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Alternatives to Exceptions
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Some Objective-C APIs, and many programmers, adhere to the traditional C error-handling pattern; the return value of a function or method is tested to determine success or failure. Some prototypical examples are shown in Listing 14-9. The programming philosophy at work here is that exceptions should be reserved for runtime errors (index out of bounds, invalid object, missing application resource, out of memory) and other programming mistakes that should ideally be eliminated from the application during development. Anticipatable failures that could reasonably be expected to occur (file not found, empty database, duplicate name) should be handled using error codes or error objects. This section describes the most commonly used techniques for dealing with errors without using exceptions, and later explains how to combine the two.
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Listin g 14-9. Examples of Error Handling
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// Simple Error NSString *imagePath = [[NSBundle mainBundle] pathForImageResource:@"picture.png"]; if (imagePath==nil) { NSLog(@"missing image resource"); return; } // POSIX Error int fd = open("filename",O_RDONLY); if (fd<0) { NSLog(@"open() failed with error %d",errno); return; } // Core Foundation Error QTUUID quickTimeUUID; OSErr err = QTCreateUUID(&quickTimeUUID,0); if (err!=noErr) { NSLog(@"could not create UUID, error %d",err); return; } // Cocoa Error NSError *error = nil; NSDictionary *attributes; attributes = [[NSFileManager defaultManager] attributesOfItemAtPath:@"filename"
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CHAPTER 14 EXCEPTION HANDLING
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error:&error]; if (error!=nil) { NSLog(@"could not get attributes of file: %@",[error localizedDescription]); [self presentError:error]; return; }
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Simple Errors
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Simple errors occur when any function or method fails to achieve its objective and returns an empty value. The -pathForImageResource: message in Listing 14-9 is a good example. This method returns the path of the desired image file, or nil if no such file could be located. Your code should probably test the results unless you ve designed your code to use absent behavior, described in 7 and either provide a default, return an NSError, throw an exception, or raise an assertion.
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POSIX Error Codes
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Most BSD and POSIX functions return a value that will indicate the success or failure of the function. In Listing 14-9, the open( ) function returns a file descriptor integer if successful, or a negative value if it fails. The code that describes the reason for the failure is read from the per-thread errno variable. Note that errno isn t actually a variable, but a preprocessor macro that expands to a function call that obtains the error code. This variable is only guaranteed to contain the error code until the next POSIX function is called, so fetch the code and save it immediately after the failure.
Core Foundation Error Codes
Many Core Foundation functions return a result code directly to the caller. Result codes have the type OSError or OSStatus, and are usually negative numbers. The success of the function is indicated by the value noErr, as demonstrated in Listing 14-9. Because functions like this return the error code, pointers to variables are used to return any additional values.
Cocoa Errors
Modern Cocoa classes and functions prefer to return NSError objects when the operation could not be completed successfully. Invariably, the sender of the message includes a pointer to a nil NSError pointer variable, as shown in Listing 14-9. If the operation fails, the method creates a new NSError object and stores its address in the sender s variable. The sender can examine its variable to see if the receiver returned an NSError. Use of NSErrors is encouraged whenever the details of the failure will be presented to the user. The NSError class incorporates a number of design features that makes it integral to a well-designed and flexible error handling system. Integrating NSError objects into your application is not a trivial task, but it is highly recommended if your goal is to provide consistent, localizable, modular, and flexible error display and recovery.
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CHAPTER 14 EXCEPTION HANDLING
This book is too short to go into all the details of NSError objects, but the following sections will give you enough of an overview to appreciate their utility. Before designing NSError management in your application, take a good look at the Error Handling Programming Guide For Cocoa. 3
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