std::hash(std::shared_ptr)

template<class T> struct hash<shared_ptr<T>>;

(since C++11)

The template specialization of std::hash for std::shared_ptr<T> allows users to obtain hashes of objects of type std::shared_ptr<T>.

For a given std::shared_ptr<T> p, this specialization ensures that.

`std::hash<std::shared_ptr<T>>()(p) == std::hash<T*>()(p.get())`.	(until C++17)
`std::hash<std::shared_ptr<T>>()(p) == std::hash<typename std::shared_ptr<T>::element_type*>()(p.get())`.	(since C++17)