Objective-C: Categories

Tue, Jul 1

As an alternative to subclassing, Objective-C categories provide a means to add methods to a class. What’s intriguing, is that any methods that you add through a category become part of the class definition, so to speak. In other words, if you add a method to the NSString class, any instance, or subclass, of NSString will have access to that method.

Defining a category is identical to defining the interface for a class, with one small exception: you add a category name inside a set of parenthesis after the interface declaration. The format is shown below:

@interface ClassToAddMethodsTo (category)
  ...methods go here

For example, below I’ve defined a category that adds a method to the NSString class. The method reverseString adds the capability to all NSString objects to reverse the characters in the string.

@interface NSString (reverse)
-(NSString *) reverseString;

As with the @interface declaration, the @implementation section changes only in that the category name is added to the definition. Below is the implementation of the interface defined above. Notice how in both cases I added (reverse) , which is the category name I assigned.

@implementation NSString (reverse)
-(NSString *) reverseString
  NSMutableString *reversedStr;
  int len = [self length];
  // Auto released string
  reversedStr = [NSMutableString stringWithCapacity:len];     
  // Probably woefully inefficient...
  while (len > 0)
    [reversedStr appendString:
         [NSString stringWithFormat:@"%C", [self characterAtIndex:--len]]];   
  return reversedStr;

What follows is a short example to showing how one might use the above category.

#import <Foundation/Foundation.h>
#import "NSString+Reverse.h"
int main (int argc, const char * argv[])
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  NSString *str  = [NSString stringWithString:@"Fubar"];
  NSString *rev;
  NSLog(@"String: %@", str);
  rev = [str reverseString];
  NSLog(@"Reversed: %@",rev); 
  [pool drain];
  return 0;

The output of the above example is shown here:

Overriding Methods

Categories can also be used to override methods the class inherits, again, providing an alternative to subclassing. You can always access the overridden method using super . I would assume from an internal (compiler/code generation) perspective, this could lead to less code/overhead as compared to creating a subclass solely to override a method (caveat: I have no proof that is true).

Dividing Source Code into Separate File

Although I haven’t given this a go, categories provide an interesting opportunity to disperse the implementation of a class across one or more files. The Objective-C Programming Guide lists several benefits of this including:

  • Opportunity to group together methods that perform similar tasks.
  • Configuring classes differently for various applications, yet maintaining one set of code.

Naming Conventions

The recommended naming convention for a category is “ClassToAddMethodsTo+CatgoryName” for instance, I used the following filenames for the interface and implementation of the above category code:

  • NSString+Reverse.h
  • NSString+Reverse.m


You cannot add instance variables to a class through a category. Also, category names must be unique across an application.

Source Code

I’ve attached the Xcode project for the above example if you’d like to give it a try.

In the next post I’ll show another example of using categories – the example will show one approach for hiding methods within in a class, as Objective-C does not offer support for private methods.

One comment

Comments are closed on this post. You can comment on this same post which has been moved to the iPhone Developer Tips blog: http://iphonedevelopertips.com/objective-c/categories.html

by john on Aug 16, 2008. #