I think you should care greatly, because this is violating OOP principles - in making assumptions about the implementation of the Objects "array" (and having the source is no excuse). Obviously the GetObject source cares somewhat about what -1 points to. I suspect it's making the terrible assumption that you'll be passing in a TObject not an integer.
-1 could only possibly point to a single byte anyway, otherwise it would overrun the address limit (no matter how bits worth of memory addressing you have - 32 / 64 / whatever). Alister Christie Computers for People Ph: 04 471 1849 Fax: 04 471 1266 http://www.salespartner.co.nz PO Box 13085 Johnsonville Wellington Jolyon Smith wrote: > Who cares where "-1" points to? - if you don't *use* TObject(-1) *as* a > TObject then it's simply smoke and mirrors to get the compiler to allow you > to store a particular pattern of 32-bits in a storage slot that is "typed" in > a particular way. > > > But yes, this code causes an error in BDS 2006, thanks to this bit of > nonsense in the VCL: > > function TListBoxStrings.GetObject(Index: Integer): TObject; > begin > if ListBox.Style in [lbVirtual, lbVirtualOwnerDraw] then > Result := ListBox.DoGetDataObject(Index) > else > begin > Result := TObject(ListBox.GetItemData(Index)); > ** if Longint(Result) = LB_ERR then Error(SListIndexError, Index); ** > end; > end; > > > The line with ** ** should simply be testing "Index" imho, and doing so > *before* attempting to retrieve the item data and casting as TObject. > > However, the same code does NOT cause an error with a combo-box! > > The equivalent code in the VCL for a combo (TCustomComboBoxStrings.GetObject) > contains this pertinent comment and modification to the error checking: > > // Do additional checking on Count and Index here is so in the event > // the object being retrieved is the integer -1 the call will succeed > if (Longint(Result) = CB_ERR) and ((Count = 0) or (Index < 0) or (Index > > Count)) then > Error(SListIndexError, Index); > > > But again, I believe a simple check on "Index" before attempting to retrieve > the item data would still be more correct an entirely separate any concerns > of the "value" of the data and the expected returned result. > > > >> -----Original Message----- >> From: delphi-boun...@delphi.org.nz [mailto:delphi- >> boun...@delphi.org.nz] On Behalf Of Alister Christie >> Sent: Tuesday, 16 February 2010 10:20 a.m. >> To: NZ Borland Developers Group - Delphi List >> Subject: Re: [DUG] Is it a bug in latest version of Delphi? >> >> I can confirm that >> ListBox1.Items.AddObject('foo', TObject(-1)); >> ShowMessage(IntToStr(Integer(ListBox1.Items.Objects[0]))); >> gives an error but >> ListBox1.Items.AddObject('foo', TObject(0)); >> ShowMessage(IntToStr(Integer(ListBox1.Items.Objects[0]))); >> does not >> >> I think casting -1 to an object is probably not the best idea - what >> memory location does this point to anyway? 0xFFFFFFFF? >> >> Alister Christie >> Computers for People >> Ph: 04 471 1849 Fax: 04 471 1266 >> http://www.salespartner.co.nz >> PO Box 13085 >> Johnsonville >> Wellington >> >> >> >> sinu sudhakaran wrote: >> >>> Hi all, >>> >>> I came across a strange bug(???) in latest versions of Delphi. >>> >>> Have a look at the following code. >>> >>> procedure TForm1.Button1Click(Sender: TObject); >>> var >>> i : integer; >>> begin >>> ComboBox1.Clear; >>> ComboBox1.Items.AddObject('All Locations', TObject(-1)); >>> ComboBox1.Items.AddObject('Only this Location', TObject(0)); >>> ComboBox1.Items.AddObject(Test Location', TObject(1)); >>> >>> i := Integer(ComboBox1.Items.Objects[0]); >>> showmessage(inttostr(i)); >>> end; >>> >>> When I tried to run this code in Delphi 6 and Delphi 2007 , I am >>> getting List Index out of bounds[0] error in Showmessage. When I >>> >> tried >> >>> -2, -3, -4… instead of -1 object , I got the proper output without >>> >> any >> >>> List index out of bounds error. Also I am getting -1 without any >>> >> error >> >>> when I tried this code in Delphi 3. >>> >>> Is it a bug in latest version of Delphi ? Any help is appreciated. >>> >>> Thank you >>> >>> Regards, >>> Sinu >>> >>> >>> >> <http://sigads.rediff.com/RealMedia/ads/click_nx.ads/www.rediffmail.com >> /signatureline....@middle?> >> >>> --------------------------------------------------------------------- >>> >> --- >> >>> _______________________________________________ >>> NZ Borland Developers Group - Delphi mailing list >>> Post: delphi@delphi.org.nz >>> Admin: http://delphi.org.nz/mailman/listinfo/delphi >>> Unsubscribe: send an email to delphi-requ...@delphi.org.nz with >>> >> Subject: unsubscribe >> _______________________________________________ >> NZ Borland Developers Group - Delphi mailing list >> Post: delphi@delphi.org.nz >> Admin: http://delphi.org.nz/mailman/listinfo/delphi >> Unsubscribe: send an email to delphi-requ...@delphi.org.nz with >> Subject: unsubscribe >> > > > _______________________________________________ > NZ Borland Developers Group - Delphi mailing list > Post: delphi@delphi.org.nz > Admin: http://delphi.org.nz/mailman/listinfo/delphi > Unsubscribe: send an email to delphi-requ...@delphi.org.nz with Subject: > unsubscribe _______________________________________________ NZ Borland Developers Group - Delphi mailing list Post: delphi@delphi.org.nz Admin: http://delphi.org.nz/mailman/listinfo/delphi Unsubscribe: send an email to delphi-requ...@delphi.org.nz with Subject: unsubscribe
