__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

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.

---

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)

Answer 2

As of Xcode 7.3.1, there is currently no difference between:

- (void)foo:(NSInteger)__unused myInt;// [syntax 1]
- (void)foo:(NSInteger __unused)myInt;// [syntax 2]
- (void)foo:(__unused NSInteger)myInt;// [syntax 3]

But the following doesn't work:

- (void)foo:(NSInteger)myInt __unused;// [doesn't work]

For usage on this, Apple recommends first syntax. (information was partially taken from this answer)

But there is difference between:

__unused NSString *foo, *bar;  // [case a] attribute applies to foo and bar
NSString *foo __unused, *bar;  // [case b] attribute applies to foo only

NSString * __unused foo, *bar; // [case c] attribute applies to foo only
NSString __unused *foo, *bar;  // [case d] attribute applies to foo and bar
CFStringRef __unused foo, bar; // [case e] attribute applies to foo and bar

If we want __unused to apply to all, syntax [a] is my personal best as it leaves no ambiguity.

If we want __unused to apply to one, syntax [b] is my personal best as it leaves no ambiguity.

The latter three solutions are acceptable but confusing in my opinion. (information was partially taken from this answer)