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 = void> struct owner_less;

  template<class T> struct owner_less<shared_ptr<T>> {
    bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const;
  };

  template<class T> struct owner_less<weak_ptr<T>> {
    bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const;
  };

  template<> struct owner_less<void> {
    template<class T, class U>
      bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const;
    template<class T, class U>
      bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const;
    template<class T, class U>
      bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const;
    template<class T, class U>
      bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const;

    using is_transparent = unspecified;
  };
}

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.

 — end note ]