How can I get rid of an "unused variable" warning in Xcode?

Question

I understand exactly why unused variable warnings occur. I don't want to suppress them in general, because they are incredibly useful in most cases. However, consider the following (contrived) code.

NSError *error = nil;
BOOL saved = [moc save:&error];
NSAssert1(saved, @"Dude!!1! %@!!!", error);

Xcode reports that saved is an unused variable, when of course it isn't. I suspect this is because NSAssert1 is a macro. The NS_BLOCK_ASSERTIONS macro is not defined, so Objective C assertions are definitely enabled.

While it doesn't hurt anything, I find it untidy and annoying, and I want to suppress it, but I'm not sure how to do so. Assigning the variable to itself gets rid of the compiler warning, but I'd rather do it the "right" way if such a thing exists.

score 112 · Answer 1 · answered May 14 '12 at 06:40

112

Using Xcode 4.3.2 and found out that this seems to work (less writing)

BOOL saved __unused;

answered May 14 '12 at 06:40

JOM

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This doesn't work for me, I get a warning that `'saved' was marked as unused but was used` – Heath Borders Jul 08 '16 at 14:32

score 110 · Accepted Answer · edited Sep 16 '12 at 23:07

I'm unsure if it's still supported in the new LLVM compiler, but GCC has an "unused" attribute you can use to suppress that warning:

BOOL saved __attribute__((unused)) = [moc save:&error];

Alternatively (in case LLVM doesn't support the above), you could split the variable declaration into a separate line, guaranteeing that the variable would be "used" whether the macro expands or not:

BOOL saved = NO;
saved = [moc save:&error];

score 38 · Answer 3 · answered Mar 27 '11 at 19:35

38

In Xcode you can set the warnings for "Unused Variables." Go to "Build Settings" for the target and filter with the word "unused"

Here is a screenshot: Builld Settings Screenshot

I suggest you only change it for Debug. That way you don't miss anything in your release version.

answered Mar 27 '11 at 19:35

Black Frog

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Did you miss the part of the question where Gregory said "I don't want to suppress them in general, because they are incredibly useful in most cases"? – Sherm Pendley Mar 27 '11 at 20:21
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@ShermPendley Regardless, this is still useful, it answered my question on how to turn it off globally. – raffian Sep 23 '12 at 22:35
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turning off warnings is a great way to miss bugs. I highly recommend against. – orion elenzil May 02 '13 at 06:06
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@orionelenzil - well, yes and no. There is a real interface problem with false positives. The warning pops up constantly while you're writing code if you haven't gotten to the part of the code where you use the variable. Over time, this will train you to ignore the warning meaning you may well miss more important errors. It would better to be able to suppress the error in blocks of code being actively edited. – TechZen Mar 11 '14 at 18:43
Hi, its working fine if i set NO. but my doubt is : i have to set Debug & Release NO ya only Unused Variable: NO? which one i have to set. If i set Totally NO, then its not create any prob during release na? – Soumya Ranjan Aug 18 '14 at 10:23

score 22 · Answer 4 · answered Aug 21 '12 at 01:28

22

NSError *error = nil;
BOOL saved = [moc save:&error];
NSAssert1(saved, @"Dude!!1! %@!!!", error);
#pragma unused(saved)

Try like this. It is working for me. It will work for you, too.

answered Aug 21 '12 at 01:28

Danny Xu

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This doesn't work for me, I get a warning that `'saved' was marked as unused but was used` – Heath Borders Jul 08 '16 at 14:31

score 16 · Answer 5 · answered Feb 19 '14 at 08:10

16

The only simple and portable way to mark variable as used is… to use it.

BOOL saved = ...;
(void)saved; // now used

You may be happy with already described compiler-specific extensions, though.

answered Feb 19 '14 at 08:10

user3125367

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I prefer this over the ugly #pragma described above. – northernman Sep 03 '14 at 05:00
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Hey bro, nice tip – dimazava Mar 27 '20 at 14:02

score 15 · Answer 6 · answered Aug 04 '17 at 13:58

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#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-variable"
    NSUInteger abc; /// Your unused variable
#pragma clang diagnostic pop

SOURCE

answered Aug 04 '17 at 13:58

Alex

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This is the cleanest approach since it disables only the warning between those #pragmas – cornr Dec 19 '17 at 13:11

score 9 · Answer 7 · answered Feb 28 '14 at 15:09

9

try with: __unused attribute. Works in Xcode 5

answered Feb 28 '14 at 15:09

João Nunes

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Trenskow · Answer 8 · 2018-07-16T09:27:36.833

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This is the way you do it in C and therefore also Objective-C.

Even though you do not have warnings enabled, it's always a good idea to mark the return value as explicitly ignored. It also goes to show other developers, that you have not just forgotten about the return value – you have indeed explicitly chosen to ignore it.

(void)[moc save:&error];

EDIT: Compilers ignore casts to void, so it should not affect performance – it's just a nice clean human annotation.

edited Jul 16 '18 at 09:27

answered Mar 30 '17 at 11:55

Trenskow

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In certain contexts you MUST use this hack to get the Objective-C code to compile. Using `(void)` is the clearest way to go, thanks! – Dan Rosenstark Sep 21 '17 at 09:26

score 5 · Answer 9 · answered Jun 11 '14 at 15:53

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You can set "No" LLVM compliler 2.0 warning on "Release" enter image description here

answered Jun 11 '14 at 15:53

ArNo

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0xFADE · Answer 10 · 2016-06-22T21:33:47.397

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Make it take up two lines. Separate the declaration and default value

BOOL enabled = NO;

// ...

BOOL enabled;

enabled = NO;

edited Jun 22 '16 at 21:33

answered Jun 22 '16 at 20:38

0xFADE

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How can I get rid of an "unused variable" warning in Xcode?

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