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    <p>Each of these examples fall into different cases.</p> <p>If you are writing methods that apply to all objects, then you open the <code>Object</code> class so all objects can access it. If you are writing methods that apply to all modules, then you open <code>Module</code>. Whenever you open a class to add methods, the methods should apply to <em>all instances of the class and nothing else</em>.</p> <p>Extending the <code>Kernel</code> module is different: people do this to add methods that should be available to every scope, but not really as methods to be explicitly called on an object, by making them private.</p> <p>When you are outside of any <code>class</code> or <code>module</code> statement, you are in the scope of the <code>main</code> object, and methods you define default to being private methods of <code>Object</code>. This is fine for small or simple programs, but you will eventually want to use proper modules as namespaces to organize your methods.</p> <p>As a final note on the subject, you always need to be sure that you really want methods you add to built-in classes and modules to be available to <em>everything in your application, including external inclusions</em> because they all share the built-ins.</p> <p>Now to apply this to answer your question. Because you are defining a method that creates accessors for class variables, you should put it in the class <code>Class</code> as it applies to all classes and nothing else. Finally, you will likely only use it in class definitions (within a <code>class</code> statement), so we should make it private:</p> <pre><code>class Class private def c_attr_accessor(name) # ... end end class User c_attr_accessor :class_variable_name # ... end </code></pre> <p>If you don't really need it in every class (maybe just a few), then create a "mixin module" to extend every class that needs this feature:</p> <pre><code>module ClassVariableAccessor private def c_attr_accessor(name) # ... end end class User extend ClassVariableAccessor c_attr_accessor :class_variable_name # ... end </code></pre> <p>Note that you are using <code>Object#extend</code> to add <code>c_attr_accessor</code> only to the object <code>User</code> (remember that classes are objects; you will hear that a lot if you are new to Ruby metaprogramming).</p> <p>There is another way to implement the last example, which works by explicitly extending its base class through the <code>Module#included(base_class)</code> "hook method" called whenever the module included, and the base class is passed to <code>base_class</code>:</p> <pre><code>module ClassVariableAccessor def included(base_class) base_class.extend ClassMethods end module ClassMethods def c_attr_accessor(name) # ... end end end class User include ClassVariableAccessor c_attr_accessor :class_variable_name # ... end </code></pre> <p>I recommend this last solution because it is the most general and uses a simple interface that should not need to be updated. I hope this is not too much!</p>
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    1. POWhen should I use "class Object", "class Module", "module Kernel" and nothing?
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    1. POWhen should I use "class Object", "class Module", "module Kernel" and nothing?
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    1. COSmall clarification: Methods defined in the top scope become private instance methods of Object, not singleton methods of the main object.
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    2. COGood point. I wasn't clear about that...
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    3. COThank you! I learned a lot from your post! :)
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