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    <p>To "attach additional behavior to a class in runtime" in most any OO language I recommend the <a href="http://en.wikipedia.org/wiki/Strategy_pattern" rel="noreferrer">Strategy</a> design pattern -- have the class (and/or its instances) hold (through a pointer in C++, a [reseatable] reference in other languages) an instance of a suitable interface / abstract class known as the "strategy interface" (with one method [virtual in C++ of course, non-final in languages that have <code>final</code>, etc -- IOW, an <em>overridable</em> method!-)] per point of extensibility), and generally supply getter and setter methods (or properties or whatever's appropriate to the specific language) to access and change that instance.</p> <p>Lastly, the class or instance must delegate all appropriate functionality through the methods of the strategy interface instance it's holding.</p> <p>I often recommend this "high-ceremony" approach (for this specific purpose) even in dynamic languages such as Python or Ruby which would also allow more informal approaches via duck typing and the ability of an object of class to directly reach into the internals of another -- such dynamic abilities are generally speaking quite useful, but for this specific purpose (as I think of "changing a class behavior at runtime") a more highly architected and controlled approach, in my experience, leads to better-maintainable and clearer code (this bit is controversial: many developers in dynamic-language communities like "monkey patching" approaches even in such situations, but a dozen years of successful development practice in dynamic languages, mostly Python, have made me otherwise inclined).</p> <p>In appropriate case you can modify the fundamental Strategy DP approach in various ways; for example, when the modifiable functionality falls neatly into a few cohesive groups, it's best to "fragment" the Strategy object into several simple and cohesive ones (DrawingStrategy, PersistenceStrategy, BusinessRulesStrategy, and so forth).</p> <p>This whole approach does not take the place of performing proper analysis and consequently proper design, as it won't allow extension of a class functionality along an axis that was not originally taken into consideration; rather, the approach is intended as a proper way to <em>architect</em> a well-thought-out design, providing "hooks" for extensibility in a well-controlled manner. If new considerations come into play it may still well be necessary to iterate and perfect the classes' design to cover such new ideas. But then, iterative development (including tweaks and extensions to the original design) is inevitable in any rich, complex real-world project -- the Strategy DP is just one arrow in your quiver to help make the process more orderly and effective.</p>
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    1. USAlex Martelli
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    1. COBeautiful answer!
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      1. USKarim Agha
 

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