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    <p>99% of the time I would recommend against removing an instance variable, because it's extra code for no extra benefit.</p> <p>When you're writing code, generally you're trying to solve a real-world problem. With the instance variable, some questions to ask are:</p> <ul> <li>What real world concept am I trying to model with the various states the variable can be in?</li> <li>What am I going to do with the values stored in the variable?</li> </ul> <p>If you're just trying to blank out the continent value stored in a <code>World</code> object, you can set <code>@continent</code> to <code>nil</code> as dustmachine says. This will work fine for the 99% of the cases. (Accessing a removed instance variable will just return <code>nil</code> anyway.)</p> <p>The only possible case (I can think of) when removing the instance variable could be useful is when you're caching a value that may be <code>nil</code>. For example:</p> <pre><code>class Player def score(force_reload = false) if force_reload # purge cached value remove_instance_variable(:@score) end # Calling 'defined?' on an instance variable will return false if the variable # has never been set, or has been removed via force_reload. if not defined? @score # Set cached value. # Next time around, we'll just return the @score without recalculating. @score = get_score_via_expensive_calculation() end return @score end private def get_score_via_expensive_calculation if play_count.zero? return nil else # expensive calculation here return result end end end </code></pre> <p>Since <code>nil</code> is a meaningful value for <code>@score</code>, we can't use <code>nil</code> to indicate that the value hasn't been cached yet. So we use the undefined state to tell us whether we need to recalculate the cached value. So there are 3 states for <code>@score</code>:</p> <ol> <li><code>nil</code> (means user has not played any games)</li> <li>number (means user played at least once but did not accrue any points)</li> <li>undefined (means we haven't fetched the calculated score for the Player object yet).</li> </ol> <p>Now it's true that you could use another value that's not a number instead of the undefined state (a symbol like <code>:unset</code> for example), but this is just a contrived example to demonstrate the idea. There are cases when your variable may hold an object of unknown type.</p>
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