std::queue<T,Container>::size
From cppreference.com
                    
                                        
                    
                    
                                                            
                    
|   size_type size() const;  | 
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Returns the number of elements in the underlying container, that is, c.size().
Parameters
(none)
Return value
The number of elements in the container.
Complexity
Constant.
See also
|    checks whether the underlying container is empty   (public member function)  |