From: Brian Candler <b.candler@...>
Date: 2009-12-15T19:07:07+09:00
Subject: Re: meta-programming

Rajinder Yadav wrote:
> How would I declared a, 'class Person' dynamically

Option 1: since you're using string eval already, you could just do the 
same.

eval "class Person; end"

Option 2:

Person = Class.new         # superclass is Object
Person = Class.new(Mammal) # superclass is Mammal

I find this isn't often done in practice though. A program which doesn't 
know in advance which classes it has can be a bit too dynamic :-) You 
would probably register your classes somewhere to be able to find them. 
In a Hash is one option; under a Module is another, so you can use

   MyModule.constants

to find them all.

You can make your classes anonymous if you don't bind them to a 
constant:

   my_klass = Class.new

> and then add methods and attributes to it?

As you've done before, using string eval, is one way.

Another way:

    Person.class_eval { define_method(:greet) { puts "Hello" } }

This means that the method is a closure, and can access variables 
outside (unlike 'def' which starts a fresh scope), and this can be 
useful sometimes.

Another way: put the method(s) of interest in module(s), then include 
the relevant ones.

    Person.class_eval { include Greeter }

ActiveRecord is a plentiful source of examples.
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