Objective-C Pocket Reference

Runtime Errors

Runtime errors include program errors like unhandled method calls or messages sent to released objects, and hardware errors like division by zero. The Object root class provides simple error-handling capability; the Cocoa framework implements exception raising and handling.

Object Error Handling

When an error occurs, the runtime sets in motion the following sequence of events:

1. The runtime calls the -error : method on the object whose method generated the error. You can override this method to customize error handling for a particular class.

2. The -error: method prepares information about the receiver and passes it to the runtime C function objc_verror( ) .

3. The objc_verror( ) function calls the runtime error handler function if there is one; otherwise it writes an error message to stderr. You can provide a handler function to customize error handling for all classes.

4. If the error handler exists (because you’ve provided one) and it returns YES, execution continues; otherwise the program calls the C function abort( ) and exits.

The GNU runtime provides a function to set your own error handler function:

objc_error_handler objc_set_error_handler(objc_error_handlerf)

Calling this function sets a new error handler and returns the previous one. The default error handler is a NULL pointer. The required signature of an error handler is declared (in objc-api.h) as:

typedef 
               BOOL (*objc_error_handler)
  (id receiver, 
   int errCode, 
   const 
               char* format, 
   va_list args);

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