Re: Unable to access an instance properties and methods
Re: Unable to access an instance properties and methods
- Subject: Re: Unable to access an instance properties and methods
- From: Randall Meadows <email@hidden>
- Date: Wed, 19 Mar 2008 12:34:35 -0600
On Mar 19, 2008, at 12:23 PM, Jens Alfke wrote:
Here's some advice on how to debug stuff like this in the future...
The exception message is a good clue:
2008-03-19 17:19:30.957 cocoabc[10832:10b] *** -[NSCFString
accident]: unrecognized selector sent to instance 0x33b040
This shows that the receiver of -accident was an NSCFString object.
I've gotten some messages that just gave an address, not a class
name. I delightfully discovered that sending -class to the raw
address was just as valid and useful. This may be a "duh" moment for
some, but eye-opening to me, and has come in quite handy recently.
The way I would track something like this down is, first, to set a
symbolic breakpoint on objc_exception_throw (via Xcode's breakpoints
window.) This is always a good thing to have. Then run the app in
the debugger and it'll hit the breakpoint when the exception's raised.
I 2nd this recommendation.
Now select the topmost application stack frame — it'll be on the
line that calls [firstNote accident]. Then you can enter "po
firstNote" in the debugger console to see the object's description.
But as you pointed out, it looks like the description of a DBNNote
object even though it's a string, so you can't really tell from
that. A more surefire test is to enter "po [firstNote class]", which
will print "NSCFString" instead of "DBNNote". That's your smoking gun.
The "po" and "p" commands are incredibly useful debugging tools,
since they let you make nearly any Objective-C call interactively on
your running code. Gdb understands a surprisingly large subset of
Objective-C syntax.
'po $eax' has become my friend lately as well. It displays the
register that contains the exception object that's already been
thrown, after you've already crashed._______________________________________________
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