Null object: Difference between revisions

nil isNil ifTrue: [ 'true!' displayNl ]. "output: true!"

foo isNil ifTrue: [ 'ouch' displayNl ].

x := (foo == nil).

x := foo isNil</lang>

notice that nil is the singleton instance of the UndefinedObject class; i.e. it is a first class object. Thus we can do:

<lang smalltalk>foo := nil.

foo class. "-> UndefinedObject"

foo respondsTo: #'bar'. "asking if a message is implemented"

foo class compile:'fancyOperation ^ 123'.

foo fancyOperation "->123"</lang>

the last example being for demonstration only - it is not considered well behaved to add arbitrary code that way, except for framework support, such as encoding, decoding marshalling etc.)