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copied!<p><code>class << self</code> is more than just a way of declaring class methods (though it can be used that way). Probably you've seen some usage like:</p> <pre><code>class Foo class << self def a print "I could also have been defined as def Foo.a." end end end </code></pre> <p>This works, and is equivalent to <code>def Foo.a</code>, but the way it works is a little subtle. The secret is that <code>self</code>, in that context, refers to the object <code>Foo</code>, whose class is a unique, anonymous subclass of <code>Class</code>. This subclass is called <code>Foo</code>'s <em>eigenclass</em>. So <code>def a</code> creates a new method called <code>a</code> in <code>Foo</code>'s eigenclass, accessible by the normal method call syntax: <code>Foo.a</code>.</p> <p>Now let's look at a different example:</p> <pre><code>str = "abc" other_str = "def" class << str def frob return self + "d" end end print str.frob # => "abcd" print other_str.frob # => raises an exception, 'frob' is not defined on other_str </code></pre> <p>This example is the same as the last one, though it may be hard to tell at first. <code>frob</code> is defined, not on the <code>String</code> class, but on the eigenclass of <code>str</code>, a unique anonymous subclass of <code>String</code>. So <code>str</code> has a <code>frob</code> method, but instances of <code>String</code> in general do not. We could also have overridden methods of String (very useful in certain tricky testing scenarios).</p> <p>Now we're equipped to understand your original example. Inside <code>Foo</code>'s initialize method, <code>self</code> refers not to the class <code>Foo</code>, but to some particular <em>instance</em> of <code>Foo</code>. Its eigenclass is a subclass of <code>Foo</code>, but it is not <code>Foo</code>; it couldn't be, or else the trick we saw in the second example couldn't work. So to continue your example:</p> <pre><code>f1 = Foo.new(:weasels) f2 = Foo.new(:monkeys) f1.weasels = 4 # Fine f2.monkeys = 5 # Also ok print(f1.monkeys) # Doesn't work, f1 doesn't have a 'monkeys' method. </code></pre> <p>Hope this helps.</p>

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