implement factory pattern for products with conditional compiling

I'd like to implement factory (or some other pattern) in a way that will allow me to compile the code without introducing type dependency.

enum CarType
{
 BMW,
 PORSCHE,
 MERC
};

class CarFactory
{
  public:
 static Car* create(CarType type)
 {
  switch(type)
  {
    case BMW : return new BMWCar();
    case PORSCHE : return new PorscheCar();
    default : return new MercCar();
  }
 }
};

When I compile CarFactory, I need to include BMWCar, PorscheCar and MercCar as a part of my compilation/linking unit.

The way my codebase is setup, we may want to ship BMWCar only, or two or all three of them. So, I cannot make the create() dependent on the type.

How can I adapt the factory pattern for this ? Also, I'd like to avoid doing ifdefs since this is just a sample of my problem. The real codebase is huge and is not a practical solution to ifdef the code.

Update: Also, I am not allowed to use:

• templates
• has to conform to c++ 98 standard
• cannot use boost

These are mostly due to customer build toolchain restrictions. I don't have a choice in changing these.

via Chebli Mohamed