__unused Flag Behavior/Usage (GCC with Objective-C)

Question

I just now learned about the __unused flag that can be used when compiling with GCC and the more I learn about it, the more questions I have...

Why does this compile without warning/error? It seems strange that I specifically tell the compiler I won't be using a variable, then when I use it, things proceed as normal.

- (void)viewDidLoad {     [super viewDidLoad];      [self foo:0]; }  - (void)foo:(NSInteger)__unused myInt {     myInt++;     NSLog(@"myInt: %d", myInt);  // Logs '1' } 

Also, what is the difference between the following two method signatures?

- (void)foo:(NSInteger)__unused myInt;  - (void)foo:(NSInteger)myInt __unused; 

Answer 1

The __unused macro (which is in fact expanded to the __attribute__((unused)) GCC attribute) only tells the compiler "don't warn me if I don't use this variable".

unused: This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC does not produce a warning for this variable. (Source: gnu.gcc.org doc)

So this GCC attribute is to avoid a warning when you don't use a variable, and NOT to trigger one when you use the variable you claimed unused.

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As regard to putting the attribute before or after the variable name in your last example, both are accepted and equivalent in your case: the compiler is just lenient about that placement for compatibility purposes (quite as you can also write both const int i or int const i)

For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes (Source: gnu.gcc.org doc)