Typically, your key-value coding compliant object relies on the default implementation of key-value coding to automatically wrap and unwrap non-object properties, as described in Representing Non-Object Values. However, you can override the default behavior. The most common reason to do so is to handle attempts to store a nil value on non-object properties.

If your key-value coding compliant object receives a setValue:forKey: message with nil passed as the value for a non-object property, the default implementation has no appropriate, generalized course of action. It therefore sends itself a setNilValueForKey: message, which you can override. The default implementation of setNilValueForKey: raises an NSInvalidArgumentException exception, but you can provide an appropriate, implementation-specific behavior.

For example, the code in Listing 10-1 responds to an attempt to set a person’s age to a nil value by instead setting the age to 0, which is more appropriate for a floating point value. Notice that the override method calls upon its object’s superclass for any keys that it does not explicitly handle.