OTOH, intrusive_ptr doesn't have a weak_ptr counterpart -- and this is much more problematic, imho.
Yes that's an important feature too, but the ctor case is also a common case (e.g. solvable by adding an extra static 'Create').
To my knowledge the following drawbacks exist for shared_ptr: - you cannot use enabled_shared_from_this in the constructor. - you cannot use shared_ptr's in the destructor. - (never make a new shared_ptr family to an already shared object). - (enabled_shared_from_this only usable if the object is created as shared_ptr, not on the stack or just on the heap).
Well, all these points either have a reasonable solution/workaround that doesn't complicate the design much, or describe a quite "marginal" use-case. But the lack of weak-ptr makes impossible an object existence tracking, which is commonly used in a wide range of tasks. In some cases this would complicate the design and make it more error-prone (eg. manual breaking of circular references), and in some other cases it renders completely impossible a use of some facilities (like Boost.Signals2).