Can a factory somehow determine all possible classes, by looking in a particular file?

问题

Is it possible to create a Factory that generates all derived classes (and class names) at runtime, based on all the classes that are specified in a particular file?

For example, given a specific file where users are constantly adding derived classes:

class Laptop: public Computer {
  public:
     virtual void Run(){mHibernating = false;}
     virtual void Stop(){mHibernating = true;}
 private:
     bool mHibernating; // Whether or not the machine is hibernating
 };

class Desktop: public Computer {
 public:
     virtual void Run(){mOn = true;}
     virtual void Stop(){mOn = false;}
 private:
     bool mOn; // Whether or not the machine has been turned on
 };

// ....[more classes derived from Computer]...

Can a Factory generate a list of possible classes to map to, based on their name?

 class ComputerFactory {
 public:
     static Computer *NewComputer(const std::string &description)
     {
         if(description == "Laptop")
             return new Laptop;
         } else if (description == "Desktop") {
             return new Desktop;
         } else if  // ...[more comparisons based on the classes defined at runtime]...
         } else return NULL;
     }
 };

回答1:

You can write a parser and have the class process the input file, but I doubt that's what you want.

You can also use the crtp pattern to your advantage:

template<class T>
struct Base
{
   Base()
   { 
      reg;
   }
   virtual std::string name() = 0;
   static bool reg;
   static bool init() 
   { 
      T t; 
      Factory::registerClass(t.name());
      return true;
   }
};

template<class T>
bool Base<T>::reg = Base<T>::init();

source

You'd then derive classes as

struct Derived1 : Base<Derived1> 
{
 ...
}

This would automatically register your class with Factory given its name (you have to implement name in non-abstract classes because it's pure).

Additionally, you can pass a callback to registerClass that knows how to create your derived class.

来源：https://stackoverflow.com/questions/23626803/can-a-factory-somehow-determine-all-possible-classes-by-looking-in-a-particular