Comments on: UIButton troubles, and Obj-C magic

By: luke

luke — Tue, 01 Feb 2011 14:39:36 +0000

Frank, Great work.

I’m also subclassing UIButton and adding an instance variable like Ariel.

Any experience of issues?

By: Ariel

Ariel — Fri, 04 Sep 2009 04:24:40 +0000

Frank, I need subclassing UIButton but also I need add some instance variable. Obviusly the sizes of classes are diferent.
I test it, and aparently run ok.

It’s OK or maybe I’ll have problems in the future, with more test?

By: bchamp

bchamp — Mon, 10 Aug 2009 00:32:03 +0000

Frank, when I tried this with XCode 3.1.2 and iPhone SDK 2.2.1, it reported that \instance variable isa is declared protected\ and refused to compile. What am I missing?

By: guyal

guyal — Wed, 31 Dec 2008 06:45:18 +0000

Right. Thanks.

By: Fredrik Olsson

Fredrik Olsson — Tue, 30 Dec 2008 21:51:42 +0000

The instance size of a Objective-C object is always the total size of it’s instance variables, including the instance variables of the super class, and nothing else.

This means that a subclass can only increase the instance size, never decrease it. And if a subclass do not introduce any instance variables, then it’s instance size is identical to the superclass.

The class name, class- and instance-methods do not change the instance size; they will change the size of the class. This is not that important to know unless you fiddle with automatic proxy generation, or mock objects. But more info is in the “Objective-C 2.0 Runtime Reference” in the developer documentation.

By: guyal

guyal — Tue, 30 Dec 2008 17:34:37 +0000

Very neat, though how is it that the two classes are the same bytewise size? Are the only size differences between your implementation and Cocoa’s superclass things like having “-32″ instead of “0″, and both are treated as a signed int for sizing purposes. As in, depending on how you calculate “size”, the fact that the subclass has a different name and superclass declaration would alone guarantee different sizes…