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about NULL

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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victorlong
Asked:
victorlong
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1 Solution
 
wizard2072098Commented:
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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victorlongAuthor Commented:
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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wizard2072098Commented:
"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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vikiingCommented:
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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victorlongAuthor Commented:
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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victorlongAuthor Commented:

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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wizard2072098Commented:
>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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wizard2072098Commented:
>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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