Getting wrong output after calling an overriden method in C++ - c++

Question

I am making a program that contains base class RentedVehicle and its derived class FuelVehicle and the derived class of FuelVehicle as Car class. My code is:
#include<iostream>
using namespace std;
class RentedVehicle {
private:
double baseFee;
public:
RentedVehicle() {};
RentedVehicle(int a): baseFee(a) {
};
virtual double getCost() {
return baseFee;
}
};
class FuelVehicle: public RentedVehicle {
private:
double kms;
public:
int mileageFees = 0;
FuelVehicle() {};
FuelVehicle(double a, int b): RentedVehicle(a) {
kms = b;
};
double getMileageFees() {
if (kms < 100) {
mileageFees = 0.2*kms;
} else if (kms >= 100 && kms <= 400) {
mileageFees = 0.3*kms;
} else if (kms > 400) {
double x = (kms - 400);
mileageFees = (0.3*kms) + (0.5*x);
}
return mileageFees;
}
double getKms() {
return kms;
}
};
class Car: public FuelVehicle {
private:
int seats;
public:
Car() {};
Car(int a, double b, double c): FuelVehicle(b, c) {
seats = a;
};
double bF = getCost();
double mF = getMileageFees();
double getCost() {
double cost = mF + (bF*seats);
return cost;
}
int getSeats() {
return seats;
}
};
int main() {
Car c1(5, 500, 20);
double y = c1.getCost();
cout << "cost is: " << y << endl;
return 0;
}
When I'm calling overriden getCost() method it's showing cost is: 4. However a/c to my given parameters the cost should be 2504.
What is the mistake?

Answer 1

On this line:
double bF = getCost();
in the Car class, you are calling the most derived version of getCost. But this function:
double getCost() {
double cost = mF + (bF*seats);
return cost;
}
will invoke undefined behavior since you are using bF in this function before it's initialized.
Instead, you can use the base class version of getCost, like this:
double bF = FuelVehicle::getCost();
which prints the expected value of 2504.
Here's a demo.

Answer 2

1- Proper use of variables is missing. bf is used before initialization. Use the scope resolution operator to clarify the code and avoid ambiguity. When you called getcost() function of the base class, it finds the function in the existing class and binds it with the caller which causes ambiguity.
2- Tip: Try to implement OOP principles more accurately. You could have made getcost() function a pure virtual function instead of a simple virtual function. The pure virtual function gives more authentication to code. In your case, the getcost() function should be made a necessary requirement for any class that derives from Rented class as the cost is an essential part of it and defining it must be made a requirement of code. This can be achieved by making get-cost() a pure virtual function.

Answer 3

Declaring the class like this:
class Car: public FuelVehicle {
private:
int seats;
public:
Car() {};
Car(int a, double b, double c): FuelVehicle(b, c) {
seats = a;
};
double bF = getCost();
double mF = getMileageFees();
Is simply syntactic sugar for:
private:
int seats;
public:
double bF;
double mF;
Car()
: FuelVehicle() // Call to default constructor of base
// , seats => Not initialized
, bF(getCost()) // uses mF but that is not initialized yet.
// uses bF but that is not initialized yet.
// uses seats but that is not initialized yet.
// Accessing uninitialized variables is UB
, mF(getMileageFees()) // Calls the base class
// But this was default initialized (see above)
// The default constructor does not initialize
// the member variables so this will
// probably return some random value but
// is technically UB.
{};
Car(int a, double b, double c)
: FuelVehicle(b, c)
// , seats => Not initialized
, bF(getCost()) // uses mF but that is not initialized yet.
// uses bF but that is not initialized yet.
// uses seats but that is not initialized yet.
// Accessing uninitialized variables is UB
, mF(getMileageFees()) // This is OK as the base class was
// initialized correctly.
{
seats = a; // Assignment to seats which defines it
// But it is already used above.
};