Path: utzoo!mnetor!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.lang.c
Subject: Information hiding
Message-ID: <10576@steinmetz.ge.com>
Date: 26 Apr 88 14:24:50 GMT
Organization: GE Corp. R & D, Schenectady,NY
Lines: 40


  One of the things which bothers me about the C language is that many
(most) compilers, and lint, complain about taking the address on an
array. Not that it isn't redundant, but there is a good reason for
allowing it as an official part of the language: information hiding.

  Information hiding is used to improve modularity, in the sense that no
program which is not manipulating the details of a data type need know
about them. This allows the data type to be redefined in a later version
of a package, or defined in a diferent way on another system, and not
require changes to the source code which uses the data type as a "black
box". Ada programmers are astonished to find that C can do information
hiding at all...

  If I want to have a type which is defined in a header file and
manipulated by a series of routines, I can do something like:
	typedef int mytype[5];		/* this is the user type */
and write routines to work with these types. However, if I need to take
the address of a type, to pass to a procedure, if I say:
	mytype a, b, c;
	.
	init_mt(&a);
I get a warning about taking the address of an array.

  Since the whole object of information hiding is to allow things to be
changed between implementations, I don't want to have the user treat
'mytype' as an array, since the next version might use a struct.
Therefore I must use a construct like:
	typedef struct {
	  int dummy[5];
	} mytype;
which allows me to take the address freely without telling the user
anything about the actual type.

  NOTE: This applies to addresses of procedures, too, in some cases.

-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me