logger.rb

class Logger
    private_class_method :new
    @@instance = nil
    
    def info(msg)
      puts msg
    end
    
    def self.instance  
      @@instance = new unless @@instance
    end
end

logger = Logger.instance
logger.info('Hello')

# -*- encoding: utf-8 -*-

class Test

  def initialize(*args, &blk)
    @instance ||= self.new(*args, &blk) # 保持一个实例.
  end

end

a = Test.new
b = Test.new

a == b
a == b

#=> ture