I have a situation where I’d like a new object not to be created when the constructor parameters fail to meet certain conditions. I realize that the object has already been created when the constructor method is called. So is it possible for the constructor to delete (destruct) the object so that the object becomes nil?
In my main program I’d like to be able to do this:
var myObj as new objType(SomeParameters)
if IsNull(myObj) then
'Take remedial action when SomeParameters are bad
end if
Reason why I want to do this is that it’s less cumbersome for the constructor to check the validity of the input parameters than for the calling program to do it.
Add a constructor to your class & make it private - this forces you to use the Shared method below.
Add a shared method to your class, call it what you will (I use GetInstance as the method name) & pass the parameters.
Validate the parameters in the shared method & either create an instance of your class & return it or return nil.
In your code you would use
var myObj as objType = objType.GetInstance(SomeParameters)
if IsNull(myObj) then
'Take remedial action when SomeParameters are bad
end if
What might be the “remedial action” you’ll take if the parameters are bad? How will your code know what to do in order to properly satisfy the constructor?
Thanks for the replies so far. Here is a bit more information. One of the parameters of the constructor is an instance of the same class ObjType. The intention is for the constructor to duplicate the argument object with a few changes. It’s possible that the parameter object may be nil, in which case I’d like the new object to be nil as well. Perhaps this is a bad idea, but it would make my life easier, if the parameter object is nil, simply to return nil for the new object.
It’s not a great idea. Why not just check for the parameter being nil beforehand? You’re going to end up checking the returned value for nil anyhow. Unless there are other reasons the parameter can be unsuitable.
If myObj Is Nil Then // checks if it's a reference to nil directly
// This is my preferred way if you want to skip operator_convert which obviously is the case if you would check for Nil
End if
// Also
if Not (myObj Is Nil) Then
// reversed
End If
And using “=” gives operator convert availability
If myObj = Nil Then // checks if it's a reference to nil directly by operator_convert (if this is available)
End If
Thanks for all of the replies. I decided that, as advised, trying to return a nil object is not a good idea, and I was able to come up with a way to avoid doing it without complicating the rest of the code.
I’m still interested in Wayne’s method, and I intend to explore it further for possible future use.
On a related note, Wayne’s method is also a way to maintain a singleton – a class where only one instance exists at a time.
Private Method Constructor()
'the Constructor is private so it can't be instantiated
'without going through the GetInstance method
End
Private Shared Property singleton as NewClass
Public Shared Function GetSharedInstance() as NewClass
If singleton=nil Then
singleton=new NewClass()
End
Return singleton
End
And sometimes you might need to reset the singleton
Public Shared Function GetSharedInstance(reset As Boolean = false) as NewClass
If singleton is nil or reset = True then
singleton=new NewClass()
End
Return singleton
End
Why not just check for the parameter being nil beforehand? You’re going to end up checking the returned value for nil anyhow. Unless there are other reasons the parameter can be unsuitable.