CODEWITHSUNDEEP
objective-cios
S.J
S.J

Reputation: 3071

confusion regarding "id" in obj-c programming guide of Apple

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

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?

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?

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.

Upvotes: 0

Views: 363

Answers (2)

Amy Worrall
Amy Worrall

Reputation: 16337

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.

Upvotes: 3

Shaggy Frog
Shaggy Frog

Reputation: 27601

didn't understand what its talking about id replaces int?

Objective-C is a superset of C.

In C, a function must have a return value. If a return value is not specified, then a function returns int by default.

Objective-C overrides this default behaviour of C and returns id as a default value from any class method that doesn't specify its return value. But at the same time leaves the default behaviour for other (C-)functions the same (i.e. int is still the default return value for everything else)

Upvotes: 1

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