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about NULL

Posted on 1998-11-30
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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?
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Question by:victorlong
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wizard2072098 earned 150 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.
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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
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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.

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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.

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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
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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?
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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).
 

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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.
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