Use multiple inheritance to discriminate useage roles?
- by Arne
Hi fellows,
it's my flight simulation application again. I am leaving the mere prototyping phase now and start fleshing out the software design now. At least I try..
Each of the aircraft in the simulation have got a flight plan associated to them, the exact nature of which is of no interest for this question. Sufficient to say that the operator way edit the flight plan while the simulation is running. The aircraft model most of the time only needs to read-acess the flight plan object which at first thought calls for simply passing a const reference. But ocassionally the aircraft will need to call AdvanceActiveWayPoint() to indicate a way point has been reached. This will affect the Iterator returned by function ActiveWayPoint(). This implies that the aircraft model indeed needs a non-const reference which in turn would also expose functions like AppendWayPoint() to the aircraft model. I would like to avoid this because I would like to enforce the useage rule described above at compile time.
Note that class WayPointIter is equivalent to a STL const iterator, that is the way point can not be mutated by the iterator.
class FlightPlan
{
public:
void AppendWayPoint(const WayPointIter& at, WayPoint new_wp);
void ReplaceWayPoint(const WayPointIter& ar, WayPoint new_wp);
void RemoveWayPoint(WayPointIter at);
(...)
WayPointIter First() const;
WayPointIter Last() const;
WayPointIter Active() const;
void AdvanceActiveWayPoint() const;
(...)
};
My idea to overcome the issue is this: define an abstract interface class for each usage role and inherit FlightPlan from both. Each user then only gets passed a reference of the appropriate useage role.
class IFlightPlanActiveWayPoint
{
public:
WayPointIter Active() const =0;
void AdvanceActiveWayPoint() const =0;
};
class IFlightPlanEditable
{
public:
void AppendWayPoint(const WayPointIter& at, WayPoint new_wp);
void ReplaceWayPoint(const WayPointIter& ar, WayPoint new_wp);
void RemoveWayPoint(WayPointIter at);
(...)
};
Thus the declaration of FlightPlan would only need to be changed to:
class FlightPlan : public IFlightPlanActiveWayPoint, IFlightPlanEditable
{
(...)
};
What do you think? Are there any cavecats I might be missing? Is this design clear or should I come up with somethink different for the sake of clarity?
Alternatively I could also define a special ActiveWayPoint class which would contain the function AdvanceActiveWayPoint() but feel that this might be unnecessary.
Thanks in advance!
