Fabulous Adventures In Coding
Eric Lippert is a principal developer on the C# compiler team. Learn more about Eric.
in order for VBScript to support variable referencing there has to be a variable to
our earlier example:
ABC = 123
that had been Change
based on what you know from two posts ago, you’d know that this passes ABC byval,
not byref. So the assignment to XYZ would
NOT change ABC in
the rule is pretty simple -- if you want to pass a variable by reference, you've got
to pass the variable, period.
series of posts was inspired by an intrepid scripter who was trying to combine our
previous two examples. He had a program
that looked something like this:
= New Foo
in fact does not change the value. This
passes the value of Blah.Bar,
not a reference to Blah.Bar.
scripter asked me "why does this not work the way I expect?" Here's
my Socratic dialog reply to him:
does this not work the way I expect?
Because your expectations are inconsistent with the real universe. Adjust your
expectations and they'll start being met!
is remarkably unhelpful. Let me rephrase: What underlying design principle did the
VBScript developers use to justify this decision to pass by value, not reference?
The fundamental principle that governs this case was "do not be unnecessarily different
from VB6." VB6 does the same thing. (Try it if you don't believe me!)
are begging the question. Why does VB6 do that?
Probably for backwards compatibility with VB5, 4, 3, 2 and 1, which incidentally was
called "Object Basic". Ah, the halcyon days of my youth.
question begging! What was the initial justification on the day that by-reference
calling was added to VB?
That is lost in the mists of time. That was like ten years ago, dude!
There are not very many of the original design team left. I was an intern at
the time and they weren't exactly consulting me on these sorts of decisions on a regular
basis. It wasn't so much "Eric, what do you think about these by reference semantics?"
as "Eric, the OLE Automation build machine needs more memory, here's a screwdriver."
you're in luck. I seem to recall back in the dim mists of time someone telling
me something about wanting to avoid copy-in-copy-out semantics on COM objects.
Suppose for example you said:
Frob = CreateObject("BitBucket.Frobnicator")
so now what happens? This isn't a VB class, this is some third party COM object. COM
objects do not have property slots, they have getter/setter accessor functions. There
is no way to pass the value of Frob.Rezrov by
reference because VB does not have psychic powers which tell it where in memory the
implementers of BitBucket.Frobnicator happened
to store the value of the Rezrov property.
that, how could you implement byref semantics? You could implement copy-in-copy-out
semantics! VB would have to create a memory location, fill it with the value
returned by get_Rezrov,
pass the address of that location to SetToFive,
and then upon SetToFive's return,
it would have to call Frob::set_Rezrov with
the new value put into the buffer.
right? Well, it gets weird once you start thinking about non-trivial functions.
Consider the case where SetToFive does
NOT change the value of the by-ref value. That call to set_Rezrov may
have side effects -- do we really want to call it if nothing changed? It seems
like that could potentially cause badness, and certainly cause poor performance.
In a "realio-trulio byref" system we'd expect zero sets if there was no change but
in copy-in-copy-out we end up with one call to the setter regardless. How could
we avoid that unwanted call?
we could create yet another temporary storage to keep the original value around and
do a comparison when SetToFive returns.
(Note that I've just waved my hands there; I'm assuming that the two values
can sensibly be compared. Comparing two
things for equality is non-trivial, but that's another posting.)
what if the temporary storage variable changed during the execution of SetToFive and
then changed back? In that case we'd expect two calls to the setter, but actually
end up with no calls!
naïve copy-in-copy-out doesn't provide particularly good fidelity with true byref
addressing. The original designers of VB decided that it was simply not worth
the trouble to do it at all. It is much easier to simply say that members
of COM objects do not get copy-in-copy-out semantics, and therefore they cannot be
passed by reference. If you're going to make that restriction for some COM objects,
it seems perverse to say "we'll do this for third party COM objects but not for VB
class objects." Thus, VBScript does not support passing object properties by