std::vector::at
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   reference       at( size_type pos );  | 
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|   const_reference at( size_type pos ) const;  | 
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Returns a reference to the element at specified location pos. Bounds checking is performed, exception of type std::out_of_range will be thrown on invalid access.
Contents | 
[edit] Parameters
| pos | - | position of the element to return | 
[edit] Return value
reference to the requested element
[edit] Exceptions
std::out_of_range if !(pos < size()).
[edit] Complexity
Constant
[edit] See also
|    access specified element   (public member function)  | |