20 General utilities library [utilities]

20.11 Smart pointers [smartptr]

20.11.5 Class template owner_­less [util.smartptr.ownerless]

1
#
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 noexcept;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
  };

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

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

    using is_transparent = unspecified;
  };
}
2
#
operator()(x, y) returns 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
]