Null

Null : Null

Parameter	Type		Description
Function result	Null	←	Null value

Description

Null returns the Null type value null.

This function allows you to assign or compare the null value to the following language elements:

Language elements	Comments
object property values	Comparing Null to an object property returns true if the property value is null, and false otherwise. To simplify code, comparing Null also returns true if the property does not exist in the object (i.e. is Undefined), see example 4.
collection elements	When a collection is expanded by adding non-adjacent elements, any intermediary elements get automatically the null value.
object variables	See (*) below
collection variables	See (*) below
pointer variables	See (*) below
picture variables	(*) Assigning the null value to such a variable type clears its contents. In this case, it has the same effect as calling the CLEAR VARIABLE command.
variant variables

Note: This command cannot be used with scalar database fields. Null values in database fields are managed by the SQL engine, and are handled through the Is field value Null and SET FIELD VALUE NULL commands,

Example 1

You want to assign and test the null value to an object property:

 var vEmp : Object
 vEmp:=New object
 vEmp.name:="Smith"
 vEmp.children:=Null
 
 If(vEmp.children=Null) //true
 End if
 If(vEmp.name=Null) //false
 End if
 If(vEmp.parent=Null) //true
 End if

Note: This example requires that the object notation is activated in the database.

Example 2

You want to assign and compare the null value to a collection element:

 var myCol : Collection
 myCol:=New collection(10;20;Null)
 ...
 If(myCol[2]=Null)
  // if the 3rd element is null
    ...
 End if

Example 3

These examples show the various ways to assign or compare the null value to variables:

  //Object variable
 var $o : Object
 $o:=New object
 $o:=Null //equivalent to CLEAR VARIABLE($o)
 If($o#Null) //equivalent to If (OB Is defined($o))
 End if

  //Collection variable
 var $c : Collection
 $c:=New collection
 $c:=Null //equivalent to CLEAR VARIABLE($c)
 If($c#Null)
 End if

  //Pointer variable
 var $p : Pointer
 $p:=->$v
 $p:=Null //equivalent to CLEAR VARIABLE($p)
 If($p=Null) //equivalent to If (Is Nil pointer($p))
 End if

  //Picture variable
 var $i : Picture
 $i:=$vpicture
 $i:=Null //equivalent to CLEAR VARIABLE($i)
 If($i#Null) //equivalent to If (Picture size($i)#0)
 End if

Example 4

Here are the different results of the Undefined command as well as the Null command with object properties, depending on the context:

 var vEmp : Object
 vEmp:=New object
 vEmp.name:="Smith"
 vEmp.children:=Null
 
 $undefined:=Undefined(vEmp.name) // False
 $null:=(vEmp.name=Null) //False
 
 $undefined:=Undefined(vEmp.children) // False
 $null:=(vEmp.children=Null) //True
 
 $undefined:=Undefined(vEmp.parent) // True
 $null:=(vEmp.parent=Null) //True

See also

Is field value Null
OB SET NULL
SET FIELD VALUE NULL

Properties

Command number	1517
Thread safe	✓