Re: [MI-L] MAPBASIC Object or not Object?

[Phil Waight]

Bill,   If you have an alias or object variable which may or may not contain a valid object, you can use a test as below before assigning it to an object variable and using it. The trick is to defer the object assignment until you are safely in a function with an error handler.   I also use NULL objects where an object variable must be returned. In this case, create it as a small object, outside the bounds of your working tables to ensure no intersects etc.   I sometimes find alias variables dont behave as I would expect (see note at function IsObjectT), but its probably because I dont fully understand the scope of their use.    Dim reci as smallint, aobj,atab,arid as alias, oLegit as object, lExists as logical  if not istableopen("testobjects") then   open table "f:\temp\testobjects" as testobjects  end if  reci = 1  fetch first from testobjects                        'testobjects has row1 record ok, deleted object; row 2 deleted; row 3 rec, obj ok   atab = "testobjects"   aobj = atab + ".obj"  while not eot(testobjects)   'oLegit = testobjects.obj   'lExists = isobject(oLegit)                   'this statement fails with "cant use uninitialised obj var in expr"   'lExists = isobjectA(aobj)                   'fails with uninit msg   'lExists = isobject(testobjects.obj)     'works as test but not flexible enough - really need alias var passed   lExists = isobjectT(str$(atab))   'works as legit object test   Print "row " + str$(reci) + " object exists: " + str$(lExists)   if (lExists) then                                 'can now safely manipulate object            oLegit = testobjects.obj   End If   fetch next from testobjects   arid = atab + ".rowid"   reci = arid  wend '------------------------------------------------------------------------------ Function IsObject (ochk as object) as logical  Dim o1 as object  onerror goto errhandler  o1 = ochk  IsObject = true  Exit Function ErrHandler:  IsObject = False End Function '------------------------------------------------------------------------------ 'If this function were to accept the table alias variable 'instead of a string, it fails on the line aobj = atabnm + ".obj" ' in an unexpected manner ("could not convert data")!!! Function IsObjectT (byval tabnm as String) as logical  Dim o1 as object, aObj as alias, sobj as string  onerror goto errhandler    aObj = tabnm + ".obj"  o1 = aObj  sobj = str$(o1)    'use obj var in _expression_ tests validity!  IsObjectT = true    'obj is ok at this point    Exit Function ErrHandler:  IsObjectT = False  ' print "IsObjectT error: " + error$()   dont really want to know End Function       Phil Waight.   _______________________________________  Email : [EMAIL PROTECTED] Web: www.spatialprojects.com.au Skype: PhilWaight     ----- Original Message ----- From: Bill Thoen To: Spencer Simpson Cc: mapinfo-l@lists.directionsmag.com Sent: Saturday, July 29, 2006 1:39 AM Subject: Re: [MI-L] MAPBASIC Object or not Object? Spencer Simpson wrote: >No, I've found that aliases (especially objects) can act strangely if you >try to extract their values more than once between two successive fetches. >Maybe newer versions of MapInfo are more robust, but it conditioned me to >always fetch aliases into variables back in the 1990s. >  > I'd have to say, "Prove it." IMHO, that's a superstition that you develpoed as you were learning and now it's just a habit. It's similar to the superstition that using defined constants is not safe. >Using "degenerate" objects to simulate null objects is useful, but it >doesn't help fetching them from a table, because you can't guarantee that a >row didn't have its object deleted. >  > That's right. And for that you can test the object itself (which is actually an alias). But once you put an object into a variable you need another way to indicate whether its valid or not. _______________________________________________ MapInfo-L mailing list MapInfo-L@lists.directionsmag.com http://www.directionsmag.com/mailman/listinfo/mapinfo-l

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