Path: utzoo!attcan!uunet!ogicse!ucsd!ucbvax!bloom-beacon!daemon
From: scs@adam.mit.edu (Steve Summit)
Newsgroups: comp.lang.c
Subject: Re: sizeof() confusion
Message-ID: <1990Nov16.035457.8474@athena.mit.edu>
Date: 16 Nov 90 03:54:57 GMT
References: <9156@latcs1.oz.au> <27432@mimsy.umd.edu> <2665@cirrusl.UUCP> <1990Nov10.123204.8968@druid.uucp> <2692@cirrusl.UUCP>
Sender: daemon@athena.mit.edu (Mr Background)
Reply-To: scs@adam.mit.edu (Steve Summit)
Organization: Thermal Technologies, Inc.
Lines: 53

In article <2692@cirrusl.UUCP> dhesi%cirrusl@oliveb.ATC.olivetti.com (Rahul Dhesi) writes:
>>>        printf("sizeof 'c'     = %d\n", sizeof 'c');
>
>>Were you suprised that "'c'" was 4?
>>You shouldn't be.  That expression evaluates to an int, not a char.
>
>If characters are promoted to ints in expressions, then why isn't
>|sizeof c| equivalent to |sizeof (int) c|?

As Doug has pointed out, the Standard contains exhaustive
specifications of exactly when the "usual arithmetic conversions"
(i.e. promoting char to int) are carried out.

Though it's an ill-defined term (appearing nowhere in the Standard)
I find it useful to think of the notion of "dereference time."
(Please, let's not restart Usenet Standard Discussion #12671 on
the word "dereference.")  "Dereference time" is when you might
need the value of something, i.e. when you're doing arithmetic on
it (or, for pointers, using the unary * operator), but _not_ when
it's on the left-hand-side of the assignment operator, or the
operand of unary &, or the operand of sizeof.  "Dereference time"
is when you have to "look inside" an identifier, to pull out its
value.

(It is instructive to consider how differently the identifiers a
and b are handled in a simple assignment expression like "a = b".
b's value is fetched, and in fact this could be done fairly early
in the course of evaluation, but "a" must remain as a location%% --
an "lvalue" -- so we'll know where to store the value of the
right-hand-side when we're through evaluating it.  Fetching a's
value early in evaluation, or in fact at any point, would be a
completely counterproductive thing to do.)

It is at "dereference time" that the usual arithmetic conversions
are applied.  "Dereference time" is also when arrays decay into
pointers.

                                            Steve Summit
                                            scs@adam.mit.edu

(Now I'll get seventeen responses poking holes in this
"dereference time" notion and noting situations in which it's
incorrect or misleading.  I _said_ it was an ill-defined term,
and I'm only suggesting using it conceptually.  I suppose I
should think of a few exceptions first and mention them, to
forestall followups.)

%% The term "lvalue" takes its name from "left hand side," not
"location," and referring to "a," or any lvalue, as a "location"
can be misleading.  The currently Official definition of lvalue
is "an expression that designates an object" (ANSI section
3.2.2.1).  Note that "dereferencing" turns an lvalue into an
rvalue.