Metaprograming: custom initialize

Question

I have to write a my_initialize method in class Class, so that it works in this manor, when used in another class:

class Person my_initialize :name, :surname end

is equivalent to :

class Person def initialize(name, surname) @name, @surname = name, surname end end

It also has to raise an ArgumentError if a wrong number of arguments is passed. For example Person.new("Mickey") is invalid. I know that my code should look something like:

class Class
  def my_initialize(*args)
    args.each do |arg|      
     self.class_eval("?????")          
    end
  end 
end

I just started to read metaprogramming, but can't find anything useful for my problem. Any ideas how to do this task?

Keep on reading until you discover define_method. :)

zwippie
– zwippie

2014-11-19 23:40:54 +00:00
Commented Nov 19, 2014 at 23:40 — zwippie
– zwippie, Commented Nov 19, 2014 at 23:40

Adrian · Accepted Answer · 2014-11-20 00:21:50Z

2

class Class
  def my_initialize(*vars)
    define_method :initialize do |*args|
      if args.length != vars.length
        raise ArgumentError, 'wrong number of arguments'
      end
      vars.zip(args).each do |var, arg|
        instance_variable_set :"@#{var}", arg
      end
    end
  end
end

class C
  my_initialize :a, :b
end

The Module#define_method method takes a method name and block and defines the method for that module. In this case, the module is C. The Object#instance_variable_set method takes an instance variable name and a value and sets it. The instance of Object in this case would be an instance of C.

By the way, it is best to avoid using methods where you pass a string of code in to be evaluated. I would recommend passing blocks instead.

answered Nov 20, 2014 at 0:21

Adrian

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1 Comment

Cary Swoveland Over a year ago

Good one, Adrian! I thought you might have to make initialize private, but no, Ruby takes care of that: C.private_instance_methods(false) => [:initialize].

Cary Swoveland · Accepted Answer · 2014-11-20 02:26:35Z

1

Here's another way that does not use define_method.

class Class
  def my_initialize(*vars)
    str = "def initialize(*args)
             raise ArgumentError if args.size != #{vars.size}
             #{vars}.zip(args).each do |var, arg|
               instance_variable_set(\"@\#{var}\", arg)
             end
           end"
    class_eval str
  end
end

class C
  my_initialize :a, :b
end

c = C.new("Betty", "Boop")
  #=> #<C:0x00000102805428 @a="Betty", @b="Boop">

C.private_instance_methods(false)
  #=> [:initialize]
c.instance_variables
  #=> [:@a, :@b]

C.new("Betty")
  #=> ArgumentError

answered Nov 20, 2014 at 2:26

Cary Swoveland

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