What does __forwarding__ in the stack trace mean?

Question

(gdb) bt
#0  0x302ac924 in ___TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION___ ()
#1  0x92077e3b in objc_exception_throw ()
#2  0x302d6ffb in -[NSObject doesNotRecognizeSelector:] ()
#3  0x3026e056 in ___forwarding___ ()
#4  0x3024a0a2 in __forwarding_prep_0___ ()
#5  0x00004ae9 in -[GameObject doesTouch:] (self=0xe893a0, _cmd=0x643ee, obj=0xe82e20) at /Users/aaa/Desktop/CPT/Game/Classes/GameObject.m:220
#6  0x00006e05 in -[StaticGrid checkTouchNearest:] (self=0xe82f20, _cmd=0x64ec3, obj=0xe893a0) at /Users/aaa/Desktop/CPT/Game/Classes/StaticGrid.m:62
#7  0x0000a393 in -[EAGLView touchesBegan:withEvent:] (self=0xe8dad0, _cmd=0x3199fa3c, touches=0x632c0b0, event=0xe14590) at /Users/aaa/Desktop/CPT/Game/Classes/EAGLView.m:459
#8  0x30910f33 in -[UIWindow _sendTouchesForEvent:] ()
#9  0x308faecb in -[UIApplication sendEvent:] ()
#10 0x309013e1 in _UIApplicationHandleEvent ()
#11 0x32046375 in PurpleEventCallback ()
#12 0x30245560 in CFRunLoopRunSpecific ()
#13 0x30244628 in CFRunLoopRunInMode ()
#14 0x32044c31 in GSEventRunModal ()
#15 0x32044cf6 in GSEventRun ()
#16 0x309021ee in UIApplicationMain ()
...

Currently I have a rare occuring error that I do not know the cause yet. I am not sure where to look at, so what I want to ask is what do the first five lines (#0 to #4) mean? I know that it claims that there are some errors, but what are those things like "___forwarding___"?

If you have some knowledge in this, please help. Thank you very much.

Mcdavid

Answer 1

The forwarding stuff is used for uhm… forwarding messages. Each object can easily forward the messages it receives to some other objects, see the excellent tutorial by Scott Stevenson. When your GameObject receives a message it does not understand, it tries to forward it. If there is no forwarding implemented, the doesNotRecognizeSelector method is called and you get the exception.

Detailed description can be found in the Apple documentation for the NSObject class:

When an object is sent a message for which it has no corresponding method, the runtime system gives the receiver an opportunity to delegate the message to another receiver. It delegates the message by creating an NSInvocation object representing the message and sending the receiver a forwardInvocation: message containing this NSInvocation object as the argument. The receiver’s forwardInvocation: method can then choose to forward the message to another object. (…) NSObject’s implementation of forwardInvocation: simply invokes the doesNotRecognizeSelector: method; it doesn’t forward any messages. Thus, if you choose not to implement forwardInvocation:, sending unrecognized messages to objects will raise exceptions.

As for your error, it seems that the GameObject gets sent some message it does not understand. This can be a simple typo or something more subtle like memory management error, you’d have to give us more information.

Reeta

+1 good answer. I would like to add that this is an attribute of Objective-C being a dynamic programming language, where methods are resolved at run-time, not at compile time. This writing code that calls methods that does not exist is not an error per-se, in run-time your application can resolve what to do, very handy for writing proxies. – Ehrman

Answer 2

First thing I would check is whether GameObject has a -doesTouch: or +doesTouch: method. I am not sure what __forwarding... is. What error message do you see in the console log?

Semblance

Doesn't the stack trace indicate that doesTouch is sending the urecognized message, rather than being the unrecognized message? – Refugee

-[GameObject doesTouch:] is indeed sending an unrecognized method. There is no way to tell what it is from the stack trace, but it will be logged when the exception is thrown, so it should be visible in the Xcode console. To get more information, catch the exception or set a breakpoint in objc_exception_throw (Run->Show->Breakpoints->Double-Click for Symbol, type objc_exception_throw, then Run->Debug). – Tinnitus