Some complex structure I can't code correctly

Hello all, I've been trying to reengineer some C code to C++, but got
stuck on some complex data structure:

On the C code, there is an "Model" struct used to store various kind of
data, the exact kind stored in the "type" member. Each kind has
different data, that is stored in a "value" array. So, if a Model is
type "Car", the maximum speed is stored in value[0]. If it's a "House",
value[0] is used to store number of inhabitants. A huge mess!

Here comes C++ to the rescue, I created a ModelBase class, with
ModelCar, ModelHouse and so on as derived clases. All models have many
common data, so this polimorphism works very nicely.

However, there is also an "Obj" struct, which represents an instance of
the data stored in a "Model". In C, it is the same mess as with Model
("type" member, "value" array)... and along new data needed for all
Objs, it has a pointer back to the Model it originated from.

So I created a ObjBase clases, with ObjCat, ObjHouse deriving from it.

And here is the problem... each Model needs to "spawn" an Obj, using a
covariant return type function, such as:
    ObjHouse* ModelHouse::spawn();
But also, each Obj needs to returns a reference to its Model, also
using covariant return type:
    ModelHouse* ObjHouse::get_model();

I have this:

    class Model;
    class Obj {
        virtual Model* get_model() = 0;
    };
    class Model {
        virtual Obj* spawn() = 0;
    };

    class ModelHouse;
    class ObjHouse : public Obj {
        ModelHouse* get_model(); // <- ERROR HERE
    };
    class ModelHouse : public Model {
        ObjHouse* spawn();
    };

Error says "invald covariant return type". Of course, up to that point,
it doesn't know that ModelHouse derives from Model, but moving
ModelHouse before ObjHouse, gives a similar error for
ModelHouse::spawn.

Any clues on how to solve this? Some pattern to represent this data? I
have tried some ideas, but nothing works.

I'm using gcc 3.4.4, btw.

Thanks.