> To get around this little problem when you need to keep lists of objects
> created dynamically, Delphi provides the TList class. You can use
> a TList or
> make a derivative from it. It would be usual to override the destructor to
> ensure all the memory is freed in your objects.
In that case, I would strongly suggest you use TCollection instead as it
maintains its items and will free them when deleted.
Regards,
Dennis.
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> Sent: Saturday, 7 July 2001 00:32
> To: Multiple recipients of list delphi
> Subject: RE: Listbox items (Was: Re: [DUG]: array of TStringList)
>
>
> Some understanding here is useful.
>
> It is not possible to create numerous objects with the same name.
>
> When you create an instance of a class (an object) you are doing several
> things:
> 1. Delphi reserves some memory for you
> 2. Delphi gives you in the variable on the left hand side, a reference to
> that piece of memory.
>
> If you then create another instance of the object using the same variable
> name, you have lost access to the bit that you had before. it is
> still there
> but you have no way of accessing it, because the variable now
> points to the
> new bit you got. You can do this ten times but at the end you will still
> only be able to access the last bit.
>
> To get around this little problem when you need to keep lists of objects
> created dynamically, Delphi provides the TList class. You can use
> a TList or
> make a derivative from it. It would be usual to override the destructor to
> ensure all the memory is freed in your objects.
>
> Now, when using a TList the above steps become:
>
> 1. Delphi reserves some memory for you
> 2. Delphi gives you in the variable on the left hand side, a reference to
> that piece of memory.
> 3. You add the value of that variable to a TList object.
>
> Then you can use the methods of TList to access all those dynamic objects
> you have created. In the days before OO we used to use linked lists of
> pointers to do the same thing. Compared to that a TList is a doddle.
>
> Addressing to your particular situation. A record is not an object. A
> Tobject is an object.
>
> I used to use records until I discovered how to use TList to link a whole
> lot of dynamically created objects. To give an example I have just updated
> an application that uses counters to count numbers in a big report, and do
> various calculations etc. It used to use various TStringLists to store all
> the counters. I updated it to use OO stuff. This is good because there are
> two different types of counter and one is a subclass of the other.
>
> All you have to do is:
> * Create an instance of your object e.g. MyObject := TMyObject.Create
> * Do something with it...set the fields or something
> * Add it to the Objects property of the list box items (which is probably
> implemented as a TList) . No typecast needed since everything is descended
> from TObject.
>
> Now you can safely reuse MyObject to create another instance, as the
> reference is now saved in the list box. (Don't free it though, as it still
> points to an instance, but if you wish you can set it to nil without
> affecting any instances of your object)
>
> When you have to do something with your object, it looks like this:
>
> MyObject := TMyObject(ListBox1.Items.Objects[X]
>
> You have to typecast it because Delphi stores it as a TObject and
> won't let
> you access any of the methods or properties other than for a
> TObject, if you
> don't do the typecast, also you will get an "invalid type" error from the
> compiler.
>
> Note that the Objects property still contains the reference until
> that list
> item is deleted.The question I am not at all sure about is whether Delphi
> frees the memory associated with each object reference when the
> list item is
> removed from the list.
>
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
> > Behalf Of Ross Levis
> > Sent: Friday, 6 July 2001 23:33
> > To: Multiple recipients of list delphi
> > Subject: Re: Listbox items (Was: Re: [DUG]: array of TStringList)
> >
> >
> > Thanks James, Ben & Reg. It's looks rather messy to me. I
> > didn't realise you
> > can create numerous duplicate objects with the same name. Is there any
> > advantage to using a Record object as opposed to a class(TObject)?
> >
> > Ross.
> >
> > James Low wrote:
> >
> > > Thanks Ben ...
> > >
> > > -----Original Message-----
> > > From: Ben Taylor [mailto:[EMAIL PROTECTED]]
> > > Sent: 6 July 2001 17:01
> > > To: Multiple recipients of list delphi
> > > Subject: RE: Listbox items (Was: Re: [DUG]: array of TStringList)
> > >
> > > > AddObject('Somestring to display', myObject);
> > > > myObject.Free; //the list owns a reference to it now
> > >
> > > you _really_ dont want to do this.. the list now has a pointer to the
> > > object, but its still the same object. if you do the free, then the
> > > list holds an invalid pointer.
> > >
> > > > myObject := Listbox1.Objects[ListBox1.Listindex];
> > > this wont compile. you'll need to cast the TObject pointer to the
> > > correct type:
> > >
> > > myObject := tRossesObject(Listbox1.Objects[ListBox1.Listindex]);
> > >
> > > that should work better..
> > >
> > > there are 'nicer' OO ways to do this but it's friday and i'm going now
> > > :-)
> > >
> > > __________________________________________________
> > > Do You Yahoo!?
> > > Get personalized email addresses from Yahoo! Mail
> > > http://personal.mail.yahoo.com/
> > >
> > ------------------------------------------------------------------
> > ---------
> > > New Zealand Delphi Users group - Delphi List -
> [EMAIL PROTECTED]
> > > Website: http://www.delphi.org.nz
> > > To UnSub, send email to: [EMAIL PROTECTED]
> > > with body of "unsubscribe delphi"
> > >
> > ------------------------------------------------------------------
> > ---------
> > > New Zealand Delphi Users group - Delphi List -
> [EMAIL PROTECTED]
> > > Website: http://www.delphi.org.nz
> > > To UnSub, send email to: [EMAIL PROTECTED]
> > > with body of "unsubscribe delphi"
> >
> > ------------------------------------------------------------------
> > ---------
> > New Zealand Delphi Users group - Delphi List - [EMAIL PROTECTED]
> > Website: http://www.delphi.org.nz
> > To UnSub, send email to: [EMAIL PROTECTED]
> > with body of "unsubscribe delphi"
> >
>
> ------------------------------------------------------------------
> ---------
> New Zealand Delphi Users group - Delphi List - [EMAIL PROTECTED]
> Website: http://www.delphi.org.nz
> To UnSub, send email to: [EMAIL PROTECTED]
> with body of "unsubscribe delphi"
---------------------------------------------------------------------------
New Zealand Delphi Users group - Delphi List - [EMAIL PROTECTED]
Website: http://www.delphi.org.nz
To UnSub, send email to: [EMAIL PROTECTED]
with body of "unsubscribe delphi"
