[fpc-pascal] type helpers

[Sven Barth]

Am Fr., 11. Jan. 2019, 11:24 hat Mattias Gaertner via fpc-pascal <
fpc-pascal at lists.freepascal.org> geschrieben:

> Hi,
>
> A type helper can change Self.
> I wondered how FPC 3.3.1 handles properties and got some
> unexpected results. Is this by design, a bug, or not-yet-implemented?
>
> {$mode objfpc}{$modeswitch typehelpers}
> type
>   TIntHlp = type helper for word
>     procedure DoIt;
>   end;
>
>   TBig = class
>   strict private
>     FW: word;
>     procedure SetW(const AValue: word);
>   public
>     property W: word read FW write SetW;
>   end;
>
> var b: TBird;
>
> procedure TBig.SetW(const AValue: word);
> begin
>   // not called by type helper
>   writeln('TBig.SetW');
> end;
>
> procedure TIntHlp.DoIt;
> begin
>   writeln('TIntHlp.DoIt START ',Self,' w=',b.w);
>   Self:=4; // changes b.FW directly
>   writeln('TIntHlp.DoIt END ',Self,' w=',b.w);
> end;
>
> begin
>   b:=TBig.Create;
>   b.w.DoIt;
> end.
>

This is by design. In this case DoIt is called on a temp variable that gets
its value from b.w, the value of b.FW does not change (same reason why the
C operators do not work on properties). Same happens with constants btw:
Word(42).DoIt will work as well.

Regards,
Sven

>
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