std::enable_shared_from_this<T>::weak_from_this
From cppreference.com
                    
                                        
                    < cpp | memory | enable shared from this
                    
                                                            
                    |   std::weak_ptr<T> weak_from_this() noexcept;  | 
(1) | (since C++17) | 
|   std::weak_ptr<T const> weak_from_this() const noexcept;  | 
(2) | (since C++17) | 
Returns a std::weak_ptr<T> that tracks ownership of *this by all existing std::shared_ptr that refer to *this.
Notes
This is a copy of the the private mutable weak_ptr member that is part of    enable_shared_from_this. 
Return value
std::weak_ptr<T> that shares ownership of *this with pre-existing std::shared_ptrs
Example
| This section is incomplete Reason: no example  | 
See also
|    (C++11)  | 
   smart pointer with shared object ownership semantics   (class template)  |