Dynamically register constructor methods in an AbstractFactory at compile time using C++ templates

Question

When implementing a MessageFactory class to instatiate Message objects I used something like:

class MessageFactory 
{
  public:
    static Message *create(in         


        

Answer 1

2: you could use a dynamic container, but then you'd also had to change how the registration etc. For example, you could use a map with an int as key and a function pointer as element:

typedef Message* ( *NewMessageFun )();

template< class tMessage >
Message* NewMessage()
{
  return new tMessage();
};

class PingMessage : public MessageImpl
{
public:
  enum{ _MESSAGE_ID = 10 };
};

class PongMessage
{
public:
  enum{ _MESSAGE_ID = 11 };
}

//factory
std::map< int, NewMessageFun > mymap;
bool Register( const int type, NewMessageFun fun )
{
  if( mymap.contains( type ) )
    return false; //already registered!
  mymap[ type ] = fun;
  return true;
}

template< class tMessage >
bool RegisterAny() //shortcut
{
  return Register( tMessage::_MESSAGE_ID, NewMessage< tMessage > );
}
//

//main
factory.RegisterAny< PingMessage >();
factory.RegisterAny< PongMessage >();
//

Or, in your current code, just use a sensible allocation size and have runtime bounds checking to see there are too much registrations. And maybe supply an 'Unregister' method.