Contents | < Browse | Browse >
14C. data hiding (EXPORT/PRIVATE/PUBLIC)
----------------------------------------
E has a very handy data-hiding mechanism. Other languages, like C++,
use data-hiding on classes, which raises the need for kludges (like
'friends'), and makes datahiding insecure (Eiffel). E's datahiding
works on the module-level, which can model class-level datahiding,
but enables more intelligent schemes also.

PRIVATE and PUBLIC let you declare a section of an object as visible
to the outer world or not; the outer world here is all code outside
the module. for the code within a module, everything is always visible.
example:


OBJECT mydata PRIVATE                   -> whole object is private
  bla:PTR TO mydata, burp, grrr:INT
ENDOBJECT

OBJECT aaargh
  blerk:PTR TO aaargh                   -> public
PRIVATE
  x:INT, y:INT, z:INT                   -> private
PUBLIC
  hmpf[10]:ARRAY OF mydata              -> public again
ENDOBJECT


an object is by default public, an occurring PRIVATE or PUBLIC
acts as a toggle-switch to the objects current visibility. In the
first object, all is private. The second object has only (x,y,z)
as private. PRIVATE and PUBLIC keywords may occur:
- in the object header line
- as a line on itself in the object-def
- preceding decls in an object-def
(i.e virtually anywhere)

Why datahiding?
If you want to know why datahiding is a good technique, you'd
probably want to read a good book on OO. But in short: it is
generally assumed that lots of problems in maintaining and
enhancing large pieces of software is the fact that it's hard to change
things because lots of code start to depend on certain structures
in your program. if you datahide an object, only the code within a
module will rely on the format of objects, and you can easily change
both representation of an object (for example changing a stack
implementation from ARRAY to a linked list) and the code that
works with it. If a lot of code of a large app depend on the fact
that the stack is an ARRAY, you won't be able to simply change
it, which will lead to kludges. In general, try to datahide as
much as possible without becoming too restrictive on the use
of your object. Using methods (below) will often enable you to
keep the whole object private.