pass empty matrix to function - c++

Question

I'm trying to pass a matrix without dimensions to a function and fill with data. Here is an example:
#include <iostream>
#include <stdlib.h>
using namespace std;
void fun(double **matrix);
int main(void)
{
double **matrix;
fun(matrix);
for(int i = 0; i < 10; i ++)
{
cout << "matrix = " << matrix[i][0] << "\t" << matrix[i][1] << endl;
}
}
void fun(double **matrix)
{
int rowCount = 10;
int colCount = 2;
matrix = new double*[rowCount];
for(int i = 0; i < rowCount; ++i)
matrix[i] = new double[colCount];
for(int i = 0; i < rowCount; i ++)
{
matrix[i][0] = 3.;
matrix[i][1] = 4.;
}
}
It compiles but when I execute it, it return the following error:
Illegal instruction: 4
Do you know why?

Answer 1

First of all if you are got a pointer it should point at something, so in order to not have a warnings just initialize it with nullptr. Second thing is that you should return a pointer to pointer.I fixed your code so you can look what you have made wrong,
#include <iostream>
#include <stdlib.h>
using namespace std;
double **fun(double **matrix);
int main(void)
{
double **matrix= nullptr;
matrix = fun(matrix);
for (int i = 0; i < 10; i++)
{
cout << "matrix = " << matrix[i][0] << "\t" << matrix[i][1] << endl;
}
getchar();
getchar();
}
double **fun(double **matrix)
{
int rowCount = 10;
int colCount = 2;
matrix = new double*[rowCount];
for (int i = 0; i < rowCount; ++i)
matrix[i] = new double[colCount];
for (int i = 0; i < rowCount; i++)
{
matrix[i][0] = 3.;
matrix[i][1] = 4.;
}
return matrix;
}

Answer 2

Your program fails because fun allocates memory, but does not return the newly allocated matrix array to main. It would be easier to do this by return value than by argument:
double** fun() {
// ...
double** matrix = new double*[...];
// ...
return matrix;
}
int main() {
double** matrix = fun();
// ...
}
There are a million other ways to do that (by passing a double*** into fun, or a reference, or by using some sort of object etc). Pick whatever you like best.