isa objc_msgSend

he objc_msgSend Function

In Objective-C, messages aren’t bound to method implementations until runtime. The compiler converts a message expression,

[receiver message]

into a call on a messaging function, ​

​objc_msgSend​

​. This function takes the receiver and the name of the method mentioned in the message—that is, the method selector—as its two principal parameters:

objc_msgSend(receiver, selector)

Any arguments passed in the message are also handed to ​

​objc_msgSend​

​:

objc_msgSend(receiver, selector, arg1, arg2, ...)

The messaging function does everything necessary for dynamic binding:

• It first finds the procedure (method implementation) that the selector refers to. Since the same method can be implemented differently by separate classes, the precise procedure that it finds depends on the class of the receiver.
• It then calls the procedure, passing it the receiving object (a pointer to its data), along with any arguments that were specified for the method.
• Finally, it passes on the return value of the procedure as its own return value.

Note: The compiler generates calls to the messaging function. You should never call it directly in the code you write.

The key to messaging lies in the structures that the compiler builds for each class and object. Every class structure includes these two essential elements:

• A pointer to the superclass.
• A class dispatch table. This table has entries that associate method selectors with the class-specific addresses of the methods they identify. The selector for the 

  setOrigin::

   method is associated with the address of (the procedure that implements) 

  setOrigin::

  , the selector for the 

  display

   method is associated with 

  display

  ’s address, and so on.

When a new object is created, memory for it is allocated, and its instance variables are initialized. First among the object’s variables is a pointer to its class structure. This pointer, called ​

​isa​

​, gives the object access to its class and, through the class, to all the classes it inherits from.

Note: While not strictly a part of the language, the ​

​isa​

​ pointer is required for an object to work with the Objective-C runtime system. An object needs to be “equivalent” to a ​

​struct objc_object​

​ (defined in ​

​objc/objc.h​

​) in whatever fields the structure defines. However, you rarely, if ever, need to create your own root object, and objects that inherit from ​

​NSObject​

​ or ​

​NSProxy​

​ automatically have the ​

​isa​

​ variable.

These elements of class and object structure are illustrated in Figure 3-1.

Figure 3-1  Messaging Framework

When a message is sent to an object, the messaging function follows the object’s ​

​isa​

​ pointer to the class structure where it looks up the method selector in the dispatch table. If it can’t find the selector there, ​

​objc_msgSend​

​ follows the pointer to the superclass and tries to find the selector in its dispatch table. Successive failures cause ​

​objc_msgSend​

​ to climb the class hierarchy until it reaches the ​

​NSObject​

​ class. Once it locates the selector, the function calls the method entered in the table and passes it the receiving object’s data structure.

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