Re: -Wparentheses lumps too much together

[Ross Smith <r-smith@ihug.co.nz>]

Paul Brook wrote:
 >James K. Lowden wrote:
>>
>> 1) most combinations of && and || don't need parentheses because
>>
>> 	(a && b) || (c && d)
>>
>> is by far more common than
>>
>> 	a && (b || c) && d
>>
>> and, moreover, broken code fails at runtime, and
> 
> I dispute these claims.
> 
> The former may be statistically more common, but I'd be surprised if the 
> difference is that big. I can think of several fairly common situations where 
> both would be used.
> 
> Any time you've got any sort of nontrivial condition, I always find it better 
> to include the explicit parentheses. Especially if a, b, c, and d are 
> relatively complex relational expressions rather than simple variables.

I second Paul's points. The precedence of && and || are not widely 
enough known that warning about it should be off by default (for people 
sane enough to use -Wall).

A couple of data points:

First, I've been writing C and C++ for a living for nearly 20 years now, 
and I didn't know that && had higher precedence than ||. I vaguely 
recalled that they had different precedence, but I couldn't have told 
you which came first without looking it up. I'd happily bet that the 
same is true of the overwhelming majority of developers who aren't 
compiler hackers. Giving && and || different precedence is one of those 
things that feels so totally counterintuitive that I have trouble 
remembering it no matter how many times I look it up. I have a firm 
coding rule of always parenthesising them when they're used together. 
(Likewise &, |, and ^, which have similar issues. I can't remember 
whether -Wparentheses warns about those too.)

Second, I just grepped the codebase I'm curently working on (about 60k 
lines of C++) for anywhere && and || appear on the same line. I got 49 
hits, 29 where && was evaluated before ||, 20 the other way around. (All 
of them, I'm happy to say, properly parenthesised.) So while 
&&-inside-|| seems to be slightly more common, I'd certainly dispute 
James's claim that it's "far more common".

-- Ross Smith