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    <p>I think your confusion comes from the fact that def does not depend on the current self, you might think about it as being a "current class" that has it's own rules.</p> <p>Following your examples:</p> <pre><code>class A # defs here go to A puts self # =&gt; A class &lt;&lt; self #defs here go to A's eigenclass end end A.class_eval do #defs here go to A end A.instance_eval do #defs here go to A's eigenclass end s = "Hello World" class &lt;&lt; s #defs here go to s's eigenclass end </code></pre> <p>Here's the portion of the chapter that talks about the issue and it's pretty clear about the behaviour </p> <blockquote> <p>class_eval and instance_eval both set self for the duration of the block. However, they differ in the way they set up the environment for method definition. class_eval sets things up as if you were in the body of a class definition, so method definitions will define instance methods In contrast, calling instance_eval on a class acts as if you were working inside the singleton class of self. Therefore, any methods you define will become class methods.</p> </blockquote> <p>The only thing I think is worth adding is that you can call instance_eval in any object, not just classes, and the behaviour doesn't change but has different consequences.</p> <p>Some relevant reading:</p> <p><a href="http://blog.jayfields.com/2007/03/ruby-instanceeval-and-classeval-method.html" rel="nofollow noreferrer">Ruby: instance_eval and class_eval method definitions</a></p> <p><a href="http://www.pragprog.com/screencasts/v-dtrubyom/the-ruby-object-model-and-metaprogramming" rel="nofollow noreferrer">Chapter 4 of this most excelent series</a></p>
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    1. COHm, I see -- that makes sense. Is there any way to inspect that "current class" in Ruby code?
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    2. COI think in `A.instance_eval`, you meant to write `#defs here go to A's eigenclass`? Or am I mistaken?
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    3. COIt's not exactly simple, I will post some relevant links in the answer
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