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copied!<p>Heres's a little additional information that may help better understand some of the other technically correct, but shorter answers.</p> <p>In the strictest sense a Class Factory is a function or method that creates or selects a class and returns it, based on some condition determined from input parameters or global context. This is required when the <em>type</em> of object needed can't be determined until runtime. Implementation can be done directly when classes are themselves objects in the language being used, such as Python.</p> <p>Since the primary use of any class is to create instances of itself, in languages such as C++ where classes are not objects that can be passed around and manipulated, a similar result can often be achieved by simulating "virtual constructors", where you call a base-class constructor but get back an instance of some derived class. This must be simulated because constructors can't really be <a href="https://en.wikipedia.org/wiki/Virtual_function" rel="nofollow noreferrer">virtual</a><sup><strong>✶</strong></sup> in C++, which is why such object—not class—factories are usually implemented as standalone functions or static methods.</p> <p><sup><strong>✶</strong> Virtual functions are normally resolved "<a href="https://en.wikipedia.org/wiki/Late_binding" rel="nofollow noreferrer">late</a>" by the actual type of object referenced, but in the case of constructors, the object doesn't exist yet, so the type must be determined by some other means.</sup></p> <p>The best implementations are those that handle new candidate classes automatically when they are added rather than having only a certain finite set currently hardcoded into the factory (although the trade-off is often acceptable if the factory is the only place requiring modification).</p> <p>James Coplien's 1991 book <a href="https://rads.stackoverflow.com/amzn/click/0201548550" rel="nofollow noreferrer"><strong><em>Advanced C++: Programming Styles and Idioms</em></strong></a> has details on one way to implement such virtual generic constructors in C++. There are even better ways to do this using <a href="http://web.archive.org/web/20010216204456/http://www.kirkrader.com/writing/technical/cpp.htm" rel="nofollow noreferrer">C++ templates</a>, but that was not covered in the book which predates their being added to the standard language definition. In fact, C++ templates themselves are class factories since they instantiate a new class whenever they're used with different actual type argument(s). <strong>Update:</strong> I located a 1998 paper he wrote for EuroPLoP titled <a href="http://web.archive.org/web/20080807045811im_/http://users.rcn.com:80/jcoplien/Patterns/C++Idioms/EuroPLoP98.html" rel="nofollow noreferrer"><strong><em>C++ Idioms</em></strong></a> where, among other things, he revises and regroups the idioms in his book into design-pattern form à la the 1994 <a href="https://rads.stackoverflow.com/amzn/click/0201633612" rel="nofollow noreferrer"><strong>Design Patterns: Elements of Re-Usable Object-Oriented Software</strong></a> book. Note especially the <a href="http://web.archive.org/web/20080807045811im_/http://users.rcn.com:80/jcoplien/Patterns/C++Idioms/EuroPLoP98.html#VirtualConstructor" rel="nofollow noreferrer">Virtual Constructor</a> section (which uses his <a href="http://web.archive.org/web/20080807045811im_/http://users.rcn.com:80/jcoplien/Patterns/C++Idioms/EuroPLoP98.html#EnvelopeLetter" rel="nofollow noreferrer">Envelope/Letter</a> pattern structure).</p> <p>See also the related answers here for the question <a href="https://stackoverflow.com/questions/456672/class-factory-in-python">Class factory in Python</a>. Also see <a href="http://www.drdobbs.com/conversations-abstract-factory-template/184403786" rel="nofollow noreferrer"><em>Abstract Factory, Template Style</em></a> which is a 2001 Dr. Dobb's article also about implementing them with C++ Templates.</p>

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