Ruby private class method

Question

I tried to implement a singleton pattern in ruby, just want to know why I can not access private class method in ruby

class Test private_class_method :new @@instance = nil def self.getInstance if(!@@instance) @@instance = Test.new end return @@instance end end

I declare "new" as a private class method, and try to call the "new" in my singleton method "getInstance"

test output

>> require "./test.rb" => true >> Test.getInstance NoMethodError: private method `new' called for Test:Class from ./test.rb:7:in `getInstance' from (irb):2 >>

You cannot access new with a receiver because it is now private. And it is you who made it private. Not clear why you are doing such contradictory thing and wondering over it. — sawa
– sawa, Commented Jul 15, 2013 at 9:13
not sure why this is down voted. @sawa, someone with java background can make this mistake easily and I think this question can be helpful to them. In JAVA, we can access any private stuff inside a class declaration, — GingerJim
– GingerJim, Commented Jul 15, 2013 at 9:18
Ruby is not Java. Don't use a method without even knowing what it does. Not reading the document is just laziness. — sawa
– sawa, Commented Jul 15, 2013 at 9:21
I know ruby is not java. I think the forum have tolerance for newbie questions, and most people is willing to help others pass their learning curves. Save your arrogance. — GingerJim
– GingerJim, Commented Jul 15, 2013 at 9:26

Chris Heald · Accepted Answer · 2013-07-15 09:00:58Z

Since ::new is a private method, you can't access it by sending a message to the class constant. It'll work to just send it to implicit self by omitting the receiver, though.

Additionally, since this is a class method, its instance variable scope is already class-level, so you don't need to use @@ class variables. An instance variable will work just fine here.

class Test private_class_method :new def self.instance @instance ||= new end end puts Test.instance.object_id puts Test.instance.object_id # => 33554280 # => 33554280

Sergio Tulentsev · Accepted Answer · 2013-07-15 09:00:14Z

private_class_method :new NoMethodError: private method `new' called for Test:Class

That's why. Private methods cannot be invoked with explicit receiver. Try using send.

@@instance = Test.send(:new)

or with implicit receiver (because self is Test)

@@instance = new

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