Path: utzoo!mnetor!uunet!husc6!rutgers!mcnc!rti!xyzzy!throopw
From: throopw@xyzzy.UUCP (Wayne A. Throop)
Newsgroups: comp.lang.c
Subject: Re: Question about type casting
Message-ID: <561@xyzzy.UUCP>
Date: 17 Jan 88 19:22:25 GMT
References: <5080009@hpfcdc.HP.COM> <5080010@hpfcdc.HP.COM>
Organization: Data General, RTP NC.
Lines: 45

> boemker@hpfcdc.HP.COM (Tim Boemker)
> I want to know why a syntactically
> correct and semantically meaningful expression is not allowed.
> If you need further provocation, consider this program:
> main()
> {
>   int A[1];
>   ( (int [1]) A ) [0] = 0;
> }

The question is ill formed.  The above cast, while it *is* syntactically
correct, is *NOT* semantically meaningful.  The real question is why
were the rules that give C syntax its semantics chosen in such a way as
to make that cast meaningless.  For a full answer, one would have to ask
dmr, but the paraphrase of the question Tim asks can be addressed:

> Why can't I cast an array as an array?

To review the rules: casts take the value of the target expression and
convert this value to a new type.  An array, when it appears in an
expression, evaluates (yields a value) with pointer type, not array
type.  Thus, the cast of what appears to be something to its own type
is, in fact, an attempt to convert a pointer to an array, not an attempt
to cast an array as an array.

Now, why were these rules chosen?  Well, the rule that really gets in
the way here is the rule about how arrays are to be treated when
evaluated.  This special treatment of arrays in its turn was based on
the unification of array subscripting and address arithmetic in C.  In
other words, it is the indirect consequence of an attempt to make C a
simpler and more conceptually unified language.  In retrospect, I think
better tradeoffs could have been made, but I don't myself pretend to be
a genius language designer, so I tend to accept C as it is, with only
modest regrets.  

--
Giving up on assembly language was the apple in our Garden of Eden:
Languages whose use squanders machine cycles are sinful.  The LISP
machine now permits LISP programmers to abandon bra and fig-leaf.
                                                --- Alan J. Perlis
Moon calls them Fortran Machines and Lisp Machines,
but I would prefer to call them fuzzy mammals and
pterodactyls myself.
                                                --- Richard P. Gabriel
-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw