On Friday 02 May 2008 08:18, Peter Dimov wrote:
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
It's not clear to me that this kind of behavior could be implemented for weak_ptr without changing its specification. Also, it doesn't allow for distinguishing between "object alive but no shared_ptr owner yet" and "shared_ptr expired or object destructed" cases. For the use case of tracking signal/slot connections, the signal would want to be able to distinguish between the two cases, temporarily blocking the connection in the first, and disconnecting it permanently in the second. What does seem to have the right semantics is for weak_from_this() to return something like a shared_ptr<weak_ptr<T> >. Every call would return a shared_ptr that points at the unique weak_ptr which will be assigned the object's owning shared_ptr. The "object alive but no owning shared_ptr" case would correspond to the weak_ptr being empty, and the "owning shared_ptr expired case" would correspond to the weak_ptr being expired. No changes to the specifications or implementations of shared_ptr/weak_ptr would be required. This is really all my shared_from_that class is (I'm starting to dislike my name choice btw). -- Frank