Tatva-Artha

Coming from Java this may be a little surprise that order of class initialization (order of method definitions) does impact the outcome of your program, as I just found out.

For example, consider 2 cases where I define a singleton class:

Case-1:

class A
  @@instance = A.new
  def self.instance
    @@instance
  end
  attr_reader :x
  private
  def initialize
    @x = 99
  end
end

Output:

>> A.instance.x
=> nil

Case-2:

class A
  private
  def initialize
    @x = 99
  end
  @@instance = A.new
  public
  def self.instance
    @@instance
  end
  attr_reader :x
end

Output:

>> A.instance.x
=> 99

The location of constructor/initialize() is different. And as we can see from output, the our constructor isn’t yet defined when A.new() was called in the first case and hence the @x instance variable didn’t get initialized the way we would have expected it to. The execution doesn’t complain since it uses the default/inbuilt constructor until our constructor gets defined.

Now I know.

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