[Lazarus] Making sources compatible with Delphi (but Lazarus is priority)

[Tony Whyman]

On 01/05/17 16:33, Sven Barth via Lazarus wrote:
> Would you please elaborate on these and which FPC version you targeted?
>
> Regards,
> Sven
>
>
>
I am working with FPC 3.0.x and an example of the solution I used 
follows. Note that with FPC, there is no problem calling a constructor 
from a placeholder, while with delphi I found it necessary to add an 
explicit coercion. I also wanted to return the interface provided by the 
object. Again with FPC, this is easy. With Delphi, I found it necessary 
to use QueryInterface to do the same.

The difference between the two seems to be that FPC waits until the 
generic is instantiated before type checking while Delphi performs type 
checking when compiling the generic itself - or thereabouts. IMHO, FPC 
is much superior to Delphi in this area.

{$IFDEF FPC}
   TOutputBlockItemGroup<_TItem,_IItem> = class(TOutputBlockItem)
{$ELSE}
   TOutputBlockItemGroup<_TItem: TOutputBlockItem; _IItem: IUnknown> = 
class(TOutputBlockItem)
{$ENDIF}
   public
     function GetItem(index: integer): _IItem;
     function Find(ItemType: byte): _IItem;
     property Items[index: integer]: _IItem read getItem; default;
   end;

....

{$IFDEF FPC}
{ TOutputBlockItemGroup }

function TOutputBlockItemGroup<_TItem,_IItem>.GetItem(index: integer): 
_IItem;
var P: POutputBlockItemData;
begin
   P := inherited getItem(index);
   Result := _TItem.Create(self.Owner,P);
end;

function TOutputBlockItemGroup<_TItem,_IItem>.Find(ItemType: byte): _IItem;
var P: POutputBlockItemData;
begin
   P := inherited Find(ItemType);
   Result := _TItem.Create(self.Owner,P);
end;


{$ELSE}

{ TOutputBlockItemGroup }

function TOutputBlockItemGroup<_TItem,_IItem>.GetItem(index: integer): 
_IItem;
var P: POutputBlockItemData;
     Obj: TOutputBlockItem;
begin
   P := inherited getItem(index);
   Obj := TOutputBlockItemClass(_TItem).Create(self.Owner,P);
   if Obj.QueryInterface(GetTypeData(TypeInfo(_IItem))^.Guid,Result) <> 
0 then
IBError(ibxeInterfaceNotSupported,[GuidToString(GetTypeData(TypeInfo(_IItem))^.Guid)]);
end;

function TOutputBlockItemGroup<_TItem,_IItem>.Find(ItemType: byte): _IItem;
var P: POutputBlockItemData;
     Obj: TOutputBlockItem;
begin
   P := inherited Find(ItemType);
   Obj := TOutputBlockItemClass(_TItem).Create(self.Owner,P);
   if Obj.QueryInterface(GetTypeData(TypeInfo(_IItem))^.Guid,Result) <> 
0 then
IBError(ibxeInterfaceNotSupported,[GuidToString(GetTypeData(TypeInfo(_IItem))^.Guid)]);
end;