class vehicle_template {
     vechicle_template(xmlstreamthing x);
     // properties found in vehicles, set from xml
};
class vehicle {
     vechicle(vehicle_template v);
     // properties vehicles have, taken from template
};
vehicle vehicle_factory(string s) {
    static map<string,vehicle_template> already_loaded;
    vehicle_template vt = already_loaded.find(s);
    if (vt == null) {
        xmlstreamthing xml = get_xml_from_object_name(s); // you need to figure this part out
        if (xml == null) {
            throw no_such_vehicle(s);
        }
        vt = new vehicle_template(xml);
        already_loaded.add(s,v);
    }
    return new vehicle(vt);
}