I can't access to the protected member of my base class-CodePudding

I am new at programming using c and having some troubles creating my constructors & objects. How can I access to my protected members like int p_iID in the Fahrzeug class? I have to access them for both of my objects seperately.

I would be so happy if you could help me out with this.

class Fahrzeug {
private:
    
protected:
    string p_sName;
    int p_iID;
    double p_dMaxGeschwindigkeit;
    double p_dGesamtStrecke;
    double p_dGesamtZeit;
    double p_dZeit;
public:
    virtual void vAusgeben(Fahrzeug* pFahrzeug1,Fahrzeug* pFahrzeug2);
    virtual void vKopf();
    virtual void vSimulieren(Fahrzeug *pFahrzeug, Fahrzeug *pFahrzeug2);
class PKW;

class PKW: public Fahrzeug{
PKW(const int p_iMaxID, string p_sName, double p_dMaxGeschwindigkeit, double p_dGesamtStrecke) {
    p_iID = p_iMaxID;
    this->p_sName = p_sName;
    this->p_dMaxGeschwindigkeit = (p_dMaxGeschwindigkeit < 0) ? 0 : p_dMaxGeschwindigkeit;
    this->p_dGesamtStrecke = p_dGesamtStrecke;
}
    void vAusgeben(PKW pkw1, PKW pkw2) {
    cout << "\n";
    PKW pkw1;
    PKW pkw2;
    pkw1.vKopf();

    cout << setw(5) << left << pkw1.p_iID<< " " << setw(10) <<pkw1.p_sName << setw(8) << " "    << setw(15) << showpoint << pkw1.p_dMaxGeschwindigkeit << setw(3) << " " << pkw1.p_dGesamtStrecke; //Here I have the issue with pkw1.p_sName
    cout << "\n";
    cout << setw(5) << left << pkw2.p_iID << " " << setw(10) << pkw2.p_sName << setw(8) << " " << setw(15) << showpoint << pkw2.p_dMaxGeschwindigkeit << setw(3) << " " << pkw2.p_dGesamtStrecke;
    cout << "\n";

}
}

CodePudding user response：

void vAusgeben(PKW pkw1, PKW pkw2) {

You probably don't want to pass your PKW objects by value (or expect object slicing). Pass const references instead:

void vAusgeben(const PKW& pkw1, const PKW& pkw2) {

Also, why are you shadowing your 2 parameters with these local variables?

PKW pkw1; // ???
PKW pkw2; // ???

CodePudding user response：

Aside from the issues raised in comments (and in another answer), there's a special rule for protected members that sometimes surprises people. An object of a derived type can access protected members of its base sub-object, but it can't access protected members of some other object. So:

struct B {
protected:
    int i;
};

struct D : B {
    void f(const B&);
};

void D::f(const B& b) {
    i = 3;   // okay, accessing my own protected member
    b.i = 3; // no, access to protected member of different object not allowed
}

In the code in the question, the function PKW::vAusgeben can access its own copies of p_sName, p_dMaxGeschwindigkeit, and p_dGesamtStrecke, but it can't access pkw1.p_sName, pkw1.p_dMaxGeschwindigkeit, or pkw1.p_dGesamtStrecke.