Thread: Can't reach protected variable in a derived class?

Hi! I don't understand why my compiler gives me this error when I'm trying to run this code:

Code:

#include <iostream>
#include <cmath>


using namespace std;


class Airplane
{
    public:
        Airplane();
        ~Airplane();


    protected:
        int fuel;
        int speed;
        int metersOverGround;
        int steeringWheel;


    private:
        void ShowData(const Airplane& ap);
};


Airplane::Airplane  () {}
Airplane::~Airplane () {}


class MultiAirplane : public Airplane
{
    public:
        MultiAirplane(int fuel, int speed, int metersOverGround, int steeringWheel);
        ~MultiAirplane();


    private:
        Airplane multiAirplane;
};


MultiAirplane::MultiAirplane(int fuel, int speed, int metersOverGround, int steeringWheel)
{
    cout << "MultiAirplane" << endl;
    this->multiAirplane.fuel = fuel; // WHY WON'T THIS WORK??
}


MultiAirplane::~MultiAirplane()
{


}


class SingleAirplane : public Airplane
{
    public:
        SingleAirplane();
        ~SingleAirplane();
};


void Airplane::ShowData(const Airplane& ap)
{
    cout << "Speed: " << ap.speed            << endl;
    cout << "Fuel:  " << ap.fuel             << endl;
    cout << "M.O.G: " << ap.metersOverGround << endl;
    cout << "S.W:   " << ap.steeringWheel    << endl;
}


int main(int argc, char* argv[])
{
    return 0;
}

error: 'int Airplane::fuel' is protected
error: within this context

The variable is protected. Yeah, that's right. But shouldn't a derived class be able to reach it? Or is it only in a function that the derived class is able to reach protected variables and isn't able to reach protected variables in the constructor?

Thanks for help!