mikeash.com pyblog/friday-qa-2010-11-6-creating-classes-at-runtime-in-objective-c.html comments

ajay babu - 2015-04-07 11:23:56

Tue, 07 Apr 2015 11:23:56 GMT

how to call the description method on the class instance

Preston - 2012-06-08 00:56:38

Fri, 08 Jun 2012 00:56:38 GMT

You can use NSGetSizeAndAlignment() to get those values when adding an ivar.

Whisper - 2012-01-04 13:46:21

Wed, 04 Jan 2012 13:46:21 GMT

Great article, thanks.

A little typo there near the end of this line:

object_getInstanceVariable(obj, "foo", &currentValue;);

Kevin L - 2011-10-25 16:49:19

Tue, 25 Oct 2011 16:49:19 GMT

Mike, sorry about the double post about ARC, didn't see your response until after I posted.

Kevin L - 2011-10-25 16:47:53

Tue, 25 Oct 2011 16:47:53 GMT

Apparently it is a bug that objc_allocateClassPair doesn't work in iOS 5 with ARC enabled. The workaround is to use the compiler flag -fno-objc-arc on the file using the objc_allocateClassPair method and it will work fine.

mikeash - 2011-10-25 14:46:25

Tue, 25 Oct 2011 14:46:25 GMT

Mr. Wight was using ARC, and I assume you are as well. It's not currently possible to safely call objc_allocateClassPair with ARC. Move your relevant code into a separate file and disable ARC for that file, and all should be well.

Kevin L - 2011-10-25 14:41:34

Tue, 25 Oct 2011 14:41:34 GMT

I'm running into the same issue with objc_allocateClassPair on both the simulator and device. Is there an alternative to using objc_allocateClassPair or a way of getting it working?

Jonathan Wight - 2011-10-18 22:23:31

Tue, 18 Oct 2011 22:23:31 GMT

As of iOS 5, on the simulator Class mySubclass = objc_allocateClassPair([NSObject class], "MySubclass", 0); will cause an EXC_BAD_ACCESS exception...

mikeash - 2010-11-26 22:03:03

Fri, 26 Nov 2010 22:03:03 GMT

It's best to ignore the formal definition of IMP and just think of it as being a generic method implementation pointer, kind of like a void * for methods. Just like you can't access the contents of a void * without casting it to a more specific type, so you shouldn't use an IMP without casting it to a more specific type.

To put it another way, compatibility with the declared type of IMP doesn't matter. What does matter is compatibility with the code that will be generated at the call site to the method, which is determined by the method declaration that you're trying to match.

Ling - 2010-11-26 00:58:15

Fri, 26 Nov 2010 00:58:15 GMT

One thing I'm curious about:
The return type of CountOfObject is NSUInteger which is not compatible with id, then why could it be safely cast to IMP?

Kyle S - 2010-11-06 22:40:30

Sat, 06 Nov 2010 22:40:30 GMT

Jason, that sounds like an excellent feature request to make for clang/LLVM.

mikeash - 2010-11-06 18:05:44

Sat, 06 Nov 2010 18:05:44 GMT

Jason: Indeed, I have rued and lamented that many times.

Jason - 2010-11-06 16:15:26

Sat, 06 Nov 2010 16:15:26 GMT

It's a shame the compiler doesn't emit @encode(...) as literal strings, so you could just say:

const char * x = @encode(NSUInteger) @encode(id) @encode(SEL) @encode(id);

Great article, and thanks for MAObjCRuntime!

Brandon Walkin - 2010-11-06 10:24:35

Sat, 06 Nov 2010 10:24:35 GMT

Great article. Looking forward to part two.

Nick - 2010-11-05 20:58:12

Fri, 05 Nov 2010 20:58:12 GMT

Another really interesting article, thanks :-)

mikeash - 2010-11-05 18:04:11

Fri, 05 Nov 2010 18:04:11 GMT

Good catch, and fixed.

Jesper - 2010-11-05 17:36:31

Fri, 05 Nov 2010 17:36:31 GMT

Nice article. The objc_allocateClassPair example should use a C string, not an NSString.