Question

id
In Objective-C, object identifiers are of a distinct data type: id. This type is the general type for any kind of object regardless of class and can be used for instances of a class and for class objects themselves.

id anObject;

Till here its simple and clear

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For the object-oriented constructs of Objective-C, such as method return values, id replaces int as the default data type. (For strictly C constructs, such as function return values, int remains the default type.)

didn’t understand what its talking about id replaces int? and in last inside brackets int remains the default value?

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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/objc.h.

id is defined as pointer to an object data structure:

typedef struct objc_object {
    Class isa;
} *id;

the above is not a obj-c syntax what is it and what it is explaining?

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Every object thus has an isa variable that tells it of what class it is an instance. Since the Class type is itself defined as a pointer:

typedef struct objc_class *Class;

the isa variable is frequently referred to as the “isa pointer.”

also didn’t understand this last para.

Answer 1

For the object-oriented constructs of Objective-C, such as method return values, id replaces int as the default data type. (For strictly C constructs, such as function return values, int remains the default type.)

Your book’s just being a bit confusing here. All it’s saying is that if you’re working with objects and you don’t want to get more specific, id is a good data type to use.

id is defined as pointer to an object data structure:

typedef struct objc_object {
    Class isa;
} *id;

the above is not a obj-c syntax what is it and what it is explaining?

That’s the internal representation of an object. Your book is trying to show that an objective C object isn’t magic: under the surface it’s just a struct. The data type id is a pointer to such a struct.

Every object thus has an isa variable that tells it of what class it is an instance. Since the Class type is itself defined as a pointer:

typedef struct objc_class *Class;

the isa variable is frequently referred to as the “isa pointer.”

In its internal data structure, each object has a pointer to another object that represents its class.

You don’t need to know this stuff if you’re just interested in programming in Objective-C (at least not while you’re learning). It does come in handy if you’re doing some advanced stuff, such as when you’re interacting with the Objective-C runtime directly.