 | | | -none- | -none- 2007-08-16 - By Warren Young
Back
Jonathan Wakely wrote:
>
> Users sharing Rows between threads already have to ensure
> the Result object doesn't go out of scope in another thread,
Well, funny you should bring give that example. The template we are
discussing in this thread was created to solve the single-threaded
version of the same problem.
This comes up when someone runs a query, gets some Rows, then wants to
get a new result set without copying the Row data to a new data
structure just to work around the existing dependency of Row on the
Result that created it. This is a common need when details of the
second query depend on the results of the first. This problem is now
solved; Rows no longer require their creator to outlive them.
The atomic inc/dec issue brought up by Joseph Artsimovich is all that
prevents the problem in your example from being solved, too. The
question is, does it *need* to be solved? Obviously I think the single-
thread variant of this problem does need to be solved. But in light of
the bottleneck argument I gave in my response to Joseph, you can guess
that I'm not seeing the value in solving the multi-thread variant.
So, tell me why I'm wrong. Why is it helpful to send off Rows to a
second thread while the first reuses the Result object for a new query?
> http://www.artima.com/cppsource/safebool.html
Thanks for the link, this was most helpful.
> I think the latest Boost.SharedPtr actually uses a pointer to member
> data, not member function.
Are you endorsing this difference, or just pointing it out?
> With a safe bool there's no need for operator!
Actually, I don't see why it was needed with the existing scheme, but it
was necessary to make something compile. I don't remember the details.
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