Jaa

A Whole Lot Of Nothing

  • Artikkeli

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Occasionally
I get questions from people who are confused over the semantics of data that are not
even there. Usually they've written
code something like

If Blah
= Nothing Then

or

If Blah
= Empty Then

or

If Blah
= Null Then

all
three of which almost certainly do not correctly express the actual intention of the
programmer. Why does VBScript have Null, Nothing and Empty,
and what are the differences between them?

Let's
start with Empty. When you declare a
variable in C, the variable's value before the first assignment is undefined:

int index;

printf("%d",
index); /* could print any integer */

In
C, the declaration reserves space for the variable, but does not clear
the contents of that space. After
all, why would it need to? You're just going to initialize it to some value yourself,
right? Why should the compiler waste
time by initializing it only to have that initialization overwritten?

That
might seem like a sensible attitude if you are one of those people who prefers that
your program be twenty or even thirty nanoseconds faster
in exchange for causing any accidental use of uninitialized memory to make your program's
behaviour completely random.

The
designers of VB knew that their users were not hard core bit twiddling performance
wonks, but rather line-of-business developers who prefer a predictable programming
environment. Thus, VB initializes variables
as they are declared and eats a few processor cycles here and there. When
you declare an integer in VB, it's initialized to zero, strings are initialized to
empty strings, and so on.

But
what about variants? Should an uninitialized
variant be initialized to zero? That
seems bogus; why should an uninitialized variant automatically become a number? Really
what we need is a special of-no-particular-type "I'm an uninitialized variant"
value, and that's Empty. And
since in VBScript, all variables are variants, all variables are initialized to Empty.

What
if in VB you compare an uninitialized variant to an uninitialized integer? It
seems sensible that the comparison would return True,
and it does. Empty compares
as equal to 0 and the empty string, which might cause false positives in our example
above. If you need to detect whether
a variable actually is an empty variant and not a string or a number, you can use IsEmpty. (Alternatively,
you could use TypeName or VarType,
but I prefer IsEmpty.)

Nothing is
similar to Empty but
subtly different. Empty says
"I am an uninitialized variant", Nothing says
"I am an object reference that refers to no object". Since
the equality operator on objects checks for equality on the default property of an
object, any attempt to say If
Blah = Nothing Then  is
doomed to failure -- Nothing does
not have a default property, so this will produce a run-time error. To
check to see if an object reference is invalid, use If
Blah Is Nothing Then.

Null is
weirder still. The semantics of Null are
very poorly understood, particularly amongst people who have little experience with
relational databases. Empty says
"I'm an uninitialized variant", Nothing says
"I'm an invalid object" and Null says
"I represent a value which is not known."

Let
me give an example. Suppose you have
a database of sales reports, and you ask the database "what
was the total of all sales in August?" but one of the sales staff has not reported
their sales for August yet. What's the
correct answer? You could design the
database to ignore the fact that data is missing and give the sum of the known sales,
but that would be answering a different question. The
question was not "what was the total of all
known sales in August, excluding any missing data?" The
question was "what was the total of all sales
in August?" The answer to that question
is "I don't know -- there is data missing",
so the database returns Null.

What
happens when you say

If Blah
= Null Then

? Well,
try this:

Sales
= 123

WScript.Echo
Sales = Null

You
get not True,
not False,
but Null! Why's
that? Well, think about the semantics
of it. You're saying "is the unknown
quantity equal to 123?" The answer to
that is not "yes", it's not "no", it's "I don't know what the unknown quantity is,
so, uh, maybe?"

Nulls
propagate themselves. Any time
you numerically manipulate a Null,
you get a Null right
back. Any sum containing an unknown addend
has an unknown sum, obviously! The correct
way to check for Null is
much as you'd do for Empty:
use IsNull (or TypeName or VarType.)

 

The
sharp-eyed among you will have noticed that I never actually answered
the question. What does happens when you say

If
Blah = Null Then

--
does VBScript run the consequence block or the (optional) "else" block?
Obviously it has to do one of the two. When it comes right down to it,
VBScript will assume falsity in this situation.

 

The
way JScript and JScript .NET handle nulls is a little bit weird; I'll talk about that
in my next entry.

Comments

  • Anonymous
    September 30, 2003
    Another very interesting post Eric. Thanks!

  • Anonymous
    September 30, 2003
    Heh. Anyone who interviewed with me from '98-'01 faced exactly that question: What is the difference between Empty, Null, Nothing and "", and when would you use each. Sadly, I would say no more than 1 in 5 or 10 got it right.

  • Anonymous
    September 30, 2003
    I am always interested in what questions people ask in interviews. What information was this question intended to elicit?

  • Anonymous
    September 30, 2003
    I believe in working at the right level of abstraction, and through that period VBScript and ASP were clearly the right level of abstraction for the sort of work my team was doing. I also believe that better developers need to understand how things work at at least one level lower.A good answer to that question involved an understand of at least Null propagation and the fact that Nothing is an object reference not a value. Better answers understood Empty's association with uninitialized variables. I don't remember if I ever got one who knew Empty compared as equal to "" or 0.A superior answer involved showing they understood at least something of the lower level of abstraction, i.e. if they mentioned the VARIANT struct or VT_EMPTY and VT_NULL.

  • Anonymous
    September 30, 2003
    Another odditity is a "missing value", which is actually a VT_ERROR with a value of DISP_E_PARAMNOTFOUND. Since JScript classic cannot represent 'Nothing' or a missing parameter, I had to write some very trivial code to expose these two concepts as properties of an ActiveX object (JArgUtil)

  • Anonymous
    October 01, 2003
    I agree strongly that devs need to understand at least one level lower, and two is better. What I find interesting is that your question tests knowledge. I don't give a darn about what the candidate knows about. I care very much about what they know how to do. I try to elicit information about skills when I interview people, not knowledge. But then again, I am looking for people who will be very flexible problem solvers over a very long term. If you're looking for people who can jump right in and solve a specific problem in a specific language, then it makes more sense to look for specific knowledge.

  • Anonymous
    October 01, 2003
    We're on exactly the same page. A big chunk of the interviews in that era were for contract positions, where I was not particularly interested in the long term and very interested in how fast they could be productive.

  • Anonymous
    January 21, 2009
    PingBack from http://www.keyongtech.com/1157170-if-question

  • Anonymous
    May 14, 2009
    Back when I started this blog in 2003, one of the first topics I posted on was the difference between

  • Anonymous
    May 24, 2009
    Когда я начал этот блог в 2003 году, один из первых постингов был про отличия между Null, Empty и Nothing