Objective-C dot notation with class methods?

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:(开发者_StackOverflow社区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

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.

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

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);