[util.smartptr.ownerless]

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

(2.1)
operator() defines a strict weak ordering as defined in [alg.sorting];
(2.2)
!operator()(a, b) && !operator()(b, a) is true if and only if a.owner_equal(b) is true.

— end note]