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) shall return x.owner_before(y). [ Note: Note that
operator() defines a strict weak ordering as defined in [alg.sorting];
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 ]