Objective-C Exceptions

Question

I have just completed an iPhone app programming course. As part of the course, I saw

• Objective-C provides exception handling using the @try directive
• The system library does not use exception handling, preferring to return nil

I asked if I should use exception handling for new code I wrote (e.g. if I write both the front-end and logic code, so the communication between them is in my hands) but I was told no, one should not use exceptions for new code. (But he failed to elaborate, then the class moved on, I thought perhaps the reason would become clear later..)

Surely exceptions are superior to return nil? You can catch particular types, you are not tempted to ignore them by ignoring the return type of a function which normally succeeds, you have text messages which can be logged, they allow your code to focus on the normal case and thus be more readable. Why to use exceptions.

So what do you think? Was my trainer right not to use Objective-C exceptions? If so, why?

Answer 1

It is unsafe to throw exceptions in circumstances where resources are not automatically managed. This is the case of the Cocoa framework (and neighbor frameworks), as they use manual reference counting.

If you throw an exception, any release call you skip over by unwinding the stack will result in a leak. This should limit you tothrowing only if you're certain that you're not going to recover since all resources are returned to the OS when a process quits.

Unfortunately, NSRunLoops tend to catch all exceptions that propagate to them, so if you throw during an event, you'll resume to the next event. This is, obviously, very bad. Therefore, it's better that you simply don't throw.

This problem is diminished if you use garbage-collected Objective-C, as any resource represented by an Objective-C object will be properly released. However, C resources (such as file descriptors or malloc-allocated memory) that are not wrapped in an Objective-C object will still leak.

So, all in all, don't throw.

The Cocoa API has several workarounds to this, as you mentioned. Returning nil and the NSError** pattern are two of them.

Clarifications for ARC

ARC users can choose to enable or disable full exception safety. When exception safety is enabled, ARC will generate code to release strong references when their scope is killed, making it safe to use exception in your code. ARC will not patch external libraries to enable exception support in them, so you should be careful where you throw (and especially where you catch), even with exception support enabled in your program.

ARC exception support can be enabled with -fobjc-arc-exceptions or disabled with -fno-objc-arc-exceptions. By default, it is disabled in Objective-C but enabled in Objective-C++.

Full exception safety in Objective-C is disabled by default because the Clang authors assume that Objective-C programs will not recover from an exception anyways, and because there is a large code size cost and a small performance penalty associated to that cleanup. In Objective-C++, on the other hand, C++ already introduces a lot of cleanup code, and people are much more likely to actually need exception-safety.

This is all from the ARC specification on the LLVM website.

Answer 2

In Cocoa and iOS programming, exceptions are used to indicate non-recoverable programmer error. When an exception is thrown by the frameworks, it indicates that the frameworks have detected an error state that is both not recoverable and for which the internal state is now undefined.

As well, exceptions thrown through framework code leave the frameworks in an undefined state. I.e. you can't do something like:

void a() {     @throw [MyException exceptionWithName: @"OhNoes!"  ....]; }  @try {     ... call into framework code that calls a() above ... } @catch (MyException e) {     ... handle e ... } 

Bottom line:

Exceptions are not to be used in Cocoa for flow control, user input validation, data validity detection or to otherwise indicate recoverable errors. For that, you use the NSError** pattern as documented here (thanks Abizem).

(Note that there is a small number of API that does violate this -- where an exception is used to indicate a recoverable state. Bugs have been filed against these to deprecate and eventually remove these inconsistencies.)

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Finally found the document I was looking for:

Important: You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access, attempts to mutate immutable objects, sending an invalid message, and losing the connection to the window server. You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime.

If you have an existing body of code (such as third-party library) that uses exceptions to handle error conditions, you may use the code as-is in your Cocoa application. But you should ensure that any expected runtime exceptions do not escape from these subsystems and end up in the caller’s code. For example, a parsing library might use exceptions internally to indicate problems and enable a quick exit from a parsing state that could be deeply recursive; however, you should take care to catch such exceptions at the top level of the library and translate them into an appropriate return code or state.