Category names should begin with the project’s three letter prefix followed by a descriptive name of functionality the extension provides. Avoid generic category names such as “Extensions, “Utilities”, or “Additions”.

Files extending the functionality of existing classes, whether through categories or alternative designs, should be named with the name of the class being extended followed by “+”, followed by the name of the category.

Methods in categories, including those declared as properties, should be prefixed with the three letter prefix and an underscore.


Categories can modify existing classes from other frameworks and libraries. Using the three letter prefix of the framework or library defining the category helps provide a unique category name and quickly indicates which library added the functionality when viewing the header to symbols in a backtrace.

Avoid generic category names for better code organization and to improve discoverability from header file names. Good names prevent a category from becoming a dumping of unrelated functionality.

Methods must be prefixed in order to help avoid potential naming clashes.

See Apple’s guidelines for more detail and rationale on declaring categories.


// StringUtilities.h 
// bad: doesn't use the full name of the original class, doesn't have a "+", no three letter prefix, and an overly-generic category name.

// NSString+XYZExcitementAdditions.h 
// good: follows the pattern of (original class)+(descriptive category name with prefix).h

@interface NSString (Utilities) // bad: no three-letter prefix present, not a descriptive category name
- (NSString *)stringWithExcitement; // bad: three letter prefix reduces likelihood of name collision
@property (nonatomic, copy) NSNumber *excitementLevel; // bad: no prefix to property name

@interface NSString (XYZExcitementAdditions) // good: three-letter prefix present, descriptive category name
- (NSString *)xyz_stringWithExcitement; // good: potential name collisions
@property (nonatomic, copy, setter=xyz_setExcitementLevel:) NSNumber *xyz_excitementLevel; // good: property name prefixed, setter name maintains prefix