Re: Unable to access an instance properties and methods

FROM : Randall Meadows
DATE : Wed Mar 19 19:34:35 2008

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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