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


