20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.2 Shared-ownership pointers [util.sharedptr]

20.3.2.4 Class template owner_­less [util.smartptr.ownerless]

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; }; }
operator()(x, y) returns x.owner_­before(y).
[Note 1:
Note that
  • operator() defines a strict weak ordering as defined in [alg.sorting];
  • two shared_­ptr or weak_­ptr instances are equivalent under the equivalence relation defined by operator(), !operator()(a, b) && !operator()(b, a), if and only if they share ownership or are both empty.
— end note]