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6. Error Handling

Initialisation methods (e.g. -init) should, upon failure to initialise the class, release itself and return nil. This may mean in certain cases, that it should catch exceptions, since the calling method will be expecting a nil object rather than an exception on failure. However, init methods should endeavour to provide some information, via NSLog, on the failure.

All other methods should cause an exception on failure*, unless returning nil is a valid response (e.g. [dictionary objectForKey: nil]) or if documented otherwise.

Failure here is a relative term. I’d interpret failure to occur when either system resources have been exceeded, an operation was performed on invalid data, or a required precondition was not met. On the other hand, passing a nil object as a parameter (as in [(NSMutableData *)data appendData: nil]), or other "unusual" requests should succeed in a reasonable manner (or return nil, if appropriate) and/or reasonable default values could be used.

If an error is recoverable or it does not damage the internal state of an object, it’s ok not to raise an error. At the very least, though, a message should be printed through NSLog.

Special care should be taken in methods that create resources like allocate memory or open files or obtain general system resources (locks, shared memory etc.) from the kernel. If an exception is generated between the allocation of the resource and its disposal, the resource will be simply lost without any possibility to release. The code should check for exceptions and if something bad occurs it should release all the allocated resources and re-raise the exception.

Unfortunately there is no nice way to do this automatically in OpenStep. Java has the "finally" block which is specifically designed for this task. A similar mechanism exists in libFoundation with the CLEANUP and FINALLY blocks.

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