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copied!<p><strong>EDIT:</strong> below code was tested with 1.8.7 and 1.9.1...it seems the situation is different again with 1.9.2 :/</p> <p>The situation actually isn't that straight forward. There are differences in behaviour depending on whether you're using 1.8 or 1.9 and whether you're using <code>class_eval</code> or <code>instance_eval</code>.</p> <p>The examples below detail the behaviour in most situations. </p> <p>I also included the behaviour of constants, for good measure, as their behaviour is similar to, but not exactly the same as, class variables.</p> <h2>Class variables</h2> <p><strong><code>class_eval</code> in Ruby 1.8:</strong></p> <pre><code>class Hello @@foo = :foo end Hello.class_eval { @@foo } #=> uninitialized class variable </code></pre> <p><strong><code>class_eval</code> in Ruby 1.9:</strong></p> <pre><code>Hello.class_eval { @@foo } #=> :foo </code></pre> <p>So class variables <strong>are</strong> looked up in 1.9 (but not in 1.8) when using <code>class_eval</code> </p> <p><strong><code>instance_eval</code> in Ruby 1.8 <em>and</em> 1.9</strong></p> <pre><code>Hello.instance_eval { @@foo } #=> uninitialized class variable Hello.new.instance_eval { @@foo } #=> uninitialized class variable </code></pre> <p>It appears class variables are <strong>not</strong> looked up in 1.8 <strong>or</strong> 1.9 when using <code>instance_eval</code></p> <p>What is also interesting is the case for <strong>constants</strong>:</p> <h2>Constants</h2> <p><strong><code>class_eval</code> in Ruby 1.8</strong></p> <pre><code>class Hello Foo = :foo end Hello.class_eval { Foo } #=> uninitialized constant </code></pre> <p><strong><code>class_eval</code> in Ruby 1.9</strong></p> <pre><code>Hello.class_eval { Foo } #=> :foo </code></pre> <p>So, as with class variables, constants are looked up in 1.9 but <strong>not</strong> in 1.8 for <code>class_eval</code></p> <p><strong><code>instance_eval</code> in Ruby 1.8</strong></p> <pre><code>Hello.instance_eval { Foo } #=> uninitialized constant Hello.new.instance_eval { Foo } #=> uninitialized constant </code></pre> <p><strong><code>instance_eval</code> in Ruby 1.9</strong></p> <pre><code>Hello.instance_eval { Foo } #=> uninitialized constant Hello.new.instance_eval { Foo } #=> :foo </code></pre> <p>It appears that constant lookup is not quite analogous to class variable look up for Ruby 1.9. A <code>Hello</code> instance <strong>does</strong> get access to the constant while the <code>Hello</code> class does not.</p>

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