有人能够发现我用这个 C++ 代码做错了什么吗？

Question

So I'm breaking my head here. I've been reading up and down and I just can't figure out why my program crashes once I reach the setLoan function inside of the for loop in main. AM I missing something, or did I implement the pointer wrong? Thanks in advanced.

 #include <string>
#include <iostream>
using namespace std;

//Vehicle Class
class Vehicle {
public:
    Vehicle();
    void setPrice(double a);
    void setMpg(int a);
    double getPrice() const;
    int getMpg() const;
    void printVehicle() const;
    Vehicle(double price, int mpg);
private:
    double price;
    int mpg;
};

//Loan Class
class Loan {
public:
    void setBank(string a);
    void setLoan(double a);
    string getBank() const;
    double getLoan() const;
    void printLoan() const;
    Loan(string bank = "", double loan = 0);
private:
    string bank;
    double loan;
};

//Car Class
class Car : public Vehicle {
public:
    Car(double price = 0, int mpg = 0, string bank = "", double loan = 0, string name = "", int element = 0);
    void setName(string a);
    void setLoan(string b, double l, int element);
    string getName() const;
    void printFull() const;
    void setNbrOfLoans(int a);
    ~Car();
private:
    string name;
    Loan* pLoan;
    int nbrOfLoans;
};

//Main
int main() {
    Car car1(24800, 22, "Citi", 21600, "Mustang", 1);
    Car car2;
    Car* pCar1 = &car1;
    Car* pCar2 = &car2;
    cout << "Enter the price of the car: ";
    double price;
    cin >> price;
    pCar2->setPrice(price);
    cout << "Enter the mpg: ";
    int mpg;
    cin >> mpg;
    pCar2->setMpg(mpg);
    cout << "Enter the name of the car: ";
    string name;
    cin >> name;
    pCar2->setName(name);
    string bank;
    double loan;
    int index;
    cout << "Enter the amount of loans you obtained: ";
    cin >> index;
    pCar2->setNbrOfLoans(index);
    for (int i = 0; i < index; i++)
    {
        cout << "Enter the name of bank " << i + 1 << ": ";
        cin >> bank;
        cout << "Enter the amount of loan " << i + 1 << ": ";
        cin >> loan;
        pCar2->setLoan(bank, loan, i);
    }
    cout << endl;
    pCar1->printFull();
    pCar2->printFull();
    return 0;
}
////////////////////////////////////////////////////////////////////////////////////////
//Vehicle class function definitions
////////////////////////////////////////////////////////////////////////////////////////
Vehicle::Vehicle() {
    price = 0;
    mpg = 0;
}

Vehicle::Vehicle(double price, int mpg) {
    price = price;
    mpg = mpg;
}

void Vehicle::setPrice(double a) {
    price = a;
}

void Vehicle::setMpg(int a) {
    mpg = a;
}

double Vehicle::getPrice() const {
    return price;
}

int Vehicle::getMpg() const {
    return mpg;
}

void Vehicle::printVehicle() const {
    cout << "Price: " << price << endl;
    cout << "MPG: " << mpg << endl;
}

////////////////////////////////////////////////////////////////////////////////////////
//Loan Class function definitions
///////////////////////////////////////////////////////////////////////////////////////

Loan::Loan(string bank, double loan) {
    bank = bank;
    loan = loan;
}

void Loan::setBank(string a) {
    bank = a;
}

void Loan::setLoan(double a) {
    loan = a;
}

string Loan::getBank() const {
    return bank;
}

double Loan::getLoan() const {
    return loan;
}
void Loan::printLoan() const {
    cout << "Bank: " << bank << endl;
    cout << "Loan: " << loan << endl;
}

////////////////////////////////////////////////////////////////////////////////////
//Car Class function definitions
////////////////////////////////////////////////////////////////////////////////////
Car::Car(double price, int mpg, string bank, double loan, string name, int element) : Vehicle(price, mpg)
{
    nbrOfLoans = element;
    Car::name = name;
    pLoan = new Loan[nbrOfLoans];
}


void Car::setName(string a) {
    name = a;
}

void Car::setLoan(string b, double l, int element) {
    pLoan[element].setBank(b);
    cout << " ";
    pLoan[element].setLoan(l);
}

string Car::getName() const {
    return name;
}

void Car::setNbrOfLoans(int a) {
    nbrOfLoans = a;
}

void Car::printFull() const {
    cout << endl << "Name: " << name << endl;
    printVehicle();
    for (int i = 0; i < nbrOfLoans; i++)
    {
        cout << pLoan[i].getBank();
        cout << endl;
        cout << pLoan[i].getLoan();
        cout << endl;
    }
}

Car::~Car() {
    delete[] pLoan;
}

Answer 1

What you are doing wrong is you are trying to access a dynamic array of size zero when you call Car::setLoan for pCar2 .

Car car2;//the nbrOfLoans member is 0 and pLoan is not pointing to anywhere
....
Car* pCar2 = &car2;//the nbrOfLoans member is 0 and pLoan is not pointing to anywhere
...
Car::Car(double price, int mpg, string bank, double loan, string name,    int element) : Vehicle(price, mpg)
{
   ...
   pLoan = new Loan[nbrOfLoans];//doesn't create anything if nbrOfLoans is 0, which is the default
}

When you first try to allocate memory for pLoan in the constructor of the second car(car2), the size of nbrOfLoans is 0, so your pointer is not pointing to anything. Later you tried to access it by calling pCar2->setLoan(bank, loan, i); .

It would be better if you could use std::vector otherwise you need to create/resize your dynamic array after you know the size that it should hold,ie after getting the value of nbrOfLoans at Car::setNbrOfLoans .

void Car::setNbrOfLoans(int a) {
   nbrOfLoans = a;

   if(pLoan!=NULL)
     delete[] pLoan;

   pLoan = new Loan[nbrOfLoans];
}