[cfe-dev] Side effect of conditional operator

[Sanghyeon Seo]

2007/11/14, Chris Lattner <clattner at apple.com>:
> ?: itself doesn't have side effects.  The warning is reasonable in
> this code:
>
>    i < j ? (sum += i) : (sum += j);
>
> The issue is that the ?: expression itself returns a value, and that
> value is being ignored.  Why not use an if statement in this case?

i = 1;

The issue is that = expression itself returns a value, and that value
is being ignored...? In this case, ?: expression as a whole clearly
has a local side effect.

The affected code is from Expat 2.0.1, xmlparse.c, lookup function,
which implements linear probing of open addressing hash table.

unsigned long h = hash(name);
unsigned long mask = (unsigned long)table->size - 1;
unsigned char step = 0;
i = h & mask;
while (table->v[i]) {
  if (keyeq(name, table->v[i]->name))
    return table->v[i];
  if (!step)
    step = PROBE_STEP(h, mask, table->power);
  i < step ? (i += table->size - step) : (i -= step);
}

Yes, it can be written as i = (i - step) % table->size; (I think).
Whether this diserves a warning is a matter of opinion.

-- 
Seo Sanghyeon