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From: lewie@EE.ECN.PURDUE.EDU (Jeff Lewis)
Newsgroups: gnu.gcc
Subject: Re: left-hand side cast
Message-ID: <8811221620.AA25541@ee.ecn.purdue.edu>
Date: 22 Nov 88 16:20:52 GMT
Article-I.D.: ee.8811221620.AA25541
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Re: left-hand side casts

First, the name is awkward, how 'bout we call 'em lvalue casts?
(where an lvalue is as described in the C book; roughly speaking,
something to which you can assign a value.)

But anyway, ...

> They are a documented bug.  In very rare situations, they may be
> used to eliminate a declared temporary by making your code less
> readable and less portable.  (They always have the latter effects.)

I'm not attempting to justify the addition of various non-standard
features to an implementation of a language, however I think the
lvalue cast is getting a bad rap here.

Say you needed to push arbitrary sized data onto a stack:

	int *sp, foo;
	short int *sp2, bar;
	unsigned char *sp3, biff;

	*sp++ = foo;
	sp2 = (short int *) sp;
	*sp2++ = bar;
	sp3 = (unsigned char *) sp2;
	*sp3++ = biff;
	sp = (int *) sp3;
	...

given lvalue casts:

	int *sp, foo;
	short int bar;
	unsigned char biff;

	*sp++ = foo;
	*((short int *) sp)++ = bar;
	*((unsigned char *) sp)++ = biff;

even better, much more direct and descriptive of what's going on:
(here caddr_t is used in the sense of any valid pointer type of
proper alignment for all values to be pushed on the stack)

	caddr_t sp;
	long int foo;
	short int bar;
	unsigned char biff;

	*((long int *) sp)++ = foo;
	*((short int *) sp)++ = bar;
	*((unsigned char *) sp)++ = biff;

Now, obviously this isn't the kind of thing you're going to code everyday,
but when, and if, you do, I find the lvalue cast much preferable to
the indirection induced by the unnecessary temporaries.

--Jeff Lewis (lewie@ee.ecn.purdue.edu, pur-ee!lewie)