Access Violation with helper functions

[Lang]

I have class A trying to access helper functions in class B.

class A has a friend class B declaration and vice verse.

However when class B tries to use the helper function I get an access violation.

class A{
     friend B;
     private:
            string a;

     public:
            string returnString(){ return a; }
};

Can anyone tell me what's wrong with that?

Thanks,
Lang

[GCDEF]

more code please.

[ahmd]

Try cleaning the solution. If you have MSVS 7 and up, click Build -> Clean Solution.

Also close it and remove the following files: *.aps, *.ncb, *.suo

[cilu]

Bad design. If A needs B's privates, and B needs A's privates, then maybe you need an C class with those parts, and make A and B derived from it. However, that depends on what exactly one needs from the other. However, you should avoid using friends at all.

[Lang]

Ok, thanks for the quick reply. GCDEF asked for more code so this is class Car.

//We fordward reference the parkingLot class because Class Car has yet to be defined
//but our parkingLot class requires it.
class parkingLot;

using namespace std;

class Car{
friend parkingLot;
private:
	string plateNumber;
	
public:
	Car (string argString){
		this->plateNumber = argString;
	}

	string getPlateNumber(){ return plateNumber; }
};

This is class parkingLot;

class parkingLot{
	friend Car;
private:
	int lot_number;		// the identification number of the parking lot
	int capacity;			// the maximum number of car it can accommodate
	int car_count;			// the number of cars currently in the lot
	double hourly_rate;		// the hourly rate 
	double max_charge;		// the maximum charge for each car
	double revenue;		// the daily revenue of this rate
	
	Car ** parking_spots;		// a dynamic array of pointers to Car, 
								// representing parking spots in the lot
	
public:
	parkingLot(int argNumber, int argCapacity, double hourlyRate, double maxCharge){
		lot_number = argNumber;
		capacity = argCapacity;
		hourly_rate = hourlyRate;
		max_charge = maxCharge;
		revenue = 0;
		car_count = 0;

		parking_spots = new Car *[capacity];
	}

	int enter(Car * argCar, int hour, int min){
		if (car_count == capacity) return 1; //parking lot full

		for (int i = 0; i < capacity; ++i){
			if (parking_spots[i] != NULL){ // we have to make sure its not null to compare license plates
				if (argCar->getPlateNumber() == parking_spots[i]->getPlateNumber()){
					return 2; //2 = car already in parking lot
				}
			}
		}

		for (int i = 0; i < capacity; ++i){
			if (parking_spots[i] == NULL){
				parking_spots[i] = argCar; //found an empty spot, break, there's no use to keep looking
				break;
			}
		}

		argCar->park(this, hour, min);
		cout << argCar->toString() << " enters Lot " << lot_number << " at " << hour << ":" << min;

		++car_count;
		return 0; // no error
	}

};

I just need class parkingLot to have the ability to call Car::getPlateNumber()

Thanks,
Lang

[GCDEF]

What I meant was code that calls the function. I see

if (argCar->getPlateNumber() == parking_spots[i]->getPlateNumber())

Are you sure argCar and parking_spots[i] are both valid pointers?

[cilu]

I just need class parkingLot to have the ability to call Car::getPlateNumber()

And why do they have to be friends for that? :confusion: That is a public methods. As long as you have an object or valid pointer to object of type Car, you can call it. No need for friends. As I already said, don't use friends. Friends are like guns. For the simple fact that you have one, but you don't have to use it.

[GCDEF]

cilu makes a good point. As long as getPlateNumber is public, which it is, you don't need a friend class. That's not the cause of your crash however. Check your pointers when you try to call the function.

[Lang]

Thanks for the help guys!

parking_spots[i] was not NULL but it was an invalid pointer. Is there anyway to detect that? Or should I just set every value in my parking_spots array to NULL?

Thanks,
Lang

[GCDEF]

After this line
parking_spots = new Car *[capacity];
you'd need to initialize it to NULLs. You're declaring a bunch of pointers, but not initializing them.

[Lang]

Yes, I tried that and it worked.

Thanks for the help everone.