From: "Jan E." <lists@...>
Date: 2012-10-03T03:12:07+09:00
Subject: Re: still learning by doing - connecting rooms in a game

Sebastjan H. wrote in post #1078397:
> Sorry if I confused anyone. It's just that I didn't want any duplication
> as Peter suggested, but then If I understand correctly I need either the
> writer or accessor to be able to change the values of instance
> variables.

No, you only need to define setter methods (i. e. methods ending with a
"=").

class A

  def initialize
    @x = 4
  end

  # getter for @x
  def x
    @x
  end

  # setter for @x
  def x=(value)
    @x = value
  end

end

a = A.new
puts a.x  # call getter
a.x = 3   # call setter; the same as a.x=(3)
puts a.x

That's exactly what attr_accessor does: It defines getter and setter 
methods for the given variables. You could actually define it yourself:

class Module

  def my_attr_accessor *vars
    vars.each do |var|
      # getter
      define_method(var) {instance_variable_get "@#{var}"}
      # setter
      define_method("#{var}=") {|val| instance_variable_set "@#{var}", 
val}
    end
  end

end

class A

  my_attr_accessor :x, :y

  def initialize
    @x = 2
  end

end

a = A.new
puts a.x
a.x = 12
puts a.x



> What I meant was, that if omit the attr_accessor completely and only
> have methods for instance variables defined, then
>
> player.hp -=20 does nothing.

You should actually get an error saying that player doesn't have a "hp=" 
method.

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