ID

In Objective-C, objects are identified by a distinct data type, id. This type is defined as a pointer to an object--in reality, a pointer to the object's data (its instance variables). Like a C function or an array, an object is identified by its address. All objects, regardless of their instance variables or methods, are of type id.

id ObjectName;

The keyword nil is defined as a null object, an id with a value of 0. id, nil, and the other basic types of Objective-C are defined in the header file objc.h, which is located in the objc subdirectory of  NextDeveloper/Headers.