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Re: [smart_ptr] Sole ownership of shared_ptr?

by Joseph Thomson :: Rate this Message:

Richard L. Aurbach wrote:

(1) Your idea of storing both a weak_ptr and a true pointer to the
object strikes me as dangerous. Any time you use the true pointer
instead of locking the weak_ptr, you run the risk that the true
pointer is not valid. This would, of course, invalidate one of the
major points in using the shared_ptr/weak_ptr mechanism in the first
place. I don't think that the overhead of converting the weak_ptr via
lock() is sufficiently high to forego the advantages of the mechanism.
But of course, only performance testing can verify that. [My own bias
is to build a safe mechanism first, then only weaken it if performance
measurements verify that this step is necessary.]

(2) If you have, in fact, built a mechanism where you *know* that it
is safe to use the true pointer instead of converting the weak_ptr,
then I wonder if starting with a shared_ptr strategy is the best way
to go in the first place. Since you only want one lock on the file
(rather than shared ownership), wouldn't it be better to use auto_ptr?
That pattern seems better suited to your overall approach -- if (and
only if) you can be sure that a user will never use a deleted true
pointer.

Thanks for your reply. Well, now that I think about it my method is not thread safe. The pointer could be deleted at any point during its use. I may re-consider since I haven't ruled out the use of multi-threading in the future. However, with a single thread, doesn't checking that the weak_ptr hasn't expired before any use of the raw pointer assure that the pointer is valid (assuming that the Object doesn't unexpectedly trigger its deletion in one of its methods)? This method still requires use of a weak_ptr for validation, and therefore auto_ptr wouldn't be sufficient, unless I am missing something?

After a quick test of lock() vs. expired() vs. no check at all in my example, with 1,000,000,000 iterations of each I get 49 seconds with no check, 59 seconds with an expired() check, and 296 seconds when locking. So although locking is 25 times slower, it seems sufficiently fast enough to use in my application.

Cheers,

Joe

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