Passing an unknown type (?) object to a function as a parameter in 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]); ?