Archive for January, 2011

attr_accessor with default in Ruby

January 31st, 2011 | Category: Programming, Ruby

If you aren’t in rails (hello to both of you) the lack of an attr_accessor with a default value is very annoying. I wrote up the following:

module ExtendAccessors
  def attr_accessor_with_default name, *default, &block
    if(default.size >= 1)
      define_method name.to_sym do
        instance_variable_set("@#{name}", default[0]) unless instance_variable_defined?("@#{name}")
        instance_variable_get("@#{name}")
      end
    elsif block_given?
      define_method name.to_sym do
        instance_variable_set("@#{name}", instance_eval(&block)) unless instance_variable_defined?("@#{name}")
        instance_variable_get("@#{name}")
      end
    else
      raise "Must either provide a default value or a default code block"
    end
    define_method "#{name}=".to_sym do |value|
      instance_variable_set("@#{name}",value)
    end
  end
end

which supports the following syntax:

class A
  extend ExtendAccessors
  attr_accessor_with_default :some_map, {}
  attr_accessor_with_default :some_object do
    AnotherObject.new(self)
  end
end
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Meaning of ’self’ in Ruby

January 25th, 2011 | Category: Programming, Ruby

This came up today. When defining classes in Ruby “self” can refer to the class or the instance of that class depending on the context and understanding which is which is important.

When defining a class “self” in the context of the class definition refers to the object representing the class being defined. When in a (non-class-level) method “self” refers to the instance of the object. So:

class SomeClass
  #dynamically add a method to the SomeClass object. 
  #self is the SomeClass class object
  #now I can call SomeClass.class_level_method
  def self.class_level_method
    self == SomeClass #this is true....
  end
 
  #define an instance level method
  def instance_level_method
    self #this is now an instance of the object
  end
end

This syntax may make more sense in terms of the following

class SomeClass
end
 
#this can be done outside of the class definition
def SomeClass.class_level_method
  echo "hi"
end
 
c = SomeClass.new
d = SomeClass.new
 
#class objects are notspecial in this regard. 
#You can do the same thing to
#a specific object instance of any other type.
def c.specific_instance_method
  #this method is defined just for this 
  #specific instance of this class
end
 
c.specific_instance_method #no problem
d.specific_instance_method #nope!
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