Solved

about NULL

Posted on 1998-11-30
8
224 Views
Last Modified: 2012-05-04
Can you tell me what is NULL?
Apart from VARIANT, can it be used in other types?
Can you also give me an example showing why we need NULL?
0
Comment
Question by:victorlong
  • 4
  • 3
8 Comments
 
LVL 1

Accepted Solution

by:
wizard2072098 earned 50 total points
ID: 1447520
NULL means "waiting to be initialized", whereas EMPTY or BLANK means "currently has no value".  

Dim v as Variant  --  memory reserved, not initialized.  NULL.

v = ""  -- memory initialized, has no value.  EMPTY/BLANK.

v = vbNull   --- memory released, no value. NULL.

It's the difference between actually holding memory or just having a pointer to a memory area that may or may not be used.

The NULL-able variable is used more often in C++, but is a powerful tool. VB used to reject the use of NULLS because there was no way to assign a null in VB until the advent of the variant. But they still have to give you the "vbNull" type in order to allow you the ability to assign a null.

The variant is the only datatype that can hold a null. Many databases will allow nulls in their records, so the VB variant is a good way to coerce a db null into something else, like an empty string.

Using variants is nice, since they do not actually take up memory until they are initialized ( v = "", or v = 0 ), and they release memory when they are set to null.
0
 

Author Comment

by:victorlong
ID: 1447521
Thank you wizard2.

>Dim v as Variant  --  memory reserved, not initialized.  NULL.

So, "Dim i as Integer" also doesn't need the 2 bytes.

>It's the difference between actually holding memory or just having a pointer to a memory area that may or may not be used.

But the memory area pointed by a varable may not be used by other varables....that equals to holded :-)

> "vbNull" type in order to allow you the ability to assign a

vbNull = Null ?

>The variant is the only datatype that can hold a null.

I see now.

Cheers.

Victor
0
 
LVL 1

Expert Comment

by:wizard2072098
ID: 1447522
"Dim v as Variant" would equate to creating a pointer to an uninitialized memory location in C++. Since the variant can contain any datatype, it is impossible to actually "malloc", or allocate, the amount of memory needed. The variant Dim simply creates a pointer to a memory area that will be allocated and populated later.

Unlike the variant, "Dim i as Integer" would also include the malloc to actually allocate the memory area, since the actual amount of memory needed for the variable is known.

The "vbNull" is the built-in constant that can be used to assign NULL to a variant.

0
Free Tool: Site Down Detector

Helpful to verify reports of your own downtime, or to double check a downed website you are trying to access.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 
LVL 3

Expert Comment

by:vikiing
ID: 1447523
Just to add two words, not only integer but any numeric and user-defined variables have their lengths defined at compile time by allocating memory for them when executable program is created; hence they are NOT NULLable.

0
 

Author Comment

by:victorlong
ID: 1447524
Hi experts

I have just tested that
   Dim v as Variant
   Print VarType(v)
print out EMPTY, instead of NULL! An EMPTY would be 0 or ""???

Cheers.

Victor
0
 

Author Comment

by:victorlong
ID: 1447525

Dim v as Variant     'v got a small space (as Empty type).
v = "string"         'v got space of 16 bytes (as String type).
v = NULL             'v got no space (as Null type).

Am I write?
0
 
LVL 1

Expert Comment

by:wizard2072098
ID: 1447526
>Dim v as Variant     'v got a small space (as Empty type).
  -- v has no allocated memory here. the small space is for the pointer, not the value associated with the pointer, so you really can't count it.
>v = "string"         'v got space of 16 bytes (as String type).
>v = NULL             'v got no space (as Null type).
 

0
 
LVL 1

Expert Comment

by:wizard2072098
ID: 1447527
>I have just tested that
>   Dim v as Variant
>   Print VarType(v)
>print out EMPTY, instead of NULL! An EMPTY would be 0 or ""???

-- the variable is technically EMPTY, but in reality it is "not initialized". That's somewhat different than "NULL". Null is a VALUE, not a STATE (I may have confused you in an earlier post). However, in VB, assigning NULL to a variant causes it to release the memory it is holding, which effectively changes the state of the variable. In C++, for example, assigning NULL to a variable does not release the memory. It just changes the variable's value. In C++ you have to release memory manually. In VB, they've given you sort of a nice "built in" way of doing it.
0

Featured Post

Free Tool: Postgres Monitoring System

A PHP and Perl based system to collect and display usage statistics from PostgreSQL databases.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Article by: Martin
Here are a few simple, working, games that you can use as-is or as the basis for your own games. Tic-Tac-Toe This is one of the simplest of all games.   The game allows for a choice of who goes first and keeps track of the number of wins for…
I was working on a PowerPoint add-in the other day and a client asked me "can you implement a feature which processes a chart when it's pasted into a slide from another deck?". It got me wondering how to hook into built-in ribbon events in Office.
Get people started with the process of using Access VBA to control Outlook using automation, Microsoft Access can control other applications. An example is the ability to programmatically talk to Microsoft Outlook. Using automation, an Access applic…
Get people started with the utilization of class modules. Class modules can be a powerful tool in Microsoft Access. They allow you to create self-contained objects that encapsulate functionality. They can easily hide the complexity of a process from…

829 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question