Robotic Tendencies » if (n00b); warning

June 1, 2006

if (n00b); warning

I wasted a non-trivial amount of time yesterday debugging code in which I’d accidentally written:

if (...);
  {
    ...
  }

Is there any situation where if (foo); can achieve something which just foo; couldn’t? Could the compiler not warn about a conditional that contained no code?

Aggravating lapses in competence aside, I’ve realised I’ve not blogged for months, so over the next few days I’m going to try and write a little about what I’ve been working on recently.

15 Responses to “if (n00b); warning”

PÃ¡draig Brady Says:
June 1st, 2006 at 13:31
That surprises me. I thought that used to be warned about?
I’ve confirmed that 4.0.2 doesn’t give any warning
with -Wall -pedantic.

It’s quite easy to do this when going to and from
shell and python code as they both use ; and :
right after the expression.
fraggle Says:
June 1st, 2006 at 13:39
I’ve certainly code like

if (blah)
;
else
{
stuff;
}

Sometimes I’ve done things like this because I want to later add something in where the first semicolon is.
Someone Says:
June 1st, 2006 at 15:18
Yes!:

if (someFlag && doSomething());

is like:

doSomething if (someFlag)

in Perl.
iain Says:
June 1st, 2006 at 15:26
It can. One of the myriad -W options to gc will turn it on, can’t remember which one though :/
Buck Says:
June 1st, 2006 at 16:12
as a matter of style, it’s sometimes been useful to me to
chain

if (…)
/* do nothing */;
else if (…)
/* do nothing */;
else {
/* do something */
}

, especially if the (…) expressions change things that get
evalutated in subsequent (…)’s

that’s not saying i’m an exemplar of good style or anything
Ben Hutchings Says:
June 1st, 2006 at 17:45
It can. Assuming you’re using gcc, use -Wextra (a.k.a -W).
Anonymous Says:
June 1st, 2006 at 19:33
Unless it has an else clause, it can’t possibly have any useful effect. If it *does* have an else clause, the compiler should *still* warn about it, because you could just as easily write it as if(!(foo)).
john doe Says:
June 1st, 2006 at 20:24
If it has an else-clause, it won’t compile.

if (a); { b }

will evaluate a, then run b. If it was

if (a); { b } else { c }

the compiler will complain about a meaningless ‘else’.
Peter Says:
June 1st, 2006 at 20:39
mcs/gmcs (Mono) and csc (.NET) give me a warning “possible mistaken empty statement”.
Adding an “else”-clause even throw out an error “Expecting `;`”

This is C# specific, though
Gunnar Says:
June 1st, 2006 at 22:41
Well, but remember that ; is by itself the smallest valid complete statement in C. Yes, getting a warning out for this behavior can be easy - but according to the language definition, it is completely legal.
slacker Says:
June 1st, 2006 at 23:13
gcc -Wall -Wextra warns about this construction.
Ari Pollak Says:
June 2nd, 2006 at 00:02
Well, you could do if(foo() && bar()) or if(foo() || bar()), and maybe you couldn’t do that with a standalone statement?
Sean Neakums Says:
June 2nd, 2006 at 00:56
-Wextra catches it (tested with gcc 4.0) but it may well add so much extra spew that you miss the real problems. Alas there’s no separate -W option for it.
Tack Says:
June 2nd, 2006 at 01:16
Been coding for 16 years and I also recently (within the last 6 months) spent a non-trivial amount of time debugging the exact same user error.

Actually I spent like 2 hours trying to figure out the problem, finally gave up and went to bed around 3am, and once I woke up and looked at the code again, saw it inside of 45 seconds.

Such is how it goes.
Adam Hooper Says:
June 2nd, 2006 at 02:39
Let’s say you want had this:

if (foo)
some_long_line();
else
some_other_long_line();

And you needed to debug something. One very quick debug might give:

if (foo)
;//some_long_line();
else
some_other_long_line();

I personally think there should be a warning, but the above would at least be mildly useful to some people.

June 1, 2006

if (n00b); warning

15 Responses to “if (n00b); warning”

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