std::vector<T,Allocator>::at
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|   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, with bounds checking. 
If pos is not within the range of the container, an exception of type std::out_of_range is thrown.
Parameters
| pos | - | position of the element to return | 
Return value
Reference to the requested element.
Exceptions
std::out_of_range if !(pos < size()).
Complexity
Constant.
See also
|    access  specified element   (public member function)  |