Reassignable reference_wrapper with empty state

I created a simple custom refernce_wrapper class template, which, in contrast to std::reference_wrapper, has some additional functionality:

1. It recognizes an empty state (which allows deferring binding).
2. It provides a rebinding option. (As noted in comments, this is actually possible with the copy assignment operator of std::reference_wrapper.)
3. It defines operator->() for direct access to members of the referenced object.

Here is a simple code that illustrates this functionality:

Derived d;

reference_wrapper<Base> rw;  // impossible with std::reference_wrapper
rw = d;                      // deferred binding
rw->memfn();                 // 'rw.get().memfn()' with std::reference_wrapper

My implementation is as follows:

template <typename T>
class reference_wrapper
{
  T* ptr_;

public:
  reference_wrapper() : ptr_(nullptr) { }
  reference_wrapper(const reference_wrapper&) = default;

  template <typename U>
  reference_wrapper(U& obj) : ptr_(&obj) { }

  reference_wrapper& operator=(const reference_wrapper&) = default;

  template <typename U>
  reference_wrapper& operator=(U& obj)
  {
    ptr_ = &obj;
    return *this;
  }

  bool empty() const { return ptr_ == nullptr; }

  T& get() const
  {
    assert(!empty());
    return *ptr_;
  }

  operator T&() const { return get(); }

  T* operator->() const 
  {
    assert(!empty());
    return ptr_;
  }

  void clear() { ptr_ = nullptr; }
};

template <typename T>
inline auto ref(T& r)
{
  return reference_wrapper<T>(r);
}

template <typename T>
inline auto cref(const T& r)
{
  return reference_wrapper<const T>(r);
}

Complete live demo link: https://godbolt.org/z/MGYnWKo7o

Notes:

1. I don't care about binding rvalues.
2. For performance reasons, I want to avoid unnecessary branching in production builds. This is why the access to the referenced value is protected with assertions.

Rationale:

Why am I not using a pointer instead?

1. I don't like raw pointers to be used in high-level application code.
2. I want the access to the referenced object to be protected with assertions.

I will appreciate any comments/suggestions for improvement if anyone finds anything wrong with this code.