Provides access to the element at the given index in the concurrent vector. This method is concurrency-safe for read operations, and also while growing the vector, as long as the you have ensured that the value _Index is less than the size of the concurrent vector.
reference operator( size_type _Index ); const_reference operator( size_type _Index ) const;
The version of operator  that returns a non-const reference cannot be used to concurrently write to the element from different threads. A different synchronization object should be used to synchronize concurrent read and write operations to the same data element.
No bounds checking is performed to ensure that _Index is a valid index into the concurrent vector.