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    <blockquote> <p>Don't you use <code>class_eval</code> to define a class method and <code>instance_eval</code> when you want to define an instance method?</p> </blockquote> <p>Unfortunately, it is not as straightforward as that.</p> <p>First take a closer look at what the examples of <code>class_eval</code> are doing. <code>class_eval</code> is a method which comes from Ruby's <a href="http://www.ruby-doc.org/core/classes/Module.html#M001650" rel="nofollow noreferrer">module class</a> so can be called on any class or module. When you use <code>String.class_eval</code> you are evaluating the given code in the context of the class. i.e. when you write <code>String.class_eval("def len; size; end")</code> it's exactly like you reopened the class and typed the code passed to <code>class_eval</code> e.g.</p> <pre><code>class String def len size end end </code></pre> <p>Thus to add a class method using class_eval you would write <code>String.class_eval("def self.empty; ''; end")</code> which has the same effect as:</p> <pre><code>class String def self.empty '' end end </code></pre> <p><code>instance_eval</code> is defined in Ruby's <a href="http://www.ruby-doc.org/core/classes/Object.html#M000334" rel="nofollow noreferrer">Object class</a> so is available on any Ruby object. In the general case it can be used to add a method to a specific instance. e.g. if we have a String <code>str</code> and say:</p> <pre><code>str.instance_eval("def special; size; end") </code></pre> <p>Then this will alias <code>special</code> to <code>size</code> just for <code>str</code> but not for any other String object:</p> <pre><code>irb(main):019:0&gt; "other".special NoMethodError: undefined method `special' for "other":String from (irb):19 </code></pre> <p>To understand what is going on with String.instance_eval remember that the class String is itself an object (an instance of the class <code>Class</code>) and that there is such a singleton instance object of every class defined. When you use <code>String.instance_eval</code> you are evaluating the given code in the context of the <code>String</code> instance object. i.e. it is equivalent to reopening String's metaclass and typing the code passed e.g.</p> <pre><code>class String class &lt;&lt; self def empty '' end end end </code></pre>
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