Re: [boost] Shared_ptr "mini garbage collector"

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-01-07 16:08:28

From: "David Abrahams" <dave_at_[hidden]>
> "Peter Dimov" <pdimov_at_[hidden]> writes:
>
> > It's true that, in general, there is no safe way to break the cycle; x1
may
> > keep a raw pointer to x2, or it might be a X invariant that X::p is
> > non-empty, causing ~X to fail. This is why the final decision to break
the
> > cycles should be left to the user, and the collector should not
> > automatically reclaim memory. Still, most reasonable classes would be
> > collect-friendly.
>
> Isn't the biggest problem one of system design?  How does the user
> write the cycle-breaking code which does different things based on the
> dynamic type of the objects being referenced?

The user doesn't need to write any cycle-breaking code. All that is needed
is to reset() the shared_ptr instances that keep the cycle alive. I.e. the
requirement is that the object's invariants must not be broken if a
shared_ptr member is suddenly reset.

It's not necessary to know the T in shared_ptr<T> in order to reset it if
shared_ptrs are layout-compatible (true on most implementations.)