[fpc-pascal] Traits Proposal

[Ryan Joseph]

> On Feb 15, 2021, at 11:41 PM, Sven Barth via fpc-pascal <fpc-pascal at lists.freepascal.org> wrote:
> 
> Again, the point of the interface would be to control which methods and properties of the delegated  type would be available. Also it would allow for seamless integration with the RTTI as it's possible to query the implemented interface of a class and to retrieve a reference to it, thus it would be possible to access this at runtime as well.
> 
> The compiler will generate a VMT for the interface with method thunks that adjust the Self pointer so that it works correctly. The GetInterfaceByEntry function uses this static data to generate an adjusted Self value that callers can use.

As far as the "default implements property" is concerned we don't actually need, or even want to utilize the interfaces themselves. The idea of leveraging the interface syntax for exporting method names is one thing but if we actually have to use the interfaces internally then we open a whole can of worms in terms of performance.  Class instantiation may be taking on some significant baggage now also as the compiler sets up these interface VMT tables, even if the user never needs or wants them.

For example this code should be a compile time indirection instead of a hidden interface cast, right? The method forwarding syntax which works by modifying the VMT can hopefully be done at compile time for the default property also. 

type
  ICircle = interface
    procedure Draw;
  end;

type
  TCircle = record
    procedure Draw;
  end;

type
  TMyShape = class(ICircle)
    private
      FCircle: TCircle;
    public
      property Circle: TCircle read FCircle implements ICircle; default;
  end;

var
  shape: TMyShape;
begin
  shape := TMyShape.Create;
  { calls shape.FCircle.Draw instead of ICircle(shape).Draw }
  shape.Draw;
end.


Regards,
	Ryan Joseph