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

Code:
//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;
Code:
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