[fpc-pascal] type helpers

Michael Van Canneyt michael at freepascal.org
Fri Jan 11 14:47:28 CET 2019

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 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.

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.


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