20 General utilities library [utilities]

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

• (2.1)

  operator() defines a strict weak ordering as defined in [alg.sorting];

• (2.2)

  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.