Example
id is the generic object pointer, an Objective-C type representing "any object". An instance of any Objective-C class can be stored in an id variable. An id and any other class type can be assigned back and forth without casting:
id anonymousSurname = @"Doe";
NSString * surname = anonymousSurname;
id anonymousFullName = [NSString stringWithFormat:@"%@, John", surname];
This becomes relevant when retrieving objects from a collection. The return types of methods like objectAtIndex: are id for exactly this reason.
DataRecord * record = [records objectAtIndex:anIndex];
It also means that a method or function parameter typed as id can accept any object.
When an object is typed as id, any known message can be passed to it: method dispatch does not depend on the compile-time type.
NSString * extinctBirdMaybe =
[anonymousSurname stringByAppendingString:anonymousSurname];
A message that the object does not actually respond to will still cause an exception at runtime, of course.
NSDate * nope = [anonymousSurname addTimeInterval:10];
// Raises "Does not respond to selector" exception
Guarding against exception.
NSDate * nope;
if([anonymousSurname isKindOfClass:[NSDate class]]){
nope = [anonymousSurname addTimeInterval:10];
}
The id type is defined in objc.h
typedef struct objc_object {
Class isa;
} *id;
