Re: Problem implementing an object factory

From:

Stephen Torri <storri@torri.org>

Newsgroups:

comp.lang.c++

Date:

Wed, 12 Sep 2007 14:35:36 GMT

Message-ID:

<YOSFi.14$Na1.9@newsfe03.lga>

On Mon, 10 Sep 2007 07:24:18 +0000, James Kanze wrote:

If he means that the user of the factory doesn't have to do
anything, the usual solution is to use the singleton idiom for
the factory (to avoid problems in order of initialization), and
then use static objects to register the the factories with the
map; if the map maps to functional objects, rather than to
pointers to functions, you need a static object for each type
anyway, so you might as well put the registration code in the
constructor.

Was this what you had in mind? I tried to create a singleton using the example I found in Modern C++ Design.
I am not sure about the private copy constructor or assignment operator.

Stephen

-------------------------

#include <map>
#include <iostream>

namespace tool
{
  namespace component
  {
    class Component
    {};

    class Apple : public Component
    {
    public:

      static const int ID = 1;

      static Component* Create()
      {
    return new Apple;
      }

    };

    class Blueberry : public Component
    {
    public:

      static const int ID = 5;

      static Component* Create()
      {
    return new Blueberry;
      }
    };
  }

  namespace infrastructure
  {

    class Component_Factory
    {
    public:
      typedef component::Component* (*CreateComponentCallback)();

      static Component_Factory& Instance()
      {
    static Component_Factory obj;
    return obj;
      }

      bool RegisterComponent ( int component_id,
                   CreateComponentCallback call_func )
      {
    return m_callbacks.insert ( CallbackMap::value_type ( component_id, call_func ) ).second;
      }

      bool UnregisterComponent ( int component_id )
      {
    return m_callbacks.erase ( component_id ) == 1;
      }

      component::Component* CreateComponent ( int component_id )
      {
    CallbackMap::const_iterator pos = m_callbacks.find ( component_id );

    if ( pos == m_callbacks.end() )
      {
        std::cerr << "Unknown Component ID" << std::endl;
        abort();
      }

    return (pos->second)();
      }

    private:
      typedef std::map<int, CreateComponentCallback> CallbackMap;

      CallbackMap m_callbacks;

      Component_Factory()
      {
    this->RegisterComponent ( tool::component::Apple::ID, &tool::component::Apple::Create );
    this->RegisterComponent ( tool::component::Blueberry::ID, &tool::component::Blueberry::Create );
      }

      Component_Factory ( Component_Factory const& rhs )
      {
    //m_callbacks ( rhs.m_callbacks.begin(), rhs.m_callbacks.end() );
      }

      Component_Factory& operator= ( Component_Factory const& rhs )
      {
    Component_Factory temp ( rhs );
    swap ( temp );

    return *this;
      }

      void swap ( Component_Factory& rhs )
      {
    std::swap ( m_callbacks, rhs.m_callbacks );
      }
    };

  }

}


int main (int, char**)
{
  tool::component::Component* apple_ptr = tool::infrastructure::Component_Factory::Instance().CreateComponent ( tool::component::Apple::ID );

  std::cout << "ID = " << static_cast<tool::component::Apple*>(apple_ptr)->ID << std::endl;

  tool::component::Component* berry_ptr = tool::infrastructure::Component_Factory::Instance().CreateComponent ( tool::component::Blueberry::ID );

  std::cout << "ID = " << static_cast<tool::component::Blueberry*>(berry_ptr)->ID << std::endl;

  tool::infrastructure::Component_Factory::Instance().UnregisterComponent ( tool::component::Apple::ID );
  tool::infrastructure::Component_Factory::Instance().UnregisterComponent ( tool::component::Blueberry::ID );

  delete apple_ptr;
  delete berry_ptr;

  return 0;
}