Objective-C 2.0 dot notation - Class methods?

Question

Please note I am specifically referring to the fact that dot notation is being used on class methods, not instance methods.

Out of curiosity, I wanted to see what would happen if I tried to use Objective-C 2.0 dot notation syntax on a class method. My experiment was as follows:

#import <Foundation/Foundation.h>

static int _value = 8;

@interface Test : NSObject
+ (int) value;
+ (void) setValue:(int)value;
@end

@implementation Test
+ (int) value
{
  return _value;
}
+ (void) setValue:(int)value
{
  _value = value;
}
@end

int main(int argc, char * argv[]) {
  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
  NSLog(@"Test.value: %d", Test.value);
  NSLog(@"[Test value]: %d", [Test value]);
  Test.value = 20;
  NSLog(@"Test.value: %d", Test.value);
  NSLog(@"[Test value]: %d", [Test value]);
  [Test setValue:30];
  NSLog(@"Test.value: %d", Test.value);
  NSLog(@"[Test value]: %d", [Test value]);
  [pool release];
  return 0;
}

I was surprised to see that this compiled, let alone executed with what is, I suppose, correct behavior. Is this documented somewhere, or just a fluke of the compiler?

I compiled using GCC on Mac OS X 10.6:

gcc --version: i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5659)

compile using: gcc ObjCClassDotSyntax.m -framework Foundation -o ObjCClassDotSyntax

run: ./ObjCClassDotSyntax

output:

2010-03-03 17:33:07.342 test[33368:903] Test.value: 8

2010-03-03 17:33:07.346 test[33368:903] [Test value]: 8

2010-03-03 17:33:07.351 test[33368:903] Test.value: 20

2010-03-03 17:33:07.352 test[33368:903] [Test value]: 20

2010-03-03 17:33:07.353 test[33368:903] Test.value: 30

2010-03-03 17:33:07.353 test[33368:903] [Test value]: 30

Answer 1

This is correct behavior. foo.method is syntactic sugar for [foo method]—a straight conversion with identical semantics. Similarly foo.prop = bar is syntactic sugar for [foo setProp:bar], again with identical semantics. This transformation is implemented in the compiler. Thus you can use dot notation to call 0-parameter methods as in foo.doSomething instead of [foo doSomething]. Of course, if you do this, you are evil.

The fact that the callee is a class instance doesn't mater because in Objective-C, classes are also objects. Using dot notation on a class calls the parameterless method on that class.

Dot notation is described in the Objective-C Programming Language document.

Answer 2

In the "evil but it works" category, I've been known to use convenience constructors with the dot notation once in a while, such as NSMutable *myArray = NSMutableArray.array