On Fri, 11 Jan 2019, Sven Barth via fpc-pascal wrote:
Am Fr., 11. Jan. 2019, 11:24 hat Mattias Gaertner via fpc-pascal < fpc-pascal@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.
IMO It makes sense to forbid this. For simple types and records, writing to self means self is treated like a var parameter (i.e. there is an address). The compiler can detect this.
For properties there is no address (it cannot be used as var param), so treating self as a var parameter would automatically forbid writing using a method - that writes to self - on a property.
The behaviour to make a copy has bitten me more than once, and I would much prefer that the compiler simply does not allow me to do something which will guaranteed not give me what I was expecting. Michael. _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
