Streams can be defined for input only, output only, or both. In our example, the
class needs to be able to read from a file and write to a file, and we therefore defined it to inherit from an input and an output stream class. We could have defined disjoint classes instead, one inheriting from
and the other from
. This would have allowed us to rely on the default methods for the direction predicates. However, given that we have defined one bi-directional stream class, we must define our own methods for the direction predicates.
(defmethod input-stream-p ((stream unicode-ls-stream))
(input-stream-p (ls-stream-file-stream stream)))
(defmethod output-stream-p ((stream unicode-ls-stream))
(output-stream-p (ls-stream-file-stream stream)))
The above code allows us to "trampoline" the correct direction predicate functionality from
, using the
accessor we defined previously.
are implemented as generic functions. Their default methods return
is respectively an input or output stream. If the user wants to implement a stream with no inherent directionality (and thus does not include
) but for which the directionality depends on the instance, then suitable methods should be provided.