Question

I’m again doing a task for school and I’m implementing it slowly, I don’t know why my park_car function is not working, I just wanted to make a test and the program crashes … here is my code.

PS: I can’t change the ***p2parkboxes because it is given in the starter file like most other variables. I just want to see the first element of Floor 0 as : HH-AB 1234. Your help is most appreciated.
PS2: I can’t use the std::string as well it isn’t allowed for the task.

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

#define EMPTY "----------"
class Parkbox{
    char *license_plate; // car's license plate
    public:
    Parkbox(char *s = EMPTY); // CTOR
    ~Parkbox(); // DTOR
    char *get_plate(){return license_plate;}
};
class ParkingGarage{
    Parkbox ***p2parkboxes;
    //int dimensions_of_parkhouse[3]; // better with rows,columns,floors
    int rows,columns,floors; // dimensions of park house
    int total_num_of_cars_currently_parked;
    int next_free_parking_position[3];
    // PRIVATE MEMBER FUNCTION
    void find_next_free_parking_position();
    public:
    ParkingGarage(int row, int col, int flr);// CTOR,[rows][columns][floors]
    ~ParkingGarage(); // DTOR
    bool park_car(char*); // park car with license plate
    bool fetch_car(char*); // fetch car with license plate
    void show(); // show content of garage floor
    // by floor
};

Parkbox::Parkbox(char *s ) { // CTOR
    license_plate = new char[strlen(s)+1];
    strcpy(license_plate, s);
    //cout << "ParkBox CTOR" << endl;
}
Parkbox::~Parkbox() { // DTOR
    delete [] license_plate;
    //cout << "ParkBox DTOR" << endl;
}

ParkingGarage::ParkingGarage(int row, int col, int flr){
    rows = row; columns = col; floors = flr;
    p2parkboxes = new Parkbox**[row];
    for (int i = 0; i < row; ++i) {
        p2parkboxes[i] = new Parkbox*[col];

        for (int j = 0; j < col; ++j)
            p2parkboxes[i][j] = new Parkbox[flr];
    }

}

ParkingGarage::~ParkingGarage(){

    for (int i = 0; i < rows; ++i) {
        for (int j = 0; j < columns; ++j)
            delete [] p2parkboxes[i][j];

        delete [] p2parkboxes[i];
    }
    delete [] p2parkboxes;
}

void ParkingGarage::show(){
    int i,j,k;
    for (i = 0 ; i < floors; i++){
        cout << "Floor" << i << endl;
        for (j=0;j<rows;j++){
            for (k=0;k<columns;k++){
                cout << p2parkboxes[j][k][i].get_plate() << "  ";
            }
            cout << endl;
        }
    }
}

bool ParkingGarage::park_car(char*s){

    p2parkboxes[0][0][0] = Parkbox(s); //test
    //p2parkboxes[0][0][0] = s; //test

    return true;
}


int main(void) {
    // a parking garage with 2 rows, 3 columns and 4 floors
    ParkingGarage pg1(2, 3, 4);
    pg1.park_car("HH-AB 1234");
    /*pg1.park_car("HH-CD 5678");
      pg1.park_car("HH-EF 1010");
      pg1.park_car("HH-GH 1235");
      pg1.park_car("HH-IJ 5676");
      pg1.park_car("HH-LM 1017");
      pg1.park_car("HH-MN 1111"); */
    pg1.show();
    /*pg1.fetch_car("HH-CD 5678");
      pg1.show();
      pg1.fetch_car("HH-IJ 5676");
      pg1.show();
      pg1.park_car("HH-SK 1087");
      pg1.show();
      pg1.park_car("SE-AB 1000");
      pg1.show();
      pg1.park_car("PI-XY 9999");
      pg1.show(); */
    return 0;
}

Answer 1

You did not declare the copy constructor for the Parkbox class. So, the line

p2parboxes[0][0][0] = Parkbox(s)

creates something (instance of Parkbox with a char* pointer) on the stack (and deletes it almost immediately). To correct this you might define the

Parkbox& operator = Parkbox(const Parkbox& other)
{
    license_plate = new char[strlen(other.get_plate())+1];
    strcpy(license_plate, other.get_plate());
    return *this;
}

Let’s see the workflow for the

p2parboxes[0][0][0] = Parkbox(s)

line.

1. First, the constructor is called and an instance of Parkbox is created on stack (we will call this tmp_Parkbox).
2. Inside this constructor the license_plate is allocated and let’s say it points to 0xDEADBEEF location.
3. The copying happens (this is obvious because this is the thing that is written in code) and the p2parboxes[0][0][0] now contains the exact copy of tmp_Parkbox.
4. The scope for tmp_Parkbox now ends and the destructor for tmp_Parkbox is called, where the tmp_Parkbox.license_plate (0xDEADBEEF ptr) is deallocated.

5. p2parboxes[0][0][0] still contains a “valid” instance of Parkbox and the p2parboxes[0][0][0].license_plate is still 0xDEADBEEF which leads to the undefined behaviour, if any allocation occurs before you call the

   cout << p2parboxes[0][0][0].license_plate;

Bottom line: there is nothing wrong with the line itself, the problem is hidden within the implementation details of the ‘=’ operator.

At this point it is really better for you to use the std::string for strings and not the razor-sharp, tricky and explicit C-style direct memory management mixed with the implicit C++ copy/construction semantics. The code would also be better if you use the std::vector for dynamic arrays.