How can I instantiate an object knowing only its name? [duplicate]

Question

Possible Duplicate:
Is there a way to instantiate objects from a string holding their class name?

In C++, I want to have my user enter the object type name to be created at run-time, and, depending on the string I get from them, the program will instantiate the correct object (in short, I'm implementing factory method pattern). However, if the program has to support a new object type, then modifying existing code is not allowed.

So is it possible to remove all the if...else if...else if... stuff from the method, and still have my program instantiate a correct object of a specific product type (out of many, which are known only at compile time)?

My searching around got me this link: Is there a way to instantiate objects from a string holding their class name? and it seems it's what I want but I can't understand the code at all.

Any help would be really appreciated.

Answer 1

This will only work if all the required classes are derived from some common base class, and you will generally be limited to using the base interface (though you can work around that with some additional effort). Here's one approach:

// Immutable core code:

#include <map>
#include <string>

class Base
{
  typedef Base * (*crfnptr)(const std::string &);
  typedef std::map<std::string, crfnptr> CreatorMap;

  static CreatorMap creators;

public:
  virtual ~Base() { }
  Base * clone() const { return new Base(*this); }

  static Base * create_from_string(std::string name)
  {
    CreatorMap::const_iterator it = creators.find(name);
    return it == creators.end() ? NULL : it->first();
  }

  static void register(std::string name, crfnptr f)
  {
    creators[name] = f;
  }
};

Now you can add new derived classes from your new code:

// your code:

#include "immutable_core.hpp"

class Foo : public Base
{
public:
  Foo * clone() const { return new Foo(*this); }
  static Foo * create() { return new Foo; }
};

Base::register("Foo", &Foo::create);

To create a class, you simply call Base * p = Base::create_from_string("Foo");.