Path: utzoo!utgpu!water!watmath!clyde!rutgers!sri-spam!ames!necntc!linus!philabs!pwa-b!mmintl!franka
From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: comp.lang.c
Subject: Re: Autoincrement question
Message-ID: <2632@mmintl.UUCP>
Date: 23 Dec 87 23:37:23 GMT
References: <1507@ogcvax.UUCP> <3333@sigi.Colorado.EDU> <7593@eddie.MIT.EDU> <2610@mmintl.UUCP> <2464@killer.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT.
Lines: 42

In article <2464@killer.UUCP> jfh@killer.UUCP (John Haugh) writes:
>In article <2610@mmintl.UUCP>, franka@mmintl.UUCP (Frank Adams) writes:
>> Slightly stronger: don't use the target of an assignment operator elsewhere
>> in the same expression.  Auto-increment and auto-decrement operators are
>> assignment operators.
>
>Way to strong.  What you (seem) to be saying is `don't do "a = a + 1"',

Oops.  Try: don't use the target of an assignment operator elsewhere
in the same expression, except in the expression to be assigned to the
variable.  Don't use the target of auto-increment and auto-decrement
operators elsewhere in the same expression.

>The expression
>
>	a += a++ + 5
>
>is still very legal.

I don't think so.  It may be that the fine details of expression order
evaluation in the ANSI draft make this unambiguous, but since I don't have a
copy handy I can't check it.  I would *not* trust compilers in general
enough to consider it portable, and I think it is in fact legal for
compilers to generate code which makes this statement equivalent to "a++"
instead of "a += 5" -- the store of the incremented value can be the last
code executed for the statement.

>> (Actually, you can use such results, provided that the assignment and the
>> other use are within different arguments of a comma operator.  That is, both
>> "f(i) , i++;" and "i++ , f(i);" are unambiguous.  But don't do this unless
>> (a) you know exactly what you are doing, and (b) you really need to.)
>
>Given the lack of f() being a macro, f(++i) and f(i++) are both `safe'.

Indeed they are; nor did I say otherwise.

This was, perhaps, not a very good example.  Try f(&i) instead of f(i) in
both cases.
-- 

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108