Thanks for reply.
Ad 2) I opened issue: https://bugs.freepascal.org/view.php?id=34037
Ad 1) Are there any plans for improvement of generic inheritance? Related to my
example, instead of
generic TFPGObjectListEx<T: TIDClass> = class (specialize TFPGObjectList<T>)
I would prefer simplier and more straightforward
generic TFPGObjectListEx<T> = class (TFPGObjectList)
which will give programmers possibility to extend classes from FGL without any
further constraints.
What if I fill feature request? :-)
Vojtěch
______________________________________________________________
Od: Sven Barth via fpc-pascal <fpc-pascal@lists.freepascal.org>
Komu: fpc-pascal@lists.freepascal.org
Datum: 26.07.2018 07:04
Předmět: Re: [fpc-pascal] Generics - extending TFPGObjectList
Am 26.07.2018 um 02:31 schrieb Vojtěch Čihák: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;
{$R *.res}
function TFPGObjectListEx.GetItemByID(AID: Integer): T;
begin
{...}
Result:=nil; //2
end;
var NameIDList: TNameIDList;
NameID: TNameIDClass;
begin
NameID:=NameIDList.Items[0]; //1 B
end.
The demo does not compile because of two errors:
1) project1.lpr(38,21) Error: Incompatible types: got "TBaseClass" expected
"TNameIDClass" at comment //1 B
It is caused by declaration at //1 A. Class TFPGObjectListEx can be really
generic only if it is declared like this:
generic TFPGObjectListEx<T: TIDClass> = class (specialize TFPGObjectList<T>) //1
A, otherwise you must retype TNameIDClass(Items[0]) and it is against the philosophy of generics.
Isn't it meaningless? Why there must be specialize to <T> when in fact it is no
specialization at all.
You are not overriding the Items property, thus it will still be the Items property
of TFPGObjectList<TBaseClass> and thus the compiler will rightfully complain at
location 1 B. Using generics does not absolve you from usual inheritance problems.
2) project1.lpr(32,11) Error: Incompatible types: got "Pointer" expected
"$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).
Well, no one said that the generics are completely bug free... Especially in
special situations that involve either Nil or type constraints (or in this case
both). Please provide a simple example and open a bug report.
Regards,
Sven
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