Program 9.2

[wellaged37]

Re: The id data type and dataValue.

1. Is it correct to say that dataValue is not a pointer?
2. Is it correct to say that dataValue is the memory location of the object?
3. Is it correct to say that the runtime system accesses the memory location of the object to determine its class?

[skochan]

1. Is it correct to say that dataValue is not a pointer?  No, it is a pointer.
2. Is it correct to say that dataValue is the memory location of the object?  Yes -- which is what makes it a pointer
3. Is it correct to say that the runtime system accesses the memory location of the object to determine its class? Yes.

Steve

[wellaged37]

Since it is a pointer then why doesn't dataValue (or id) have an asterisk?
Doesn't a pointer have its own memory address and the value it holds at that memory address is the address of an object?  I know you made a point of the fact, in the book, that dataValue doesn't have an asterisk but I wondered why it doesn't.

[skochan]

Since it is a pointer then why doesn't dataValue (or id) have an asterisk?
Doesn't a pointer have its own memory address and the value it holds at that memory address is the address of an object?  I know you made a point of the fact, in the book, that dataValue doesn't have an asterisk but I wondered why it doesn't.

I was afraid you would ask that question!   

The asterisk is "hidden" by the way id is defined.  It's actually a typedef (described in Chapter 10--see the example of NumberObject on page 219) that looks like this:

Code: (Objective-C)

typedef struct objc_object { 
    Class isa; 
} *id; 

Without getting into too much detail, id is defined here as a pointer to an objc_object structure (described in Chapter 12).   Let's leave it at that for now!   

Cheers,

Steve Kochan

[wellaged37]

Thanks Steve. I will leave it at that.
John