Friend functions in Ruby

friend.rb

# See http://www.ruby-doc.org/core-1.9.3/Kernel.html#method-i-set_trace_func for docs on this function

require 'ostruct'

module Friendly
  def self.included(base)
    base.extend(ClassMethods)
  end
  
  def method_missing(name, *args, &block)
    self.class.disable_trace_function
    
    friends = self.class.friends
    call_history = self.class.call_history
    called_by_friend = friends.include?(call_history[0].id) || friends.include?(call_history[0].class_constant)
    # puts friends.inspect
    # puts call_history.inspect
    # puts called_by_friend.inspect
    if self.private_methods.include?(name) && called_by_friend
      return self.send(name, *args, &block)
    else
      super(name, *args, &block)
    end
  ensure 
    self.class.enable_trace_function
  end
  
  module ClassMethods
    def friend(method_symbols_and_or_classes)
      @call_history ||= [OpenStruct.new(id: nil)] * 1
      
      @trace_function = 
        proc do |event, file, line, id, binding, class_constant|
          trace_info = OpenStruct.new(
            event: event, 
            file: file, 
            line: line, 
            id: id, 
            binding: binding, 
            class_constant: class_constant, 
            time: Time.now.to_f
          )
          
          @call_history = @call_history[1..-1] << trace_info if we_care_about?(event, class_constant)
        end
      
      enable_trace_function
      
      @friends ||= []
      @friends += [method_symbols_and_or_classes].flatten
    end
    
    attr_reader :friends, :call_history, :trace_function
    
    def enable_trace_function
      set_trace_func(@trace_function)
    end
    def disable_trace_function
      set_trace_func(nil)
    end
    
    def we_care_about?(event, class_constant)
      event == "call" && !(class_constant.to_s =~ /Friendly/)
    end
  end
end 


 

rectangle_example.rb

require_relative 'friend'

class Rectangle
  include Friendly
  
  def initialize(width, height)
    @width = width
    @height = height
  end
  
  friend :duplicate

  def from_square(square)
    #return new(square.side, square.side)
    @width = square.side
    @height = square.side
    return self
  end
  
  private 
  def width
    @width
  end
  
  def height
    @height
  end
end


rect = Rectangle.new(1, 2)
puts "Rectangle has declared any function named 'duplicate' as a friend.\n\n"
puts "Rectangle width is private:"
val = rect.width rescue puts("\t#{$!}")
puts "Rectangle height is private:"
val = rect.height rescue puts("\t#{$!}")

def duplicate(rectangle)
  Rectangle.new(rectangle.width, rectangle.height)
end
puts
puts "'duplicate' uses rectangles privates, but does not fail:"
new_rect = duplicate(rect) 
puts "\t#{rect.inspect}"
puts "\t#{new_rect.inspect}"

square_example.rb

require_relative 'friend'

class Rectangle
  include Friendly
  
  def initialize(width, height)
    @width = width
    @height = height
  end
  
  friend :duplicate

  def self.from_square(square)
    return new(square.side, square.side)
    @width = square.side
    @height = square.side
    return self
  end
  
  private 
  def width
    @width
  end
  
  def height
    @height
  end
end

class Square
  include Friendly
  friend Rectangle 
  
  def initialize(side)
    @side = side
  end
  
  private
  def side
    @side
  end
end

puts "Square declares the Rectangle class to be it's friend.\n\n"
square = Square.new(3)

puts "Square#side is private:"
puts square.side rescue puts("\t#{$!}")
puts
puts "But a Rectangle can use it:"
rect = Rectangle.from_square(square)
puts "\t#{rect.inspect}"
puts "\t#{square.inspect}"