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I am (trying to) learn Objective-C and I keep coming across a phrase like:

-(id) init;

And I understand id is an Objective C language keyword, but what does it mean to say "the compiler specifically treats id in terms of the pointer type conversion rules"?

Does id automatically designate the object to its right as a pointer?

jscs
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GPP
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    It's the opposite of the ego. For some people that's `void*` – Hot Licks Nov 02 '11 at 20:46
  • Here is a discussion relevant to the question: [SO question](http://stackoverflow.com/questions/8972221/would-it-be-beneficial-to-begin-using-instancetype-instead-of-id/14652187#14652187) – dmitri Jul 21 '15 at 04:12
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    Maybe you're here because [nobody told you about iD](https://doomwiki.org/wiki/Doom_music) – jrh Feb 10 '21 at 21:59

5 Answers5

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id is a pointer to any type, but unlike void * it always points to an Objective-C object. For example, you can add anything of type id to an NSArray, but those objects must respond to retain and release.

The compiler is totally happy for you to implicitly cast any object to id, and for you to cast id to any object. This is unlike any other implicit casting in Objective-C, and is the basis for most container types in Cocoa.

joerick
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    Supplement: `id `'s definition is `typedef struct objc_object *id;` , so it pointer to a `Objc` object. – zhongwuzw Jan 09 '18 at 14:56
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    is it the same as `NSObject`? means, can i replace `id` with `NSObject` when it's a return type? – ThunderWiring Jun 15 '18 at 18:50
  • Apple doc: https://developer.apple.com/documentation/objectivec/id?language=objc – Vraj Pandya May 25 '20 at 01:42
  • @ThunderWiring - A precise answer would be NO, since you could assign an NSProxy to a variable typed as an id. (NSProxy does not descend from NSObject.) – Mario Jun 04 '21 at 17:03
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id is a pointer to any Objective-C object (objc_object). It is not just a void pointer and you should not treat it as so. It references an object that should have a valid isa pointer. The values that can be stored in id are also not just limited to NSObject and its descendants, which starts to make sense of the existence of the NSObject protocol as well as the NSProxy class which does not even inherit from NSObject. The compiler will allow you to assign an object referenced by type id to any object type, assign any object type to id, as well as send it any message (that the compiler has seen) without warning.

Joe
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31

id is a generic type. This means that the compiler will expect any object type there, and will not enforce restrictions. It can be useful if you're expecting to use more than one class of objects there; you can then use introspection to find out which class it is. id automatically assumes a pointer, as all objects in Objective-C are passed as pointers/references.

Some Additional Resources:
id vs NSObject vs id*
Objective-C Programming (Wikibooks)
Introspection
Dynamic Typing

FeifanZ
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    The first blog link is now dead, but can be found at [archive.org](https://web.archive.org/web/20110724104417/http://unixjunkie.blogspot.com/2008/03/id-vs-nsobject-vs-id.html), the last two links now seem to redirect to a suspicious looking site, please replace them with archive.org links: [Introspection](https://web.archive.org/web/20120622135114/http://cupsofcocoa.com/2011/02/13/objects-part-5-introspection/), [Dynamic Typing](https://web.archive.org/web/20120706231341/http://cupsofcocoa.com/2011/02/20/objects-part-6-dynamic-typing/) – jrh Jan 27 '21 at 22:14
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    Last two links were to my old blog … I used to have the domain for it Just updated the links to their current Wordpress subdomain – FeifanZ Jan 29 '21 at 03:13
  • Thanks for the update, can you edit the first link as well? It's still dead. An archive.org link is better than nothing, I guess. – jrh Jan 29 '21 at 03:17
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  • id is a data type of object identifiers in Objective-C, which can be use for an object of any type no matter what class does it have. id is the final super type of all objects.

In java or c# we use like this

 Object data = someValue;


 String name =(Object)data;

but in objective c

id data= someValue;



NSString *name= data;
chancyWu
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Yes and no. It's true that having id x designates x as a pointer, but saying that the pointer type conversion rules apply is wrong, because "id" has special type conversion rules. For example, with a void * pointer you can't do this:

void *x;
char *y = x; // error, this needs an explicit cast

On the contrary, it's possible with id:

id x;
NSString *y = x;

See more usage of type id in objective c examples.

In addition in the "modern" Objective C it's preferred to use instancetype instead of "id" on "init" methods. There's even an automatic conversion tool in Xcode for changing that. Read about instancetype: Would it be beneficial to begin using instancetype instead of id?

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battlmonstr
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