cpp/numeric/valarray/operator at

Retrieve single elements or portions of the array.

The overloads that return element sequences create a new std object. The non- overloads return classes holding references to the array elements.

The selected elements(s) must exist:
 * for overloads, if is not less than , the behavior is undefined; and
 * for overloads, if the argument does not specify a valid subset of , the behavior is undefined.

Return value
@1,2@ A reference to the corresponding element. @3,5,7,9@ A std object containing copies of the selected items. @4,6,8,10@ The corresponding data structure containing references to the selected items.