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copied!<p><strong>Update</strong>: This answer made the assumption that some kind of <a href="https://en.wikipedia.org/wiki/File_format#Magic_number" rel="nofollow">magic</a> existed that could be read and passed to the factory, but that's apparently not the case. I'm leaving the answer here because a) I may update it, and b) I like it anyway.</p> <hr> <p>Not hugely different from your own answer, not using C++11 techniques (I've not had a chance to update it yet, or have it return a smart pointer, etc), and not entirely my own work, but this is the factory class I use. Importantly (IMHO) it doesn't call each possible class's methods to find the one that matches - it does this via the map.</p> <pre><code>#include <map> // extraneous code has been removed, such as empty constructors, ... template <typename _Key, typename _Base, typename _Pred = std::less<_Key> > class Factory { public: typedef _Base* (*CreatorFunction) (void); typedef std::map<_Key, CreatorFunction, _Pred> _mapFactory; // called statically by all classes that can be created static _Key Register(_Key idKey, CreatorFunction classCreator) { get_mapFactory()->insert(std::pair<_Key, CreatorFunction>(idKey, classCreator)); return idKey; } // Tries to create instance based on the key static _Base* Create(_Key idKey) { _mapFactory::iterator it = get_mapFactory()->find(idKey); if (it != get_mapFactory()->end()) { if (it->second) { return it->second(); } } return 0; } protected: static _mapFactory * get_mapFactory() { static _mapFactory m_sMapFactory; return &m_sMapFactory; } }; </code></pre> <p>To use this you just declare the base-type, and for each class you register it as a static. Note that when you register, the key is returned, so I tend to add this as a member of the class, but it's not necessary, just neat :) ...</p> <pre><code>// shape.h // extraneous code has been removed, such as empty constructors, ... // we also don't technically need the id() method, but it could be handy // if at a later point you wish to query the type. class Shape { public: virtual std::string id() const = 0; }; typedef Factory<std::string, Shape> TShapeFactory; </code></pre> <p>Now we can create a new derived class, and register it as creatable by <code>TShapeFactory</code>...</p> <pre><code>// cube.h // extraneous code has been removed, such as empty constructors, ... class Cube : public Shape { protected: static const std::string _id; public: static Shape* Create() {return new Cube;} virtual std::string id() const {return _id;}; }; // cube.cpp const std::string Cube::_id = TShapeFactory::Register("cube", Cube::Create); </code></pre> <p>Then we can create a new item based on, in this case, a string:</p> <pre><code>Shape* a_cube = TShapeFactory::Create("cube"); Shape* a_triangle = TShapeFactory::Create("triangle"); // a_triangle is a null pointer, as we've not registered a "triangle" </code></pre> <p>The advantage of this method is that if you create a new derived, factory-generatable class, you don't need to change any other code, providing you can see the factory class and derive from the base:</p> <pre><code>// sphere.h // extraneous code has been removed, such as empty constructors, ... class Sphere : public Shape { protected: static const std::string _id; public: static Shape* Create() {return new Sphere;} virtual std::string id() const {return _id;}; }; // sphere.cpp const std::string Sphere::_id = TShapeFactory::Register("sphere", Sphere::Create); </code></pre> <p>Possible improvements that I'll leave to the reader include adding things like: <code>typedef _Base base_class</code> to <code>Factory</code>, so that when you've declared your custom factory, you can make your classes derive from <code>TShapeFactory::base_class</code>, and so on. The Factory should probably also check if a key already exists, but again... it's left as an exercise.</p>

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