On May 3, 2006, at 10:06 AM, [EMAIL PROTECTED] wrote:
On May 03, 2006, at 15:17 UTC, Jason Essington wrote:
BAH! I forgot, RB doesn't have a proper Object that is the base of
all objects :-(
No, that has nothing to do with it. You can declare an array of
type Object. But that simply doesn't help. Let's use the classic
"fruits" hierarchy to see why:
Suppose you have a method that can operate on an array of Fruits.
Perhaps all fruit objects have a Sweetness property, and this
method sorts fruits by their sweetness:
Sub SortBySweetness( fruits() as Fruit )
That's fine, but now suppose you have elsewhere in your code an
array declared as:
Dim apples() as Apple
An Apple is a Fruit, so you can pass this to your SortBySweetness
method, right? Wrong. The SortBySweetness code knows only that
that its array is defined to contain fruits, so it would be
perfectly valid for it to do something like:
fruits.Append New Orange
Caution! The following is just how I would expect things to work, and
is absolutely not how RB works!
Actually not, since the array of apples passed to a method as an
array of fruits is still an array of apples, any attempt to append a
fruit that is not an apple should simply raise an
IllegalCastException at runtime.
In this case an Orange is a fruit, but it is definitely not an Apple
so the exception is a perfectly reasonable thing to expect.
This is conceptually similar to doing something like:
Sub badFruit(ByRef f As Fruit)
f = new Orange()
End Sub
Dim a as new Apple()
badFruit(a)
Rather than getting a compile exception at badFruit(a), I would
expect to get an IllegalCastException at f = new Orange().
Now, if you could pass your apples array in, you'd end up with an
array declared to contain only Apples, but actually containing an
Orange. The result would be a crash.
Not if the assignment (append) failed when trying to cast the fruit
to the proper type.
So you can see, just because an Apple IsA Fruit, does NOT mean that
an array of Apples IsA array of Fruits.
B-B-B-But an array of Apples IsA array of Fruits! A rather specific
array of Fruits, but still an array of Fruits.
That's why you can't make a generic CloneArray method that would
work on any array of Cloneable objects, and is probably why RB
doesn't contain such functionality.
Well, conceptually, it should be as simple as something like:
Sub CloneArray(oldArray() As Object, newArray() As Object)
Redim newArray(0)
For Each o As Object in oldArray
If Not o IsA Cloneable Then
msgbox "To clone an array, all of the elements must be Cloneable"
Exit
End If
newArray.Append(o.clone())
Next
Exception IllegalCastException
msgBox "Your arrays are not entirely compatible"
End Sub
It has to be written for the specific type care about. (It's also
why most of the methods in my open-source ArrayUtils module don't
operate on object arrays, even when it might naively seem like they
could.)
Well, It isn't naivety that makes me think that it should work, it is
that it does work exactly that way in my primary language (Java).
So, unfortunately, what I thought was a fairly simple problem, in
REALity ends up being a bit more complicated, but with the benefit
that it is much harder to shoot yourself in the foot.
-jason
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