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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