Vala: How to refer an abstract class' subclass?

Question

Consider the following:

abstract class Singleton : Object {
    private static Singleton _instance = null;
    public static Singleton instance() {
        if (instance == null) {
            instance = // constructor call goes here
        }
        return instance;
    }
}

class Foo : Singleton {
    Foo() {}
}

var test = Foo.instance();

I would like to implement a singleton in an abstract class. My question is: How can I refer to the subclass constructor from Singleton.instance()?

score 0 · Accepted Answer · answered Apr 09 '17 at 13:09

Inheritance and the singleton pattern don't mix well:

C++ Singleton class - inheritance good practice

Why don't you just make your Foo class static (make all members static):

public class Foo {
    // Static constructor
    public static Foo() {}
    // Static methods
    public static void bar () {}
    // Static attributes, etc.
}

This usually works as well. It does not work so well if your "singleton" has a costly construction and you want to avoid it being always constructed.

Vala: How to refer an abstract class' subclass?

1 Answers1