Before the 3.2.2 release I looked into mantis 0038496 and now I come back to it.

I noticed that custom variants are completely undocumented, is this know (IOW should I file a bug?). What I wanted to look up are the rules for implementing binaryop, since that is where the problem is.

The bug is for the following code:

{$ifdef fpc}
  {$mode delphi}
{$else}
  {$apptype console}
{$endif}
uses variants,fmtbcd;
var
   fBCD1: TBcd;
   V1, V2, V3:Variant;
   L1: Integer;
begin
   L1:=123;
   fBCD1:=1234.345;
   V1:=123;
   V2:=VarFmtBCDCreate(fBCD1);
   V3:=V1 + V2;
   writeln(v3);
   readln;
end.

The addition finally ends up in  TFMTBcdFactory.BinaryOp which is declared as follows

procedure TFMTBcdFactory.BinaryOp(var Left: TVarData; const Right:
TVarData; const Operation: TVarOp);

i.e.  not a 3 operand action, but a two operand.  This probably means that in the case of v3:=v1+v2 this is encoded as v1:=v1,v2,add, modifying V1.

The fmtbcd binaryop code however says that the left type (integer)  is not either double or a a FMTBCD custom variant and thus throws an exception.

I think it is allowed to change the result type of the variant passed in binaryop(), but am not 100% sure, and also not sure if there are other things to keep an eye for (e.g. finalization of the existing value).

Does anybody know open source custom variant examples other than fmtbcd ?

Please do. I am aware of this issue, but it always falls between the other issues. If you happen to have some sample programs (however small) I can use to document this, please attach them to the issue.

I know that Synopse makes heavy use of the customvariant (I believe their JSON document it a customvariant), maybe it can be a source of inspiration.

Michael.

https://synopse.info/forum/viewtopic.php?id=5894

Op zo 30 mei 2021 10:44 schreef Michael Van Canneyt via fpc-devel <

chakracore-delphi (experimental/variants branch) also has a custom variant (descendant of TInvokeableVariantType) to wrap javascript values/objects so you can access properties and call methods on javascript objects at runtime (similar to how you can use ActiveX/Automation objects through Variants when not compiling against a type library).

https://github.com/tondrej/chakracore-delphi
Blog post:
https://tondrej.blogspot.com/2020/10/variant-support-in-chakracore-delphi.html

Cheers,
Ondrej

On Sun, 30 May 2021 at 11:39, Don Alfredo via fpc-devel <

The only one I'm aware of that makes use of BinaryOp is Python4Delphi ( https://github.com/pyscripter/python4delphi/blob/master/Source/VarPyth.pas ). mORMot derives its TSynInvokeableVariantType and thus its TDocVariant from TInvokableVariantType, but does not override the operator methods.

Regards,
Sven

Don Alfredo wrote

https://synopse.info/forum/viewtopic.php?id=5894 <https://synopse.info/forum/viewtopic.php?id=5894>

I'm already working on a fix for this, cause I had seen that thread a few days ago already. It would have been nice however if you had found this out during the RC phase which had lasted long enough.

Also why is looking through the code of TSynInvokeableVariantType the first time I hear about the swapped order of parameters? Did you report this as a bug? And if not, why not? This could have been fixed with 3.2.2 as well! (And I'm indeed working on fixing it)

Also: no top posting, please.

Regards,
Sven

Op 2021-05-30 om 13:02 schreef Sven Barth via fpc-devel:

I think I saw this one before in March. I also remember the conclusion: this should be fixed in LefPromotion(), but FPC doesn't implement calling that method :-)