How to Elegantly Serialize Classes to a Stream in C Using the Factory Pattern?

Serialization to a Stream for a Class Demystified

When faced with the need for object serialization in C , many resort to a familiar pattern that involves class ID-based switching:

<code class="cpp">class Serializable {
  public:
    static Serializable *deserialize(istream &amp;is) {
        int id;
        is >> id;
        switch(id) {
          case EXAMPLE_ID:
            return new ExampleClass(is);
          //...
        }
    }

    void serialize(ostream &amp;os) {
        os << getClassID();
        serializeMe(os);
    }

  protected:
    int getClassID()=0;
    void serializeMe(ostream &amp;os)=0;
};

While this approach may seem intuitive, it suffers from potential pitfalls:

The Factory Pattern Approach

A more elegant and extensible solution is to employ the factory pattern. Instead of relying on class IDs, a factory class creates objects based on a registered set of key-function pairs. The functions return instances of the corresponding classes.

<code class="cpp">template<typename K, typename T>
class Factory {
    typedef T *(*CreateObjectFunc)();
    std::map<K, CreateObjectFunc> mObjectCreator;

    template<typename S> 
    static T* createObject(){ 
        return new S(); 
    }

    template<typename S> 
    void registerClass(K id){ 
        mObjectCreator.insert( std::make_pair<K,CreateObjectFunc>(id, &createObject<S> ) ); 
    }

    T* createObject(K id){
        typename std::map<K, CreateObjectFunc>::iterator iter = mObjectCreator.find(id); 
        if (iter == mObjectCreator.end()){ 
            return NULL;
        }
        return ((*iter).second)();
    }
};</code>

In this implementation, each class of interest is registered with the factory, associating its class ID with a creation function. When an object is to be deserialized, the factory is queried for the corresponding class ID. If found, the creation function is invoked to instantiate the object.

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