Question

I’m having trouble in getting the singleton pattern to initialize a instance variable in smalltalk. (here is a link to another implementation for clarification)

this is what I have:

new  ^UniqueInstance ifNil: [UniqueInstance := self basicNew.                         UniqueInstance: instanceVar := Object new. ]. 

that last line (UniqueInstance: instanceVar := Object new.) doesn’t work, but that’s basically what I need to do: instantiate instanceVar as an Object before returning UniqueInstance back to the caller.

Notice that this ‘new’ method is used as a classinstantiation, and that libraries is a instance variable of UniqueIsntance (the isntance of the wanted class).

Can anyone point me in the right direction?

Answer 1

Try simpler:

YourClass class>>singleton         UniqueInstance ifNil: [UniqueInstance := self basicNew initialize].        ^UniqueInstance 

then on instance side of your class implement an appropriate #initialize method, for example:

YourClass>>initialize            someInstvar := someInitalValue.          ^self 

Update:: Name of the class method accessing the singleton varies, it can be #default, #current, or #singleton. I mostly use later.