Null is a special data type with only one possible value: null. This value is returned by an expression that does not contain any value.
In the 4D language and for object field attributes, null values are managed through the
Null function. This function can be used with the following expressions for setting or comparing the null value:
- object attributes
- collection elements
- variables of the object, collection, pointer, picture, or variant type.
Undefined is not actually a data type. It denotes a variable that has not yet been defined. A function (a project method that returns a result) can return an undefined value if, within the method, the function result ($0) is assigned an undefined expression (an expression calculated with at least one undefined variable). A field cannot be undefined (the
Undefined command always returns False for a field). A variant variable has undefined as default value.
Here are the different results of the
Undefined command as well as the
Null command with object properties, depending on the context:
C_OBJECT($vEmp) $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