std::enable_shared_from_this::weak_from_this
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                    | std::weak_ptr<T> weak_from_this(); | (1) | (since C++17) | 
| std::weak_ptr<T const> weak_from_this() const; | (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    enabled_shared_from_this. 
Return value
std::weak_ptr<T> that shares ownership of *this with pre-existing std::shared_ptrs
Exceptions
2) 
noexcept specification:  
noexcept
  Example
| This section is incomplete Reason: no example | 
See also
| (C++11) | smart pointer with shared object ownership semantics (class template) |