This is one of those cases where a long time 4D programmer (me) noticed my long time approach to coding needed a refresh to take advantage of new tools. New tools require new techniques.
From the start 4D has managed variables so they are never “empty”. When a new variable was created or declared it was always initialized to the equivalent of zero for its data type. I think this was part of the effort, back in the early days of consumer database apps, to make “scripting”, as it was called then, easy for non programmers. You never had to worry about a variable having no value. It always had a value of zero at least and you could use it right away. Most other languages have Null. Null indicates something has no value. In the case of numbers zero is a perfectly good value and different from not having any value. 4D objects support Null. 4D developers for the most part do not. Because we’ve never had to. You still don’t have to. There are work around methods such as OB is defined. There’s nothing really wrong with using it except it forestalls getting familiar with using Null and is more limited in some cases. Consider this block of code: C_OBJECT($obj) $ok:=($obj=Null) // $ok = true $ok:=Not(OB Is defined($obj)) // $ok = true $ok:=OB Is empty($obj) // $ok = true $obj:=New object $ok:=($obj=Null) // $ok = false $ok:=Not(OB Is defined($obj)) // $ok = false $ok:=OB Is empty($obj) // $ok = true $obj.a:=123 // any key:value $ok:=($obj=Null) // $ok = false $ok:=Not(OB Is defined($obj)) // $ok = false $ok:=OB Is empty($obj) // $ok = false OB is empty is the most restrictive test because empty can be Null or empty can be undefined but Not(OB is empty) can only mean something is there. Testing $obj=Null is equivalent to the longer command test in every instance. I find it easier to type and easier to comprehend reading the code. Here are the equivalent statements for clearing $obj: CLEAR VARIABLE($obj) $obj:=Null The results are the same. A ‘cleared’ object variable is a Null object variable. There is one more really compelling use case where testing for Null is superior: C_OBJECT($obj) $ok:=($obj.a.b.c[2]=Null) // $ok = true $ok:=Not(OB Is defined($obj.a.b.c[2])) // error This works compiled as well as interpreted and fill a big void (as it were) in being able to test for valid objects. Testing an object path against Null tells you if the path is valid or not. It does not tell you anything about what part of the path fails, however. HTH -- Kirk Brooks San Francisco, CA ======================= *We go vote - they go home* ********************************************************************** 4D Internet Users Group (4D iNUG) FAQ: http://lists.4d.com/faqnug.html Archive: http://lists.4d.com/archives.html Options: https://lists.4d.com/mailman/options/4d_tech Unsub: mailto:[email protected] **********************************************************************
