Comparing Array References to Nil

· Feb 23, 01:57 PM

The inability to compare an array reference to Nil is a longstanding REALbasic annoyance. It is now possible to do so in REALbasic 2008r1, albeit obliquely.

Update: As of REALbasic 2008 r4, you can compare array references to Nil using any of the operators =, <>, Is.

In REALbasic 2008r1, the Variant class now supports storage of array references. So to compare an array reference to Nil, assign the reference to a Variant and test it.

dim s() as String = SomeFunctionThatReturnsAnArray
 dim v as Variant = s
 if v = nil then
   //s is nil
 else
   //s is not nil
 end if

You can eliminate the code duplication to follow with a global method IsNil.

Function IsNil(v as Variant) as Boolean
   return v.IsNull
 End Function

It appears that Variants are always initialized, so this code will not result in a NilObjectException when v is nil.

Now you can write code like the following.

dim s() as String = SomeFunctionThatReturnsAnArray
 if IsNil(s) then
   //s is nil
 else
   //s is not nil
 end if

One needs to be able to compare array references to nil because array references are mutable. 

dim s() as String = SomeFunctionThatReturnsAnArray

Firsthand experience suggests that it’s possible that the implementor of SomeFunctionThatReturnsAnArray can and will forget to return an array reference, resulting in s being set to nil. Subsequent operations involving s may result in a NilObjectException.

if UBound(s) > -1 then

or an application crash

dim foo as String = s(0).

There is a feedback request for comparison of array references to nil. Until that request is implemented, the IsNil function above might save you some crash-hunting.

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Comment

  1. Nice function and thanks in particular for pointing out that people forgetting to return an array can create nil array references. I had forgotten that was possible in RB and so was not as bothered about the inability to compare arrays to nil.

    — Andy Dent · Mar 6, 11:15 AM · #

  2. Thanks. And now I’ve fixed the code formatting in the article.

    — charles · Mar 6, 11:29 AM · #

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