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Issue 4.5

COLUMN

Avoiding NilObjectExceptions

Issue: 4.5 (May/June 2006)
Author: Charles Yeomans
Author Bio: Charles is the author of "I Declare: Calling External Functions in REALbasic", available online at http://www.declareSub.com/
Article Description: No description available.
Article Length (in bytes): 6,208
Starting Page Number: 37
Article Number: 4513
Related Web Link(s):

http://www.declareSub.com/

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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