[fpc-pascal] Abstract classes ignored

[Sven Barth]

Am 17.04.2021 um 20:30 schrieb Ryan Joseph via fpc-pascal:
>  From the documentation: "An abstract class is a class that cannot be instantiated directly. Instead, a descendent class must always be instantiated. However, for Delphi compatibility, the compiler ignores this directive."
>
> Why does this get ignored and what does this have to do with Delphi? I personally would like for this to actually work and give a compiler error if you instantiate the abstract class.
The compiler will generate a warning in case you instantiate a class 
that is abstract or has abstract methods. You can escalate these 
warnings to errors if you need:

=== code begin ===

program tabstract;

{$MODE OBJFPC}
{$WARN CONSTRUCTING_ABSTRACT ERROR}

type
   TAbstract1 = class abstract
   end;

   TAbstract2 = class
     procedure Test; virtual; abstract;
   end;

var
   a1: TAbstract1;
   a2: TAbstract2;
begin
   a1 := TAbstract1.Create;
   a2 := TAbstract2.Create;
end.

=== code end ===

This will generate two (different) errors.

With this we're allowing more than Delphi does (which warns in the 
second case, but the warning can not be escalated to error and does not 
warn for the first case at all), but Delphi compatbility is more 
important here, thus the default is a warning and not an error. This 
will not be changed.

Regards,
Sven