Metaprogramming and class_eval

ruby-talk
1

Hi

Whats the differents between this three classes. Why the
my_attr_accesor dosent create getters and setter clases.

Thanks in advance

···

#----------------------------------------------------------------------
#
#----------------------------------------------------------------------

class Class
  def my_attr_accessor( *args )
     args.each do |name|
       self.class.class_eval do
          attr_accessor :"#{name}"
       end
     end
  end
end
=> nil

class MyNewClass
  my_attr_accessor :id, :diagram, :telegram
end
=> [:id, :diagram, :telegram]

class MyClass
  attr_accessor :id, :diagram, :telegram
end
=> nil

class Another_class
  def initialize
    ["id","diagram","telegram"].each do |name|
       self.class.class_eval do
          attr_accessor :"#{name}"
       end
     end
  end
end
=> nil

mnc = MyNewClass.new
=> #<MyNewClass:0xb7b97d0c>
mc = MyClass.new
=> #<MyClass:0xb7b950fc>
ac = Another_class.new
=> #<Another_class:0xb7b82358>

ac.diagram
=> nil
mc.diagram

--
-------------------------------------
Pedro Del Gallego

Email : pedro.delgallego@gmail.com

2

Hi,

You seem to have an extra .class in your modification to Class. You can get
away with this:

class Class
def my_attr_accessor( *args )
    self.class_eval do
       attr_accessor *args
    end
end
end

Explanation: When you're invoking my_attr_accessor from within the class def
of MyNewClass, 'self' is the class object MyNewClass (an instance of Class).
You need to call class_eval on this object. When the block passed into
class_eval is executed, 'self' is again the class object MyNewClass. This is
what allows you to access the private 'attr_accessor' method.

Mushfeq.

···

On 3/11/07, Pedro Del Gallego <pedro.delgallego@gmail.com> wrote:

Hi

Whats the differents between this three classes. Why the
my_attr_accesor dosent create getters and setter clases.

Thanks in advance

#----------------------------------------------------------------------
#
#----------------------------------------------------------------------

class Class
  def my_attr_accessor( *args )
     args.each do |name|
       self.class.class_eval do
          attr_accessor :"#{name}"
       end
     end
  end
end
=> nil

class MyNewClass
  my_attr_accessor :id, :diagram, :telegram
end
=> [:id, :diagram, :telegram]

class MyClass
  attr_accessor :id, :diagram, :telegram
end
=> nil

class Another_class
  def initialize
    ["id","diagram","telegram"].each do |name|
       self.class.class_eval do
          attr_accessor :"#{name}"
       end
     end
  end
end
=> nil

mnc = MyNewClass.new
=> #<MyNewClass:0xb7b97d0c>
mc = MyClass.new
=> #<MyClass:0xb7b950fc>
ac = Another_class.new
=> #<Another_class:0xb7b82358>

ac.diagram
=> nil
mc.diagram

--
-------------------------------------
Pedro Del Gallego

Email : pedro.delgallego@gmail.com

3

You are already where you need to be...

class Class
   def my_attr_accessor( *args )
     attr_accessor *args
   end
end

T.

···

On Mar 11, 7:41 pm, "Mushfeq Khan" <mushfeq.k...@gmail.com> wrote:

Hi,

You seem to have an extra .class in your modification to Class. You can get
away with this:

class Class
def my_attr_accessor( *args )
    self.class_eval do
       attr_accessor *args
    end
end
end