The class template owner_less allows ownership-based mixed comparisons of shared and weak pointers.
namespace std { template<class T> struct owner_less; template<class T> struct owner_less<shared_ptr<T> > { typedef bool result_type; typedef shared_ptr<T> first_argument_type; typedef shared_ptr<T> second_argument_type; bool operator()(shared_ptr<T> const&, shared_ptr<T> const&) const; bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const; bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const; }; template<class T> struct owner_less<weak_ptr<T> > { typedef bool result_type; typedef weak_ptr<T> first_argument_type; typedef weak_ptr<T> second_argument_type; bool operator()(weak_ptr<T> const&, weak_ptr<T> const&) const; bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const; bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const; }; }
operator()(x,y) shall return x.owner_before(y). [ Note: Note that
operator() defines a strict weak ordering as defined in [alg.sorting];
under the equivalence relation defined by operator(), !operator()(a, b) && !operator()(b, a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.
— end note ]