std::experimental::propagate_const<T>::operator bool
From cppreference.com
                    
                                        
                    < cpp | experimental | propagate const
                    
                                                            
                    |   constexpr explicit operator bool() const;  | 
(library fundamentals TS v2) | |
Checks whether the wrapped pointer-like object, t_, is null.
Parameters
(none)
Return value
true if the wrapped object is not null, otherwise false. Equivalent to (bool)t_.
Example
| This section is incomplete Reason: no example  | 
See also
|   returns a pointer to the object pointed to by the wrapped pointer  (public member function)  |