Objective-C dot notation with class methods? - objective-c

Question

Note, I'm specifically referring to the fact that dot notation is being used with class methods, not instance methods.
Out of curiosity, I wanted to see what would happen if I tried to use Objective-C dot notation syntax with 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 was 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 NSMutableArray *myArray = NSMutableArray.array

Answer 3

The Underscore library further abuses this syntax by returning blocks from class methods, resulting in code like this:
NSArray *elements = Underscore.array(array)
.flatten
.uniq
.unwrap;
To understand how this works, look at the definition of Underscore.array:
+ (USArrayWrapper *(^)(NSArray *))array
{
return ^(NSArray *array) {
return [USArrayWrapper wrap:array];
};
}
So:
Underscore.array(array)
...is equivalent to this:
NSArray *array = #[];
USArrayWrapper * (^arr)(NSArray *) = [Underscore array];
USArrayWrapper *result = arr(array);