Creating a 2d Dynamic array with pointers - c++

Question

So for an assignment I have to create a program that will create magic squares when the user inputs an odd number, I have most of the program done but for some reason when I try to populate the squares I get Unhandled exception at 0x00326315 in magic square.exe: 0xC0000005: Access violation reading location 0x00000000
I'm using classes and have the declaration for square as int **square;
Here is the code
#include<iostream>
#include<iomanip>
#include"MagicSquare.h"
using namespace std;
MagicSquare::MagicSquare(): size(0), maxNum(0), row(0), col(0), curNum(0) //Constructor initialize variables
{
}
MagicSquare::~MagicSquare() //Destructor
{
for (int i = 0; i < size; i++)
{
delete[] square[i];
}
delete[] square; //Delete the dynamically allocated memory
}
void MagicSquare::getSize() //Get the size of the magic square
{
cout << "Please enter an odd number for the number of rows/columns: ";
cin >> size;
while (size % 2 == 0) //Check to see if the number entered is odd
{
cout << "The number you entered is not odd, please enter an odd number: ";
cin >> size;
}
int **square = new (nothrow) int*[size];
for (int i = 0; i < size; i++)
{
square[i] = new (nothrow) int[size];
}
maxNum = size * size;
iCount = new (nothrow) int[size];
jCount = new (nothrow) int[size];
}
void MagicSquare::populateSquare()
{
for (int i = 0; i < size; i++)
{
for (int j = 0; j < size; j++)
{
square[i][j] = 0; //This is where the error occurs
}
}
curNum = 1;
col = (size - 1) / 2;
square[row][col] = curNum;
curNum += 1;
for (int i = 1; i <= maxNum; i++)
{
row = row - 1;
col = col + 1;
if (col >= size)
col = 0;
if (row < 0)
row = size - 1;
square[row][col] = curNum;
curNum += 1;
}
}
Header file
class MagicSquare
{
private:
int **square;
int size;
int maxNum;
int row;
int col;
int curNum;
int *iCount;
int *jCount;
public:
MagicSquare();
~MagicSquare();
void getSize();
void populateSquare();
void printSquare();
};
source file
#include"MagicSquare.h"
#include<iostream>
using namespace std;
int main()
{
MagicSquare mySquare;
int choice = 1;
while (choice == 1)
{
mySquare.getSize();
mySquare.populateSquare();
mySquare.printSquare();
cout << "\n\nWould you like to create another magic square? 1 for yes, 0 for no: ";
cin >> choice;
while (choice != 1 || choice != 0)
{
cout << "\nInvalid input: \nWould you like to create another magic square? 1 for yes, 0 for no: ";
cin >> choice;
}
}
system("pause");
return 0;
}

Answer 1

You are defining a local variable called square in your getSize() method here:
int **square = new (nothrow) int*[size];.
So you make space for the local variable but never for the class's field.
Change this line to
square = new (nothrow) int*[size];
Also seriously consider checking the results of the calls.

Answer 2

Access violation reading location 0x00000000 is telling you, that you are trying to access a NULL-pointer.
A reason could be, that at least one call of new failed. you should check when allocating the array:
int **square = new (nothrow) int*[size];
if(square == NULL)
//Handle error here
for (int i = 0; i < size; i++)
{
square[i] = new (nothrow) int[size];
if(square == NULL)
//Handle error here
}
But i guess thats not the reason. If I saw it right, you have two functions:
void MagicSquare::getSize()
void MagicSquare::populateSquare()
But the int **square is created in getSize, so if you call populate square, this variable does not exist anymore.
if your class is:
class MagicSquare
{
private:
int **square;
public:
//Methods
}
in getSize you have to store the address in the classes member variable, not a local one you just created:
square = new (nothrow) int*[size]; //without int **