gistfile1.m

#import <Foundation/Foundation.h>

@interface News : NSObject {
    NSString *_title;
}
@end

@implementation News
NSString *_excerpt;

-(id)initWithTitle:(NSString*)title excerpt:(NSString*)excerpt {
    self = [super init];
    if (self) {
        _title = [[NSString alloc] initWithString:title];
        _excerpt = [[NSString alloc] initWithString:excerpt];
    }
    return self;
}

-(NSString*)description {
    return [NSString stringWithFormat:@"%@, %@",_title,_excerpt];
}
@end

int main(int argc, char *argv[]) {
  @autoreleasepool {
        News *b = [[News alloc] initWithTitle:@"A" excerpt:@"X"];
        News *a = [[News alloc] initWithTitle:@"B" excerpt:@"Y"];
        NSLog(@"%@",a);
        NSLog(@"%@",b);
    }
}

Result is

2013-01-16 11:38:12.746 Untitled[3888:707] B, Y
2013-01-16 11:38:12.748 Untitled[3888:707] A, Y

The output of clang -rewrite-objc News.m looks like this:

struct News{
    struct NSObject_IMPL NSObject_IVARS;
    NSString *_title;
};
/* @end */

// @implementation News
NSString *_excerpt; // <-- global field to the class

and seems to work as a class variable, which I thought Objective-C didn't have. Where is this documented by Apple? I wish there was a complete specification of the language to look up this stuff.

Any variable declared outside of @interface … {} or @implementation … {} is treated as a regular C variable. In your example, _excerpt is a global (file scope) variable with static storage duration and could equivalently be placed at the top of the file before @interface, or between @interface and @implementation, or between the implementation of two methods — it’s the same mechanism where file scope variables in C are defined outside of a function block.

Variables with static storage duration can be used to realise class variables, a concept that doesn’t exist in Objective-C.

Thank you, that clears it up. :)