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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: typesafe downward casting
Message-ID: <12068@dog.ee.lbl.gov>
Date: 14 Apr 91 17:04:46 GMT
References: <13100@ms.maus.de>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Distribution: world,comp
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 22
X-Local-Date: Sun, 14 Apr 91 10:04:46 PDT

In article <13100@ms.maus.de> Kai_Henningsen@ms.maus.de (Kai Henningsen) writes:
>... C uses Casts for two completely different purposes: one, to convert
>values (as in (double)1) ... and another, to make the compiler *interpret*
>the *same* value different (as in (int *)"blabla").

The latter is a conversion as well---for instance, when sizeof(int) ==
sizeof(long), `int x; (long)x' converts x by very carefully taking each
bit, turning it over twice, and putting it in the same place it came
from.  The result, of course, is just the original value again.

Yes, I am kidding about `turning it over twice'.

Seriously, even casts that do not cause any change in actual bit
pattern are still conversions, and the result of every cast is a value,
not an object.  Many compilers delete identity-conversions too early in
compilation, and wind up permitting assignments that should be
rejected, which makes people think casts are like unions.  Nonetheless,
every cast is a conversion, and every cast produces a new value, even
if the compiler figures out a way to do this using no machine code.
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov