Frank Mori Hess: ...
1. Allowing only (expired) weak_ptr instances to be created in the constructor; 2. Bringing the weak_ptr instances back to life once a shared_ptr takes ownership.
This however seems to require a relatively major surgery to sp_counted_base and friends; I'm not sure I'm comfortable with that.
It sounds like you're heading in the direction of the "shared_from_that" idea I floated earlier:
http://lists.boost.org/Archives/boost/2008/04/135372.php
which would allow client code to distinguish between the cases of "shared_ptr not available yet" and "shared_ptr expired".
To be clear what I'm talking about, I've implemented a little proof of concept of "shared_from_that" and "enable_shared_from_that", and a little test program.
What I had in mind is illustrated with the following: X * px = new X; // px->shared_from_this(); // throws bad_weak_ptr weak_ptr<X> wx = px->weak_from_this(); // OK assert( wx.use_count() == 0 ); // expired shared_ptr<X> sx( px ); assert( wx.use_count() == 1 ); // shares ownership with sx Also, not really related, but food for thought: the old behavior allows me to do something like the following pseudocode: void load( Archive & ar, shared_ptr<X> & px ) { X * p; ar >> p; try { px = p->shared_from_this(); } catch( bad_weak_ptr ) { px.reset( p ); } }