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POIncremental Refactoring Toward Null Object: C++ overloading
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copied!<p>We've got a large legacy C++/ObjC++ application that could benefit from Null Object to replace thousands of redundant checks:</p> <pre><code>Thing *thing=CurrentSelection(); if (thing) thing->Drill() </code></pre> <p>If, when nothing was selected, CurrentSelection returned an instance of NotAThing</p> <pre><code>class NotAThing public: Thing { public: virtual ~NotAThing(){} virtual bool IsValid() const { return false;} virtual void Drill() {} }; </code></pre> <p>We could get rid of almost all those checks. For a well contained class with few clients, Null Object can be a Good Thing. But this class is ubiquitous; dozens of classes depend on it and there are thousands of checks.</p> <p>Can we incrementally refactor <em>toward</em> Null Object?</p> <p>One approach would be to overload operators on NotAThing to lie:</p> <p>We'd need to overload !, !=, and probably the casts to bool and int. And that still won't catch the explicit comparison "thing==nil". And I can already hear Scott Meyers (<em>Effective C++</em>) warning that this was lies madness. At minimum, it's a code smell. </p> <p>And, while we might incrementally back out some of these and lean on the compiler in order to find and remove unneeded nil tests, I worry about coercion chains causing confusing and unexpected results -- one reason to avoid those casts.</p> <p>On the other hand, there’s all those nil checks. They’re a code smell, too.</p> <p>What’s the wisdom for large-scale refactoring toward null object? The mechanics in Fowler and in Kerievsky are sound, but they’re designed for classes with a small number of clients</p>

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