nothing
The value nothing represents an object whose object reference (which seems to be a memory address) is 0.
An object variable is considered
nothing if its reference count is zero. For such variables,
objPtr returns 0.
Checking if an object is nothing
There is no
isNothing() function in VBA (see
is…() functions).
An object can be tested for nothingness with the is nothing construct:
if obj is nothing then
…
end if
The opposite is possible with not obj is nothing (not with obj is not nothing):
if not obj is nothing then
…
end if
Set obj = Nothing | IUnknown::Release
Setting an object variable to nothing (
set obj = nothing) is basically the same as calling
IUnknown::Release for the object.
empty
The value of a (
variant) variable is empty if it has not yet been assigned a value. Thus, empty signals that the variable is not initialized.
Such a variable evaluates to
0 (zero) in numeric and to
"" (empty string) in string context. Therefore, a variable that is not a variant (such as a
long) cannot be empty (but
empty can be assigned to such a variable).
In order to test whether a variant is empty, the special function
isEmpty(…) can be used.
TODO
What is the relationship to vbEmpty?
null
The value null indicates that a variable contains invalid and/or inexistent data.
The
null value can only be assigned to a
variant variable.
Any operation that involves null values evaluates to null.
option explicit
sub testEmptyAndNull(var as variant)
if isEmpty(var) then
debug.print "| var is empty"
end if
if isNull(var) then
debug.print "| var is null"
end if
debug.print "| var = " & var
debug.print ""
end sub
sub main()
dim dbl as double
dim var as variant
' dbl = null ' null can only be assigned to variants
' ' Otherwise, a Run-time error 94 is thrown: Invalid use of Null
' If a variant is null can be tested with isNull:
testEmptyAndNull var
'
' | var is empty
' | var =
var = null
testEmptyAndNull var
'
' | var is null
' | var =
var = 42
testEmptyAndNull var
'
' | var = 42
end sub
Checking for a null value
To determine if a variable is
null, the
function isNull(…) is used:
if isNull(v) t hen
…
end if
TODO
What is the relationship to vbNull.