Objective-C Classes Are also Objects

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/DefiningClasses/DefiningClasses.html#//apple_ref/doc/uid/TP40011210-CH3-SW18

In Objective-C, a class is itself an object with an opaque type called

Class

. Classes can’t have properties defined using the declaration syntax shown earlier for instances, but they can receive messages.

The typical use for a class method is as a factory method, which is an alternative to the object allocation and initialization procedure described in Objects Are Created Dynamically. The

NSString

class, for example, has a variety of factory methods available to create either an empty string object, or a string object initialized with specific characters, including:

+ (id)string;

+ (id)stringWithString:(NSString *)aString;

+ (id)stringWithFormat:(NSString *)format, …;

+ (id)stringWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError **)error;

+ (id)stringWithCString:(const char *)cString encoding:(NSStringEncoding)enc;

As shown in these examples, class methods are denoted by the use of a

+

sign, which differentiates them from instance methods using a

-

sign.

Class method prototypes may be included in a class interface, just like instance method prototypes. Class methods are implemented in the same way as instance methods, inside the

@implementation

block for the class.