On Apr 6, 2008, at 5:48 PM, Joseph Thomson wrote:
Hi everyone,

Over the past few days I have been trying to integrate Boost smart pointers
into my system, which is basically a manager (ObjectManager) class which
creates, destroys, and keeps track of instances of another class (Object).

The manager keeps a std::map of instances of type shared_ptr.  The manager
has a createObject() method which returns a shared_ptr instance to the user,
thus the user becomes an owner of the object, alongside the manager.  My
problem arises because I want objects to _always_ be destroyed when the
manager method destroyObject() is called.  I realise that part of the reason
to use smart pointers is to prevent data from being deleted while a
reference to it still exists, but I figure they are still worth using just
for the safety aspect.

I am also in the position of attempting to integrate shared_ptr into my application framework, so I am definitely a neophyte in this area. (So please take my comments with a grain of salt.) However, I have a couple of thoughts.

(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.

Cheers,

Rick Aurbach, Ph.D.
President and Chief Engineer
Aurbach & Associates, Inc.
8233 Tulane Avenue, Suite B
St. Louis, MO 63132

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"If it ain't broke and you haven't fixed it recently,  it probably needs to be rewritten."  - Richard Aurbach (1948- )
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