Categories in Objective-C

Add methods to a class you do not own — even NSString.

#import <Foundation/Foundation.h>

// A category adds methods to a class you do not own -- even NSString.
@interface NSString (Shelf)
- (NSString *)shelfLabel;
@end

@implementation NSString (Shelf)
- (NSString *)shelfLabel {
    return [[self uppercaseString] stringByAppendingString:@" //"];
}
@end

int main(void) {
    @autoreleasepool {
        NSLog(@"%@", [@"ambient" shelfLabel]);
        NSLog(@"%@", [@"krautrock" shelfLabel]);
    }
    return 0;
}

How it works

1. @interface NSString (Shelf) opens the category.
2. The implementation block pairs with the same name.
3. Every NSString instance now answers shelfLabel.

Keywords and builtins used here

• NSString
• int
• return
• self
• shelfLabel
• void

The run, in numbers

Lines

20

Characters to type

427

Tokens

82

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 76 seconds.

Type this snippet

Step 2 of 3 in Protocols & dynamism, step 20 of 27 in Language basics.

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