problem with inheritance

[ogward]

Hi!

I have a base class called vehicle and two sub classes Car and Bike.
I have a handler file that has a couple of functions like addVehicle,showAll, showCars and showBikes.

Now to my problem, when the bikes are presented it looks like this.
Model: BMW Year: 2005 Cost: 2000
Model: Kawasaki Year: 2030 Cost: 40000
and so on....

At the end I want all costs to be added together and presented with a total cost.
So I've written something like this

for(int i=0;i<this->nrOfPresents;i++)
		{
			if(typeid(*this->prsnt[i])==typeid(Company))
			{
				this->prsnt[i]->present();
			}
		}

but when I try to access my function that returns the price for each bike it cant see it it only shows me the functions in the base class and not any function from the sub classes.
If I didn't use inheritance I would have done something like this

for(int i=0;i<this->nrOfPresents;i++)
   {    
        int totalCost+=this->prsnt[i]->getCost();
   }

How do I make this work, something tells me it should be easy?

Best Regards Ogward

[Disch]

Since all vehicles have a cost, getCost should be part of your parent class Vehicle, and not part of your child classes. Then this problem disappears.

Only things specific to cars and bikes should be in Car and Bike. Things that apply to all vehicles should be in Vehicle.

[ogward]

I know that, but only the bikes should have the cost.
if it was the way you said then it wouldnt be a problem...
the parent class vehicle has model and year but the bike class has cost as well and car has doors.

[Disch]

I know that, but only the bikes should have the cost.

So cars don't cost anything? Are you sure that's right?

[ogward]

I just made this up, the program is not about vehicles, its about something else.
But it should work as I stated above.

I'm positive it should be like that.
So how do I access the getCost() function that is in the child class?

[Disch]

In that event you have to downcast. But I'm almost certain that's not what you're supposed to do. Downcasting is actually quite rare and is a sign of a design flaw.

Perhaps if you post the actual classes and not these vehicle/car/etc classes I could be of more help.


If you want to go with the downcasting approach, you can do it like this (but again... this is generally a BAD IDEA and should be avoided):

for(int i=0; i < nrOfPresents; ++i)
{
    Bike* p = dynamic_cast<Bike*>(prsnt[i]);
    if(p)
        totalcost += p->getCost();
    else
        {}  // p will be null if prsnt[i] was not a Bike
}

[ogward]

ok, the actual program is like this.

parent class Present
child class Private
child class Company

So there are presents for the ppl in a company and for ppl that are not in the company, and they want to know the cost for the company gifts but not for the private gifts.

the thing you explained, does it have something to do with clone() or typeid?

[Disch]

Well if it were me I wouldn't make a class hierarchy out of this -- but whatever.

Anyway yeah my solution above will work. I suppose it isn't so horrible after all.

the thing you explained, does it have something to do with clone() or typeid?

No. It's downcasting. I'm not using typeid or clone anywhere in there.

[ogward]

Thanks a lot for the help!