[fpc-devel] Status report for "class helpers"
DrDiettrich1 at aol.com
Sun Jan 30 09:03:46 CET 2011
Paul Ishenin schrieb:
> For some reason the visibility is completely ignored here. I can add a
> class helper to the strict private or even move it to another unit - it
> is always visible.
IMO class helper methods always should become visible as part of the
class, regardless of where they are declared.
From the Delphi wiki:
"You can define and associate multiple helpers with a single type.
However, only zero or one helper applies in any specific location in
source code. The helper defined in the nearest scope will apply. Class
or record helper scope is determined in the normal Delphi fashion (for
example, right to left in the unit's uses clause)."
I.e. a single Helper reference (field) in the class definition is
sufficient. This field cannot be persistent, instead it has to be
initialized for every use case: whenever a helper is found in the
compiled unit, or in one of its used units, it overwrites the Helper
reference. Adding the helper elements to the class STB is not a valid
solution, because another helper can become active at any time, during
The ancestors of the Helper class must be searched as well.
"Note that the class helper function MyFunc is called, because the class
helper takes precedence over the actual class type."
That's bad, because this disallows to treat the helper like an ancestor,
accessible in all derived classes :-(
1) Unit A defines an class helper for TFoo, then uses in the
implementation section unit B, which also defines an helper for TFoo.
According to the above rules the helper in A should become inaccessible
and useless then.
2) Similarly multiple class helpers can be defined in the implementation
section, which are used in the immediately following code, until the
next helper declaration.
3) What about derived classes? Since derived classes are compiled
independently, they never will use a reintroduced method in an class helper.
4) When class helpers are evaluated at compile time, according to the
static object types (like properties are), then they could be handled
just like the well known typecast hack:
type TMyFoo = class(TFoo)
with following casts of every TFoo reference into TMyFoo.
But TFoo derived types must be handled in the same way, so that I see no
way around an extended search inside the TFoo STB.
5) What's the meaning of the "ancestor list"? IMO interfaces cannot be
added by an class helper, and multiple inheritance is quite unlikely.
Will the ancestor list allow to override interface *implementations*?
More information about the fpc-devel