Frank Mori Hess:
On Saturday 07 March 2009, Peter Dimov wrote:
I can't think of a good way to pass this shared_ptr to the enable_shared_from_this base since we have no control over T.
But we do have control over T. That is, if the user wants to define a class T which uses shared_from_this in its constructor, we can require them to fulfill certain requirements, right? Maybe something like providing a special constructor signature which accepts the owning shared_ptr, plus maybe an extra tag object to insure it doesn't get confused with other constructor overloads. Although if the T constructor is receiving the shared_ptr directly, it wouldn't have any need to call shared_from_this().
This is one possible solution. Define a "token class" weak_this into which make_shared can stuff the owning shared_ptr, so that when make_shared<X>( weak_this(), 1, 2 ) is called, X::X receives a non-empty weak_this. This works but requires cooperation from X. In principle, X may be derived from Y which in turn is derived from enable_shared_from_this<Y>. Y knows about weak_this but X may not. Another possible solution is to stick the "weak this" into a thread-local static member of enable_shared_from_this<Y>. This also works but (absent compiler support for thread locals) doesn't fit into the header-only nature of shared_ptr.