Passing an unknown type (?) object to a function as a parameter in objective-c - objective-c

Question

How do I pass an unknown type (?) object to a function and then set it to a local ones value in objective-c? The unknown object is an n-th item of a JSON value. I use the JSON framework.
[giveMeTheNthObj:[items objectAtIndex:indexPath.row]]
...
- (void)giveMeTheNthObj:(id)o {
Classname *local = o;
}
The above fails. I think because the local is a pointer that should point to a Classname type. What should be the type of the local variable?
The error log:
GNU gdb 6.3.50-20050815 (Apple version gdb-1705) (Fri Jul 1 10:50:06 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
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This GDB was configured as "x86_64-apple-darwin".Attaching to process 37438.
2011-09-30 16:39:21.649 Navig[37438:b303] -[UILabel copyWithZone:]: unrecognized
selector sent to instance 0x4b949a0
2011-09-30 16:39:21.652 Navig[37438:b303] *** Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '-[UILabel copyWithZone:]: unrecognized selector
sent to instance 0x4b949a0'
*** Call stack at first throw:
(
0 CoreFoundation 0x00dde5a9 __exceptionPreprocess + 185
1 libobjc.A.dylib 0x00f32313 objc_exception_throw + 44
2 CoreFoundation 0x00de00bb -[NSObject(NSObject) doesNotRecognizeSelector:] + 187
3 CoreFoundation 0x00d4f966 ___forwarding___ + 966
4 CoreFoundation 0x00d4f522 _CF_forwarding_prep_0 + 50
5 CoreFoundation 0x00d3edca -[NSObject(NSObject) copy] + 42
6 UIKit 0x000de18b -[UIViewController setTitle:] 95
7 Foundation 0x007b05e5 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 285
8 UIKit 0x0022c30c -[UIRuntimeOutletConnection connect] + 112
9 CoreFoundation 0x00d548cf -[NSArray makeObjectsPerformSelector:] + 239
10 UIKit 0x0022ad23 -[UINib instantiateWithOwner:options:] + 1041
11 UIKit 0x0022cab7 -[NSBundle(UINSBundleAdditions) loadNibNamed:owner:options:] + 168
12 UIKit 0x000e2628 -[UIViewController _loadViewFromNibNamed:bundle:] + 70
13 UIKit 0x000e0134 -[UIViewController loadView] + 120
14 UIKit 0x000e000e -[UIViewController view] + 56
15 UIKit 0x000de482 -[UIViewController contentScrollView] + 42
16 UIKit 0x000eef25 -[UINavigationController _computeAndApplyScrollContentInsetDeltaForViewController:] + 48
17 UIKit 0x000ed555 -[UINavigationController _layoutViewController:] + 43
18 UIKit 0x000ee7aa -[UINavigationController _startTransition:fromViewController:toViewController:] + 326
19 UIKit 0x000e932a -[UINavigationController _startDeferredTransitionIfNeeded] + 266
20 UIKit 0x000f0562 -[UINavigationController pushViewController:transition:forceImmediate:] + 932
21 UIKit 0x089b9c36 [UINavigationControllerAccessibility(SafeCategory) pushViewController:transition:forceImmediate:] + 77
22 UIKit 0x000e91c4 -[UINavigationController pushViewController:animated:] + 62
23 Navig 0x00002dfd -[RootViewController tableView:didSelectRowAtIndexPath:] + 269
24 UIKit 0x000a7b68 -[UITableView _selectRowAtIndexPath:animated:scrollPosition:notifyDelegate:] + 1140
25 UIKit 0x0009db05 -[UITableView _userSelectRowAtPendingSelectionIndexPath:] + 219
26 Foundation 0x007b779e __NSFireDelayedPerform + 441
27 CoreFoundation 0x00dbf8c3 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 19
28 CoreFoundation 0x00dc0e74 __CFRunLoopDoTimer + 1220
29 CoreFoundation 0x00d1d2c9 __CFRunLoopRun + 1817
30 CoreFoundation 0x00d1c840 CFRunLoopRunSpecific + 208
31 CoreFoundation 0x00d1c761 CFRunLoopRunInMode + 97
32 GraphicsServices 0x010161c4 GSEventRunModal + 217
33 GraphicsServices 0x01016289 GSEventRun + 115
34 UIKit 0x0003ec93 UIApplicationMain + 1160
35 Navig 0x00002439 main + 121
36 Navig 0x000023b5 start + 53
)
terminate called throwing an exceptionsharedlibrary apply-load-rules all
Current language: auto; currently objective-c
(gdb)

Answer 1

When you say "The above fails."; how exactly does it fail? Exception raised at run-time, warning during compile, etc? Does this help:
- (void)giveMeTheNthObj:(id)o {
if ([o isKindOfClass:[Classname class]]) {
Classname *local = (Classname *)o;
}
}

Answer 2

You want NSString *local = NSStringFromClass([o class]);?