class input_filter :
in_obj_channel -> io_obj_channel ->
input_filter filters the data read from it through the
io_obj_channel (usually a
pipe after the data have been
retrieved from the passed
input_filter object never generates
However, if the passed
io_obj_channel raises such
an exception, the exception will fall through the calling chain.
If the filter is closed, the
io_obj_channel will be closed, too,
but not the source
in_obj_channel (so you can still read further
data from it).