Frank Mori Hess:
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Here's a proof-of-concept patch to svn trunk for enable_shared_from_this.hpp and shared_ptr.hpp. After applying the patch, you should be able to use shared_from_this() successfully from a constructor. It's only a proof-of-concept because it includes some quick-and-dirty nastiness (it makes the shared_count in shared_ptr public!).
A few comments. I liked your original formulation with two shared_ptr instances more. It's better to avoid direct use of shared_count. The aliasing constructor may prove helpful. I don't see why pn needs to be made public. The sp_enable_shared_from_this logic needs to go into a member function (_init? _adopt?) of enable_shared_from_this. I'm unclear on the purpose of pn.swap(...) in sp_enable_shared_from_this. It seems to me that this would overwrite the user's deleter, breaking get_deleter. I don't see why it's needed. sp_enable_shared_from_this may need to be changed to take a shared_ptr instead of a shared_count. I'm disturbed by the fact that we now provide an easy (beginner-accessible) way to create dangling pointers. Consistent shared_ptr use is supposed to make this hard. I'd be interested to hear your ideas about the new precondition of shared_from_this and its new thread safety. Overall, the direction seems promising, and I thank you for the work so far. A final note: changes to enable_shared_from_this unfortunately expose the fact that the shared_ptr/weak_ptr test suite is incomplete; it doesn't perform the full array of tests with types that derive from enable_shared_from_this. I suspect that we need at least partial coverage here to catch regressions.