Re: Delete a list item from a separate thread safe?

Joe Greer <>
Mon, 28 Apr 2008 22:02:13 +0200 (CEST)
"Doug Harrison [MVP]" <> wrote in

On Mon, 28 Apr 2008 18:53:25 +0200 (CEST), Joe Greer
<> wrote:

But does this also happen in case of addition of a new item from a
separate thread to the list. Will this also cause invalidation of
the iterators ??

No, new items won't have iterators etc referring to them, so they
can't be invalidated. No other iterators will be invalidated either
by an insertion of a new item.

During the list update, the list is in an indeterminate state, and
trying to modify iterators to existing items may not work. For
example, a list iterator may contain a pointer to a list item, and the
item's prev/next fields may be changing during the update. So while
the iterator isn't "invalidated" by the update, it may be invalid for
some operations /during/ the update, hence my advice to synchronize if
iterators are used to iterate in a thread X while a thread Y is
modifying the list. Thus, unless you use the iterator merely for
dereferencing (in which case you might as well just use a pointer or
reference), you need to synchronize.

A lot depends upon how he wants to use the list. IMO insertion, deletions,
iteration are all operations that need synchronization. The first two are
obvious, the last is not so. You usually want to present a snapshot of the
current state of the list to the invoker and you don't want it out of date
before you are finished. Otherwise, you can end up processing a list that
never actually existed and any point in time. If it's all synchronized,
you don't have to worry about the iterators becoming invalid.

Of course, I am not coding his app, but iterators iterate, they don't point
(at least in my mind). So, if I am going to hold a reference to an object,
I will use a pointer, not an iterator. (Otherwise, one can run into the
case you mention.) A pointer is usually better than a reference for this
IMO because you can set it to null when you are done and it just screams
"worry about whether I exist" whereas a reference screams "I always exist".
Anyway, those are all nice design choices he can make.


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