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Object-Oriented Thinking
Avoiding NilObjectExceptions
Issue: 4.5 (May/June 2006)
Author: Charles Yeomans
Article Description: No description available.
Article Length (in bytes): 5,026
Starting Page Number: 37
RBD Number: 4513
Resource File(s): None
Related Web Link(s):
http://www.declareSub.com/
Known Limitations: None
Excerpt of article text...
NilObjectExceptions are among the most common errors in REALbasic programming. In this issue's column, we take a crack at preventing them.
When you call a method or access a property of an object via a reference, REALbasic first checks to see if the reference is nil, which means that the reference does not refer to a valid object. Attempting to call a method or access a property using a nil reference is an error. Because REALbasic does not know how to handle this error in all situations, it raises a NilObjectException to give the caller a chance to handle the error. If the exception is not handled, the runtime displays a dialog and quits. This, by the way, is not a crash. A crash is what would happen if the runtime attempted to use the nil reference.
The rule for checking object references for nil is this: did you initialize the reference yourself using the New operator? If not, then you need to check the reference before using it, or be prepared to handle a NilObjectException. Note that functions like NewAppleEvent require checking. I note that I am still not being as careful as possible. Ideally, the New operator would either succeed or raise an exception, and this behavior would be documented.
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Article copyrighted by REALbasic Developer magazine. All rights reserved.
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