From: "David Abrahams" <[EMAIL PROTECTED]> [get_deleter]
> I think that considering the alternatives require: > > 1. Periodic map sweeps (we might as well be doing GC ;->), or > > 2. Solving the constructor forwarding problem for tacking on > additional data to the pointed-to class > > IMO it's worth giving serious consideration to deleter > introspection. Looks like I have to apply my mad "diligent reader" skills to the above. Problem statement: A library lib1 contains the following functions: shared_ptr<X> lib1::f(); lib1::g(shared_ptr<X> p); The library needs to communicate a piece of private data from f to g, i.e. lib1::g needs to determine whether p has been created by lib1::f, and if so, what is the private data associated with p. Solution #1: put the private data in X. lib1::f can return a shared_ptr<X> that stores an instance of a class Ximpl, derived from X, that contains the private data. lib1::g can use dynamic_cast<Ximpl*> to detect the presence of private data. Problem: if X is not defined by lib1, transparently wrapping it in a Ximpl requires forwarding constructor arguments from Ximpl's constructors to X's constructors (X may be noncopyable.) Solution #2: use a map<weak_ptr<void>, Data>. lib1::f can use the generated shared_ptr<X> as a key to a global map, storing the private data there. lib1::g can use p to find the associated data in the map. This solution is more general since it doesn't require lib1 to create the shared_ptr<X> itself. Even external shared_ptr instances can have associated private data. Problem: the map needs to be periodically swept to eliminate expired weak_ptr keys. Solution #3: use the two-argument shared_ptr constructor and put the private data in the deleter. Problem: given a shared_ptr, there is currently no way to obtain a reference to the deleter, if there is one. Correct? Now the interface questions. Q: Why a free function? A: a member would require the p.template get_deleter<D>() syntax when p is dependent on a template parameter. Q: Why use a generic 'get_deleter' name for the free function? A: ??? _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
