Previously we learn how to add methods from outside the class : Objective-C : Categories

Objective-C permits a class to wholly replace another class within a program. The replacing class is said to "pose as" the target class.

Note: Class posing was declared deprecated with Mac OS X v10.5, and is unavailable in the 64-bit runtime.

For the versions still supporting posing, all messages sent to the target class are instead received by the posing class.

There are several restrictions 

• A class may only pose as one of its direct or indirect superclasses.

• The posing class must not define any new instance variables that are absent from the target class (though it may define or override methods).

• The target class may not have received any messages prior to the posing.

• Posing, similarly with categories, allows global augmentation of existing classes. Posing permits two features absent from categories:

• A posing class can call overridden methods through super, thus incorporating the implementation of the target class.

• A posing class can override methods defined in categories

For example,

@interface CustomNSApplication : NSApplication
@end
 
@implementation CustomNSApplication
- (void) setMainMenu: (NSMenu*) menu
{
     // do something with menu
}
@end
 
class_poseAs ([CustomNSApplication class], [NSApplication class]);

This intercepts every invocation of setMainMenu to NSApplication.