Using the GCC __unused attribute with Objective-C

Question

Is it possible to use the __unused attribute macro on Objective-C object method parameters? I've tried placing it in various positions around the parameter declaration but it either causes a compilation error or seems to be ignored (i.e., the compiler still generates unused parameter warnings when compiling with -Wall -Wextra).

Has anyone been able to do use this? Is it just unsupported with Objective-C? For reference, I'm currently using Apple's build of GCC 4.0.1.

Answer 1

Okay, I found the answer... it appears to be a bug with the implementation of Apple's gcc 4.0. Using gcc 4.2 it works as expected and the proper placement is the following:

-(void)someMethod:(id) __unused someParam;

It's documented in the Objective-C release notes if anyone is interested: http://developer.apple.com/releasenotes/Cocoa/RN-ObjectiveC/index.html#//apple_ref/doc/uid/TP40004309-DontLinkElementID_6

As a note, your answer will compile, Louis, but as I stated in my question it won't actually do anything or suppress the unused warning issued by the compiler.

EDIT: I filed a bug report with apple for this rdar://6366051.

Answer 2

A common idiom is to use the following:

#define UNUSED(x) (void)x
void SomeFunction(int param1, int param2)
{
  UNUSED(param2);
  // do stuff with param1
}

The UNUSED(param2) statement doesn't generate any object code, eliminates warnings about unused variables, and clearly documents the code as not using the variable.