[fpc-pascal] Question about interfaces and patch

[memsom]

Sorry to be late in the conversation...

>> Here is a simple example how it should be used
>>
>> type
>>  IReportable = interface
>>    function Report: string;
>>    // let's say that this should write report and
>>    // return status description as string
>>  end;
>>
>>  ILogged = interface
>>    procedure Log(aStr: string);
>>  end;


Surely, using Delphi's way of things:

  type TMyReport = class(TInterfacedObject, IReportable, ILogged)
         ... //an implementation of each interface
       end;


>> procedure MyXYZPart.DoReport(aRep: IReportable);
>> begin
>>  if (aRep <> nil) then begin
>>    if (aRep is ILogged) then
>>      (aRep as ILogged).Log(aRep.Report)
>>    else
>>      aRep.Report
>> end;


Again :


 procedure MyXYZPart.DoReport(aRep: IReportable);
 var
   tmp: ILogged;
 begin
  if (aRep <> nil) then begin
    if (aRep.QueryInterface(ILogged, tmp) = S_OK) then
      tmp.Log(aRep.Report) //or you could just do the as cast..
    else
      aRep.Report
 end;



> I see what you want, but if you would do
>
> procedure MyXYZPart.DoReport(aRep: TInterfacedObject);
>
> begin
>    if (aRep <> nil) and arep is IReportable then begin
>      if (aRep is ILogged) then
>        (aRep as ILogged).Log(aRep.Report)
>      else
>        aRep.Report
> end;

Converting interfaces to instances to object instances is a bad idea... it
screws with reference counting, for a start...> This still does not
explain why you would need to do

>    AClassInstance := AnInterface;

I think the above is a really bad idea. Especially when the implementing
class is private... e.g.

interface

type
  IPublic = interface
    procedure whatever;
  end;

  TFactory = class
    function newPublic: IPublic;
  end;

implementation

type
  TPrivate = class(TInterfacedObject, IPublic)
    ...
  end;

  function TFactory.newPublic: IPublic;
  begin

    Result := TPrivate.Create; // or implement some kind of class
                               // pooling etc ..
  end;


How would you be able to get at the class? Only as TObject or
TInterfacedObject. Bad idea.

Matt