Using the result of an assignment as a condition without parentheses

Question

I have this inside a custom UIActionSheet class

if (otherButtonTitles != nil) {
    [self addButtonWithTitle:otherButtonTitles];
    va_list args;
    va_start(args, otherButtonTitles);
    NSString * title = nil;
    while(title = va_arg(args,NSString*)) { // error here
        [self addButtonWithTitle:title];
    }
    va_end(args);
}

I have this error

! using the result of an assignment as a condition without parentheses

pointing to this line

while(title = va_arg(args,NSString*)) {

why is that?

thanks.

Solution 1

This is probably not an error as you said, it's a warning.

The compiler is warning you that you should surround assignments within parentheses when it is within a conditional to avoid the ol' assignment-when-you-mean-comparison mistake.

To get past this rather pedantic compiler warning, you can simply surround the assignment within another pair of parentheses:

while((title = va_arg(args,NSString*))) {
//...
}

Solution 2

It should be a warning and not an error. It's trying to warn in case you're using = but you meant ==.

In this case, you're not meaning to use == because you're calling va_arg() multiple times to iterate through otherButtonTitles and assigning it to the temp var title, so just add another set of parentheses. The warning will go away.

while((title = va_arg(args,NSString*))) {

Solution 3

compiler thinks that you're doing assignment by mistake, hence suggesting this error. You should wrap your statement into ONE MORE PAIR of parenthesis like this:

while((title = va_arg(args,NSString*)))

Then compiler first will evaluate the assignment, then the result of that assignment will be passed into the while condition

Solution 4

It does seem a little picky, but have you tried:

while((title = va_arg(args,NSString*)) != NULL)

?

View more solutions

Comments

[Duck]

thanks!!! That solved the problem. The most strange part is that only LLVM is picky about it. GCC compiles it without warning. I am starting to use LLVM to compile, instead of GCC and I am receiving some picking errors here and there...

[BoltClock]

@Digital Robot: I remember reading somewhere that specifically says LLVM checks for this. I think it's good to be picky about these things sometimes :)

[Brad Larson]

@Digital Robot - GCC will do this with the -Wparentheses option enabled (the Missing Braces and Parentheses warning in Xcode). I believe LLVM does this by default now.

[SinisterMJ]

LLVM has a ton more warnings for things that are usually errors - liek this example, most people with "=" in an if statement have messed something up. Also, it checks the types of what you are passing in to a string format to make sure they match - really helpful. Note that some people have seen wierd issues when using LLVM though, so just be aware you might run into problems and have to switch back to LLVM-GCC at some point.

[Duck]

Yes, I have triple checked and everything is running fine with the binary compiled with LLVM. No problems so far.

[Max MacLeod]

strangely the issue appears as a warning in the editor but not in the build results

[dylam]

Why does surrounding the assignment in parentheses make a difference?

[Greg Schmit]

@dylam it is a clear signal that you want to use an assignment in the conditional, and not an accidental forgot-an-extra-equals-sign mistake. It's not that context cannot help you determine this also, but the extra parenthesis serve as a flag that this is intended. It's not an error; it's a warning.