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copied!<p>In fact, it's a more general issue than serialization called <code>Virtual Constructor</code>.</p> <p>The traditional approach is to a <code>Factory</code>, which based on an ID returns the right derived type. There are two solutions:</p> <ul> <li>the <code>switch</code> method as you noticed, though you need to allocate on the heap</li> <li>the <code>prototype</code> method</li> </ul> <p>The prototype method goes like so:</p> <pre><code>// Cloneability class Base { public: virtual Base* clone() const = 0; }; class Derived: public Base { public: virtual Derived* clone() const { return new Derived(*this); } }; // Factory class Factory { public: Base* get(std::string const& id) const; void set(std::string const& id, Base* exemplar); private: typedef std::map < std::string, Base* > exemplars_type; exemplars_type mExemplars; }; </code></pre> <p>It is somewhat traditional to make the <code>Factory</code> a singleton, but it's another matter entirely.</p> <p>For deserialization proper, it's easier if you have a virtual method <code>deserialize</code> to call on the object. </p> <p><strong>EDIT:</strong> How does the Factory work ?</p> <p>In C++ you can't create a type you don't know about. The idea above is therefore that the task of building a <code>Derived</code> object is given to the <code>Derived</code> class, by way of the <code>clone</code> method.</p> <p>Next comes the <code>Factory</code>. We are going to use a <code>map</code> which will associate a "tag" (for example <code>"Derived"</code>) to an instance of an object (say <code>Derived</code> here).</p> <pre><code>Factory factory; Derived derived; factory.set("Derived", &derived); </code></pre> <p>Now, when we want to create an object which type we don't know at compile time (because the type is decided on the fly), we pass a tag to the factory and ask for an object in return.</p> <pre><code>std::unique_ptr<Base> base = factory.get("Derived"); </code></pre> <p>Under the cover, the <code>Factory</code> will find the <code>Base*</code> associated to the <code>"Derived"</code> tag and invoke the <code>clone</code> method of the object. This will actually (here) create an object of runtime-type <code>Derived</code>.</p> <p>We can verify this by using the <code>typeid</code> operator:</p> <pre><code>assert( typeid(base) == typeid(Derived) ); </code></pre>

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