
linked to in the future.
And yes, I plan to extend the "Code examples" list further. :D
Now what is my current status?
* class helper syntax is supported (including inheritance - which is a
pseudo inheritance internally)
* calling methods that are provided by a class helper works
* only the last available class helper is used (regarding the search
algorithm see below)
* enumerators provided by class helpers are used
* overloading methods of the extended class with the ones provided by
the class helper does NOT work currently (I need to investigate how
"overload" works at all ^^)
* in mode Delphi "override" and "virtual" are ignored (they are
disallowed for mode ObjFPC), but as I remove their corresponding flags I
might have a problem with the following example that compiles in newer
Delphis (at least Delphi 2007 does not compile this, because of an
incorrect implementation in the compiler):
==== source begin ====
type
THelper1 = class helper for TObject
procedure Foo; virtual;
end;
THelper2 = class helper(THelper1) for TObject
procedure Foo; override;
end;
==== source end ====
* I have experienced one time that Delphi chokes on properties. I'll
need to investigate that further (one can define them, but Delphi does
not seem to find them)
* I have yet to test whether the restriction "Create must be first
statement in constructors" holds true (I've found a code sample that
suggests its not true)
* I have yet to read about "class constructors" and whether they can be
used (and how)
This basically sums up my status.
Regarding the algorithm I use to find the last available class helper
(this is implemented in
compiler/symtable.pas/search_last_objectpascal_classhelper):
Delphi uses the last class helper that is available in the current scope
for method resolution (and the "parents" of that class helper). For this
I walk the symtable stack and search for a def where
is_objectpascal_classhelper returns "true". This can potentially lead to
a bad runtime behavior if a project does not use any class helpers, but
includes many units (the compiler is a good example of that).
Do you (especially @Devs) think that this is a sufficient approach or
should this be done another way? (Note: I have not yet profiled
compiling the compiler once with that search enabled and once without)
Perhaps searching for class helpers should be made dependent on a
modeswitch? E.g. "useclasshelpers"
That might lead to the potentially confusing behavior that a class
helper is not even found in its own unit though, although I think that
this would be a consequent behavior (the switch needs to be documented
good enough though ^^).
This switch could be enabled by default in mode Delphi.
Also as Delphi searches a class helper based on the last available class
helper instead of the last available method as Objective C categories
do, I think that I don't need to generate that static symbols (done by
create_class_helper_for_procdef in symdef.pas). Any objections for
disabling that for Object Pascal class helpers again?
Two other questions regarding compiler internals:
Do I need to consider something regarding WPO? Or is all that done
automatically by the compiler already (e.g. by me referencing the symbol
of a class helper method)?
What is the difference between the "Message*" and "CGMessage*"
procedures? Or asked differently: when should I use which?
Regards,
Sven
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