REALbasic Tip: Improving code readability with the IsA operator

[Geoff Perlman <geoff at realsoftware dot com>]

At the time you are writing a line of code, it's probably very clear 
what it's doing. However, weeks or months later, it might not be 
obvious anymore. Commenting your code helps but writing very clear code 
in the first place is even better. The clearer the code is, the less 
commenting is required. This tip will show a technique to make your 
code clearer for one specific case.
Many functions in REALbasic return an object. GetOpenFolderItem is an 
example of such a function. It returns a folderitem that represents the 
file selected by the user in the Open File dialog box. If the user 
clicks the Cancel button, GetOpenFolderItem returns NIL. Most of the 
time you'll need to make sure that what was returned is not nil before 
proceeding. The following code shows an example of this:
 Dim f as folderitem
  f = GetOpenFolderItem("????")
  if f <> nil then
    'continue processing
  end if

Testing the value returned to determine if it's not nil works but it's 
a double-negative. What you really want to know is whether or not 
you've got a valid object. You can test for this using the IsA operator 
and make your code easier to read at the same time. Here's an example 
of the same code using the IsA operator:
  Dim f as folderitem
  f = GetOpenFolderItem("????")
  if f IsA object then
    'continue processing
  end if

This works in all versions except v5.0. If you have v5.0, download the 
latest release (v5.2.1) from our web site.
--
Geoff Perlman
President and CEO
REAL Software, Inc.
512-328-7325 x711 (voice)
512-328-7372 (fax)


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