You can lead a horse to water, but you can't make it drink.
That assumes that your code hasn't in the meantime added another list
item to the list view and that the memory manager hasn't
coincidentally re-used the memory of a previously freed item when
allocating the memory for the new item ...
Yes, this is going to be a rare occurrence... freakishly rare, if it
even happens at all. But it *_could_* happen, and if/when it does it
will almost certainly be a Heisenbug... exhibiting some weird
behaviours but no crashes and virtually impossible to reliably reproduce.
It's up to you... J
*From:* delphi-boun...@delphi.org.nz
[mailto:delphi-boun...@delphi.org.nz] *On Behalf Of *Ross Levis
*Sent:* Tuesday, 1 February 2011 18:45
*To:* 'NZ Borland Developers Group - Delphi List'
*Subject:* Re: [DUG] Validating a TListItem exists
ListView.Items.IndexOf() does the trick, so I'll do that in the meantime.
Cheers.
*From:* delphi-boun...@delphi.org.nz
[mailto:delphi-boun...@delphi.org.nz] *On Behalf Of *Jolyon Smith
*Sent:* Tuesday, 1 February 2011 2:05 PM
*To:* 'NZ Borland Developers Group - Delphi List'
*Subject:* Re: [DUG] Validating a TListItem exists
If the original item has been Free'd, then it either:
a) Points to nothing (literally, empty unused memory)
or
b) Points to something else entirely
You might be extremely lucky and find that
c) If points to nothing, but the memory previously used for the list
item is entirely or partially temporarily as yet unchanged
Whatever you find in one particular case, possibly even in one
particular execution run of that one particular case, is not reliable
for every case or for every execution run.
There are ways to do what you wish to do, but I am fairly certain that
every single one will involve more work and more risk than simply
assuming that /Anushka has already spilled the oil/, and consider a
deleted list item as lost and gone forever, even if it might not be.
*From:* delphi-boun...@delphi.org.nz
[mailto:delphi-boun...@delphi.org.nz] *On Behalf Of *Ross Levis
*Sent:* Tuesday, 1 February 2011 13:30
*To:* 'NZ Borland Developers Group - Delphi List'
*Subject:* Re: [DUG] Validating a TListItem exists
I was hoping to avoid that. I was thinking there may be a way of
validating that it is still a TListItem the pointer is pointing to,
something like an "is" operation, but it looks like some work required
to avoid the issue completely.
Cheers.
*From:* delphi-boun...@delphi.org.nz
[mailto:delphi-boun...@delphi.org.nz] *On Behalf Of *Jolyon Smith
*Sent:* Tuesday, 1 February 2011 10:59 AM
*To:* 'NZ Borland Developers Group - Delphi List'
*Subject:* Re: [DUG] Validating a TListItem exists
AFAIK when an item is deleted from a listview it is also free'd.
Unavoidably.
If you have a reference to a list item that may have been deleted from
the list view, then any attempt to use that reference subsequent to
it's being deleted is likely to fall foul of an access violation (best
case: at least you would get an error to respond to) or be referencing
some other memory that is re-allocated and no longer actually
representing the list item it used to refer to (worst case: since this
would be lead to silent unexpected behaviours).
I think the best you can do is add a handler to the
Listview.OnDeletion event and respond to the notifications you will
receive at the point at which any list items are actually deleted.
*From:* delphi-boun...@delphi.org.nz
[mailto:delphi-boun...@delphi.org.nz] *On Behalf Of *Ross Levis
*Sent:* Monday, 31 January 2011 16:24
*To:* 'NZ Borland Developers Group - Delphi List'
*Subject:* [DUG] Validating a TListItem exists
I'm storing some TListItem's in a TList. Some of these items could be
removed from the listview at some stage. I know the items should be
removed from the TList when removed from the listview, but don't ask.
Is there a simple method to determine if a listitem reference is still
valid and exists, or do I need to use listview.items.indexof which
could be slow.
Cheers.
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