Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!ut-sally!im4u!rutgers!sri-unix!sri-spam!mordor!lll-tis!ptsfa!ihnp4!alberta!auvax!rwa
From: rwa@auvax.UUCP (Ross Alexander)
Newsgroups: comp.lang.c
Subject: Re: type-indexed arrays (was: enum - enum ?)
Message-ID: <184@auvax.UUCP>
Date: Fri, 12-Jun-87 15:09:09 EDT
Article-I.D.: auvax.184
Posted: Fri Jun 12 15:09:09 1987
Date-Received: Sun, 21-Jun-87 01:42:22 EDT
References: <139@starfire.UUCP> <516@haddock.UUCP> <20540@sun.uucp> <526@haddock.UUCP>
Organization: Athabasca U., Alberta, Canada
Lines: 51
Summary: about a[ short ]...

In article <526@haddock.UUCP>, karl@haddock.UUCP (Karl Heuer) writes:
> Some machines can support a[short] with no trouble, but that would raise
> the question of which types a conforming implementation is *required*
> to support (and hence can be used in a portable program) [...]

and a little further along,

> Now, we could assert that [the compiler] must support *all* types
> as subscripts, and that "int a[int]" will, if necessary [...]

and yet further,

> Incidentally, I am not suggesting that this feature be incorporated into the
> new standard -- I realize there's very little hope for that.  However, I *do*
> believe it is a good idea (at least for enum) [...]

I can think of only one little quibble with the above:  C arrays are
considered to begin with the zero'th element and end at the N-1'th
element, right?  So to declare something as 

	int	array[ short ];

implies that array[] has members indexed by -1, -2, -3, and so on
('short' is a signed type, no?).  So Karl will also have to think of
some way to introduce the declaration of array index bases, as well
as array index ranges, a la Algol-60 or Pascal; and this then
introduces a rather large can of worms when pointers to based arrays
are passed around.  Say I declare an array

	int	demo[ 21 : -10 ];

which I define to mean 'an array of ints, called demo, 21 ints long,
first element is demo[ -10 ]'.  Fine.  Now, what is the value of the
token 'demo' ?  Is it '& demo[ -10 ]' ?  Is it '& demo[ 0 ]' (this is
K&R's idea of 'demo', I think) ?  And further, when I then code up
something like 'frobozz( 2, "hi there", demo );' and frobozz() looks
like

	double frobozz( a, b, c );
		int a;
		char * b;
		int c[];
	{ /* and so on... */

what is the allowable range of indicies of c[] ? -10 .. +10 ?  0 ..
20 ?  Beats _me_ :-).

As a quick out (because I _do_ like the basic idea), how about
saying that subscript types must be unsigned?

...!ihnp4!alberta!auvax!rwa	Ross Alexander @ Athabasca University