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PORuby's def and instance_eval vs. class_eval
 

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    <p>I'm reading the Metaprogramming section of <em>Programming Ruby 1.9</em> and I'm having trouble understanding what's going on <em>internally</em> between <code>class_eval</code>/<code>class_exec</code> vs. <code>instance_eval</code>/<code>instance_exec</code>.</p> <p>So first of all, my understanding is that <code>def</code> adds a method to the method table of <code>self</code> (the class object):</p> <pre><code>class A puts self # =&gt; A def foo; 42; end # added to the method table of self, so becomes an instance method end A.new.foo # =&gt; 42 </code></pre> <p>And if we use <code>class_eval</code>, we get the same behavior:</p> <pre><code>A.class_eval do puts self # =&gt; A def bar; 42; end # same as above end A.new.bar # =&gt; 42 </code></pre> <p>But somehow in the <code>instance_eval</code> case, things are different:</p> <pre><code>A.instance_eval do puts self # =&gt; A def baz; 42; end # added to the method table of an anonymous # singleton class of self, so becomes a class method end puts A.baz # =&gt; 42 s = 'string' s.instance_eval do ... end # same behavior, so now def creates an instance method </code></pre> <p>So I understand the functional difference between <code>class_eval</code> and <code>instance_eval</code>.</p> <p>But the contexts inside the <code>class_eval</code> and <code>instance_eval</code> blocks look <em>exactly</em> the same to me -- in particular, <code>self</code> points to the same object, and the <code>local_variables</code> are the same. So what's going on inside the blocks (internally) that's making <code>def</code> act different?</p> <p>Is there some piece of documentation I could read? The RDoc for <a href="http://ruby-doc.org/core/classes/Object.html#M000334" rel="noreferrer">instance_eval</a> and <a href="http://ruby-doc.org/core/classes/Module.html#M001650" rel="noreferrer">class_eval</a> doesn't help. Looking at the source, <a href="http://ruby-doc.org/core/classes/Object.src/M000334.html" rel="noreferrer">instance_eval</a> seems to set up a singleton class object whereas <a href="http://ruby-doc.org/core/classes/Module.src/M001650.html" rel="noreferrer">class_eval</a> doesn't -- but is this difference visible outside the C code, on the Ruby level?</p>
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