Re: Accessing result of assignment

From: Michael Wojcik (mwojcik_at_newsguy.com)
Date: 09/10/04 

Date: 10 Sep 2004 13:28:36 GMT

In article <843a4f78.0409100326.41d984ba@posting.google.com>, oldwolf@inspire.net.nz (Old Wolf) writes:
>
> int c;
> memset(x, c = 0, sizeof x);

That looks OK to me. N869 6.5.16p4 says that accessing the result of
the assignment operator *after the next sequence point* causes UB;
that's what made

   if ((a=b) == f(c))

undefined (because there's a sequence point after the arguments to f
are evaluated). In your example, the result of the assignment
operator (in "c = 0") is accessed before the sequence point that
occurs when all of memset's arguments have been evaluated, because
accessing it is part of evaluating memset's arguments; hence no UB.

So, curiously enough,

   if ((a=b) == c)

is legal, but eg

   if ((a=b) == (c,d))

is not, because of the sequence point caused by evaluating the first
operand of the comma operator.

OK, now someone should point out where I got it wrong...

(I'm glad Mark re-posted his question. This was a tricky one, and
I certainly hadn't noticed it before.) 

-- 
Michael Wojcik                  michael.wojcik@microfocus.com
The lark is exclusively a Soviet bird.  The lark does not like the
other countries, and lets its harmonious song be heard only over the
fields made fertile by the collective labor of the citizens of the
happy land of the Soviets.  -- D. Bleiman

Relevant Pages

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