Hello,
I needed to extend TFPGObjectList and I found two wierd things (FPC 3.1.1
r39507):
program project1;
{$mode objfpc}{$H+}
uses
Classes, FGL;
type
TBaseClass = class (TObject)
end;
TIDClass = class (TBaseClass)
ID: Integer;
end;
TNameIDClass = class (TIDClass)
Name: string;
end;
generic TFPGObjectListEx<T: TIDClass> = class (specialize
TFPGObjectList<TBaseClass>) //1 A
function GetItemByID(AID: Integer): T;
end;
TNameIDList = class (specialize TFPGObjectListEx<TNameIDClass>)
end;
{$R *.res}
function TFPGObjectListEx.GetItemByID(AID: Integer): T;
begin
{...}
Result:=nil; //2
end;
var NameIDList: TNameIDList;
NameID: TNameIDClass;
begin
NameID:=NameIDList.Items[0]; //1 B
end.
The demo does not compile because of two errors:
1) project1.lpr(38,21) Error: Incompatible types: got "TBaseClass" expected
"TNameIDClass" at comment //1 B
It is caused by declaration at //1 A. Class TFPGObjectListEx can be really
generic only if it is declared like this:
generic TFPGObjectListEx<T: TIDClass> = class (specialize TFPGObjectList<T>) //1
A, otherwise you must retype TNameIDClass(Items[0]) and it is against the philosophy of generics.
Isn't it meaningless? Why there must be specialize to <T> when in fact it is no
specialization at all.
2) project1.lpr(32,11) Error: Incompatible types: got "Pointer" expected
"$gendef4" at comment //2
The line must be changed to
Result:=T(nil);
which seems bizarre to me, I've never seen retyping "nil". Even more, both TFPGObjectList and
TFPGObjectListEx are constrained to TObject and to TIDClass so there is safety, TFPGObjectListEx can be only
specialized with types that have "nil" (<T> can never be <Integer>, for example).
Thanks,
Vojtěch.
PS: The demo crashes.
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