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

Norbert Unterberg <nunterberg@newsgroups.nospam>
Wed, 05 Mar 2008 08:08:25 +0100
Bruce. schrieb:

"Doug Harrison [MVP]" <> wrote in message

On Tue, 4 Mar 2008 16:49:29 -0600, "Bruce." <> wrote:

Essentially this means thread 1 will be processing item 3 while it is
deleted by thread 2. Then thread 1 will increment the iterator to step
item 3 (now deleted) to item 4 (now 3).

Will the iterator in thread 1 still step correctly if the item it's
from has been deleted?

Nope. Deleting an item in a list invalidates all iterators, references,
pointers to the item.

Thanks Igor and Doug.

That's bad news for how I was planning to impliment this. Essentially the
items will be added to the list in one thread (a thread in a exe calling my
dll) and time stamped.

I need to periodically scan the list in another thread (lives in the dll)
and "age" the entries, deleting old entries past a certain expiration time.

Can my use of an STL list be salvaged or do I go back to the drawing board?

You need a better locking strategy. You need to decide how to handle cleanups
while another task is iterating the list. One way could be to lock the whole
iteration process and not only the increment operation. Or you need to trigger
the clean-up operation in the same iterating task after it finished the list.

So yes, you need to get back to the drawig board. Either you need a list type
that supports concurrent operations (but you still need to lock the current
"active" list item), or you need better list access algorighms. Maybe you need
to encapsulate the list with an abstraction layer.


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