Obtain a reference to the value the object is referencing.
A reference to the underlying object
Obtain a reference to the value the object is referencing. This will copy the underlying value (if necessary) so changes to the value do not affect other
write() does not have the same preconditions as
write() returns a reference to the underlying object’s value, thus requiring that an underlying object exist.
operator=() on the other hand will perform an allocation if one is necessary.