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copied!<p>A lot of the magic you see in Rubyland has to do with metaprogramming, which is simply writing code that writes code for you. Ruby's <code>attr_accessor</code>, <code>attr_reader</code>, and <code>attr_writer</code> are all simple metaprogramming, in that they create two methods in one line, following a standard pattern. Rails does a whole lot of metaprogramming with their relationship-management methods like <code>has_one</code> and <code>belongs_to</code>.</p> <p>But it's pretty simple to create your own metaprogramming tricks using <code>class_eval</code> to execute dynamically-written code.</p> <p>The following example allows a wrapper object to forwards certain methods along to an internal object:</p> <pre><code>class Wrapper attr_accessor :internal def self.forwards(*methods) methods.each do |method| define_method method do |*arguments, &block| internal.send method, *arguments, &block end end end forwards :to_i, :length, :split end w = Wrapper.new w.internal = "12 13 14" w.to_i # => 12 w.length # => 8 w.split('1') # => ["", "2 ", "3 ", "4"] </code></pre> <p>The method <code>Wrapper.forwards</code> takes symbols for the names of methods and stores them in the <code>methods</code> array. Then, for each of those given, we use <code>define_method</code> to create a new method whose job it is to send the message along, including all arguments and blocks.</p> <p>A great resource for metaprogramming issues is <a href="http://viewsourcecode.org/why/hacking/seeingMetaclassesClearly.html" rel="noreferrer">Why the Lucky Stiff's "Seeing Metaprogramming Clearly"</a>.</p>

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