Tips for debugging EXC_BAD_ACCESS

[Michel Fortin]

On 2012-10-14 19:38:21 +0000, Jacob Carlborg <doob at me.com> said:

> On 2012-10-11 20:19, Michel Fortin wrote:
> 
>> Most likely, the object objc_msgSend is called on has been deallocated,
>> which would mean that windowSendEvent is called with a deallocated
>> NSWindow object as its first argument.
> 
> I found the problem now, it's really embarrassing. I had missed passing 
> the argument of any super call taking one argument:
> 
> https://github.com/d-widget-toolkit/dwt-mac/commit/d6674c1074e8a58600cb5052a79b784ae0d3b366

By 
> 
the way, that line is half-fishy:

	super_struct.super_class = cast(objc.Class) OS.objc_msgSend(id, OS.sel_superclass);

It'll 

work as long as you have only one level of derived classes, and only as 
long as you don't have a class that overrides the superclass method 
(which would be weird, I acknowledge). You should be aware of this if 
you're creating new object classes, especially the first part (the 
second part is only relevant if you wish to implement some kind of 
proxy objects).

Theoretically, I think it'd be better to use directly functions from 
the Objective-C runtime[1], which also avoids the dynamic dispatch 
overhead of objc_msgSend:

	super_struct.super_class = class_getSuperclass(object_getClass(id));

Note however that this is still not equivalent to calling:

	[super method:arg0];

because this last one gets the class pointer at compile-tome from its 
static symbol, so there's no overhead at all (and it works with derived 
classes of derived classes too).

[1]: 
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html

-- 


Michel Fortin
michel.fortin at michelf.ca
http://michelf.ca/