std::span<T,Extent>::size
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   constexpr index_type size() const noexcept;  | 
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Returns the number of elements in the span.
Return value
The number of elements in the span.
See also
  constructs a  span (public member function)  | |
|    returns the size of the sequence in bytes   (public member function)  |