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POWhen do programmers use Empty Base Optimization (EBO)
 

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    <p>I was reading about Empty Base Optimization(EBO). While reading, the following questions popped up in my mind:</p> <ol> <li><p>What is the point of using Empty class as base class when <strong>it contributes nothing to the derived classes (neither functionality-wise, nor data-wise)</strong>?</p></li> <li><p>In <a href="http://www.informit.com/guides/content.aspx?g=cplusplus&amp;seqNum=319">this article</a>, I read this:</p></li> </ol> <blockquote> <p>//S is empty<br/> class struct T : S <br/> {<br/> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int x;<br/> };</p> <p>[...]</p> <p>Notice that we didn’t lose any data or code accuracy: when you create a standalone object of type S, the object’s size is still 1 (or more) as before; only when S is used as base class of another class does its memory footprint shrink to zero. To realize the impact of this saving, imagine a vector that contains 125,000 objects. <strong>The EBO alone saves half a megabyte of memory!</strong></p> </blockquote> <p>Does it mean that if we don't use "S" as base class of "T", we would <strong>necessarily</strong> consume double of megabyte of memory? I think, the article compares two different scenarios which I don't think is correct.</p> <p>I would like to know a real scenario when EBO can proven to be useful.(means, in the same scenario, we would <em>necessarily</em> be at loss IF we don't use EBO!).</p> <h1>Please note that if your answer contains explanations like this :</h1> <p>The whole point is that an empty class has non-zero size, but when derived or deriving it can have zero size, then I'm NOT asking that, as I know that already. My question is, why would anyone derive his class from an empty class in the first place? <strong>Even if he doesn't derive and simply writes his class (without any empty base), is he at loss in ANY way?</strong></p>
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    1. CO@Suma... what did you change in my post? :-/.. I'm unable to figure out..
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    2. COWhy do you say the base class contributes nothing functionality-wise?
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    3. CO@jalf .... so far I didn't see any example.. by the way, by functionality I meant functions that do something, rather than nothing
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