[fpc-devel] RFC: Delphi style class helpers
pascaldragon at googlemail.com
Mon Dec 20 21:37:13 CET 2010
I have taken the time to implement a first version of Delphi's class
helpers. Thanks to Jonas' code for Objective C categories it was a
rather easy to get that working.
I have yet to test the compatibility of this implementation with Delphi
and this is where the FPC community comes in. I'd like you (@all list
readers) - if possible - to test this extension with tasks you'd use
them in and also to test what behaves differently from Delphi if you
have a current Delphi version available (I believe you need Delphi 2007
or newer for this).
Especially at the core devs, but maybe also for other interested persons
I have the following questions regarding the implementation of this feature:
- should class helpers have access to protected fields of the extended
class? I propose not or this would beat the complete idea of "sealed" (I
also believe that Delphi doesn't allow it as well)
- should class helpers be able to be instantiated or even referenced in
any way? (including forward declarations)
- should class helpers be allowed to override/reintroduce methods?
- should class helpers be able to extend other class helpers (as class
helpers are implemented as child classes of the extended class this
might currently be possible)?
- should class helpers be able to implement interfaces?
- can/should "message" methods be forbidden?
- should abstract methods be forbidden?
- should a class helper for class X hide a method which was introduced
in a subclass of X?
- can class helpers extend generic classes?
- can class helpers BE generic?
I have attached the patch which implements the class helper syntax in
the trunk compiler. Please note that this might not be the final
version, so please do not rely on the syntax and especially the behavior.
Class helpers adhere to the following syntax (I'm still thinking whether
I should slap Borland/Codegear for that...)
[ClassName] = class helper for [ExtendedClass]
class procedure SomeClassMethod;
The methods are implemented like in a normal class, but "Self" refers to
an instance of the extended class in normal methods and to the class
itself if used inside a class method (of course other methods
implemented by the class helper are available as well... basically a
class helper is a child class of the extended class).
The methods implemented by class helpers are automatically available if
the class helper is available in the current scope. The methods are
called as if they belong to the extended class:
// class methods
c := [ExtendedClass].Create;
For the case you are still looking for a usecase: On the Lazarus mailing
list was posted such one by ugaciaka in this mail:
A possible answer for his first question would be the following
(untested and maybe also properties would work...):
TCheckListBoxHelper = class helper for TCheckListBox
function GetCheckedCount: Integer;
function TCheckListBoxHelper.GetCheckedCount: Integer;
Result := 0;
// Count and Checked belong to the extended TCheckListBox
for i := 0 to Count - 1 do
if Checked[i] then
// somewhere in a piece of code that has access to a TCheckListBox instance
c := SomeCheckListBox.GetCheckedCount;
Please feel free to comment, criticise, test and discuss this feature
and more especially its implementation.
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