Use of asterisk in variable names - iphone

Question

The book "iPhone Programming. The Big Nerd Ranch Guide" cites the following method (page 96)
(void)mapView:(MKMapView *)mv didAddAnnotationViews:(NSArray *) views {
MKAnnotationView *annotationView = [views objectAtIndex:0];
id <MKAnnotation> mp = [annotationView annotation];
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance([mp coordinate], 250, 250);
[mv setRegion:region animated:YES];
}
I'm confused because of the asterisk usage. The line that begins with "MKAnnotationView" and the following one can be represented in an abstract fashion by:
ObjectType variableName = [object message];
Questions:
In the first case an asterisk precedes the variable name, but not in the second. Why?
In the case where the asterisk is used, should not be the pointer the assigned to nil?
Thanks.

Answer 1

I tend to think of it as what variable types require an asterisk, not what variable names require an asterisk. Objective C doesn't allow you to allocate objects on the stack like so:
// Declare an NSObject. Won't work.
NSObject myObject;
Instead, all objects must be dynamically allocated on the heap using pointers like so:
// Declare a pointer to an NSObject. Will work.
NSObject* myObject = [[NSObject alloc] init];
id is a special Objective C keyword that just means "A pointer to some Objective C object". This may or may not inherit from NSObject and is dynamically typed. What's important to note is that, while there is no asterisk, this is still a pointer to an object:
// Same as before. Will work.
id myObject = [[NSObject alloc] init];
The only difference is that the compiler has no information about what myObject is.
As a finishing note, id <MKAnnotation> is exactly the same as a regular id, but with some extra information for the compiler. Read it as "a pointer to some Objective C object that behaves like an MKAnnotation". MKAnnotation, in this case, is the name of a Protocol whose required methods you are declaring that particular id to implement.

Answer 2

id is already defined as a pointer to a struct. If you look at its definition in objc.h, you would that id is defined as,
typedef struct objc_object {
Class isa;
} *id;
Since it is already a pointer to an objc_object, you can create pointers to objects without using the asterisk as,
id myObject;
Also saying that an object is type id gives the compiler absolutely no information about the object except its class which comes from the isa property.
An NSObject on the other hand is defined as,
#interface NSObject <NSObject> {
Class isa;
}
To create a pointer to an object of NSObject or one of its subclass (such as MKAnnotationView), you would declare it as,
NSObject *myObject;
MKAnnotationView *myObject;
We are putting the asterisk here to denote that it is a pointer.
Specifying the protocol(s) next to the type gives the compiler more information for static-type checking.
You should check out this article for a brief introduction to the differences between id and NSObject. For an in-depth understanding, checkout this article on the Objective-C runtime.

Answer 3

ObjectType is normally something like "pointer to a MKAnnotationView", which is represented in Objective-C as it is in C: "MKAnnotationView *". Exceptions include the "id" type, various integer and floating point types (including their typedefs), enums (which are really integer types), and some small structs like CGRect.