I'd rather have an optionally-owned pointer type so I can handle writing virtual methods that can return a value aliasing an already existing value or create a new one on demand. Otherwise you either have to roll your own or bloat your API:

  virtual std::flexptr<Thing> get_expensive_thing();

vs:

  virtual bool has_expensive_thing();
  virtual const Thing& get_expensive_thing();
  virtual std::unique_ptr<Thing> build_expensive_thing();

It'd probably have shared_ptr semantics but you'd have to treat it as a const ref for lifetime purposes, which might make it distasteful to the std library folks.