2D Array of pointers to structs. Memory access issue - c++

Question

I am attempting to write a program to compute the Hungarian method for a set of Jobs:Workers.
I know how I am going to code the bulk of the program, but I am stuck at loading my matrix.
I have created a class to store info in, and I have created a 2d array of pointers to objects of this class. Each entry of the matrix should hold a cost value assigned to that Worker:Job combo.
Every time I try to load the costs into the objects within the array, I keep getting a memory access violation, and I can't figure out why. Any help would be greatly appreciated.
Thanks,
Chris
using namespace std;
class costPtr{
int cost;
int fillRow;
int fillColumn;
public:
costPtr()
{
int fillRow = 0;
int fillColumn = 0;
cost = 0;
}
void fillCost(int costInput)
{
cost = costInput;
}
int printCost() const
{
return cost;
}
};
void fillMatrix(costPtr*** matrix, int workers, int jobs);
void methodMain();
void printMatrix(costPtr*** matrix, int workers, int jobs);
int main()
{
methodMain();
}
void methodMain()
{
int jobs, workers;
cout << "How many jobs are there: ";
cin >> jobs;
cout << "How many workers are there: ";
cin >> workers;
costPtr*** matrix = new costPtr**[workers];
for (int i = 0; i < workers; i++)
matrix[i] = new costPtr*[jobs];
fillMatrix(matrix, workers, jobs);
printMatrix(matrix, workers, jobs);
}
void fillMatrix(costPtr*** matrix, int workers, int jobs)
{
int cost;
for (int i = 0; i < workers; i++)
{
for (int j = 0; j < jobs; j++)
{
cout << "What is the cost for worker " << i + 1 << " doing job " << j + 1 << ": ";
cin >> cost;
(matrix[i][j])->fillCost(cost);
cout << endl;
}
}
}
void printMatrix(costPtr*** matrix, int workers, int jobs)
{
cout << endl << endl;
for (int i = 0; i < workers; i++)
{
for (int j = 0; j < jobs; j++)
cout << (matrix[i][j])->printCost() << " ";
cout << endl;
}
}

Answer 1

The specific cause of the error is here:
(matrix[i][j])->fillCost(cost);
You have declared the pointer, but then you give a command to the object that the pointer points to-- which does not exist. You never constructed it.
The deeper problem is that you attempted this with an array of arrays of pointers, before you tried it with a pointer. When you write code, you should start with something small and simple that works perfectly, then add complexity a little at a time, testing at every step. (For some reason, this rule never comes up in programming courses.)

Answer 2

If you want it to be a "2d" matrix, it should be costPtr** matrix.
// An array of pointers
costPtr** matrix = new costPtr*[workers];
// Each element in the array is (a pointer to) another array
for (int i = 0; i < workers; i++)
matrix[i] = new costPtr[jobs];
Of course, there are always the standard containers like std::vector. You can have a vector of vectors:
// Create and initialize a workers x jobs matrix
std::vector<std::vector<costPtr>> matrix(workers, vector<costPtr>(jobs));
matrix[2][3].printCost(); // Example usage
No explicit memory allocations or pointers.
Edit: After re-reading I see you wanted a "2d array of pointers" so costPtr*** matrix is correct, but you need one more step:
costPtr*** matrix = new costPtr**[workers];
for (int i = 0; i < workers; i++) {
matrix[i] = new costPtr*[jobs];
for (int j = 0; j < jobs; j++) {
matrix[i][j] = new costPtr;
}
}

Answer 3

Take a look at the dynamic allocation of you matrix container. To reach your goal, every "star" in the container type must have a "new".
You successfully dealt with the first 2 pointers, like so:
costPtr*** matrix = new costPtr**[workers];
for (int i = 0; i < workers; i++)
matrix[i] = new costPtr*[jobs];
Now all you need to do is deal with the third pointer:
for (int i =0; i < workers; i++)
for (int j = 0; j < jobs; j++)
matrix [i][j] = new costPtr;
Without the last pointer allocation, all elements would point to a null.