multidimensional array initialization using pointers

Question

I want to have a function which initializes dynamic 2d arrays in cpp like below

void initArrays(int n,double **a,double **b,double **c) {
a = new double*[n];
for (int i = 0; i < n; i++)
 a[i] = new double[n];
b = new double*[n];
for (int i = 0; i < n; i++)
 b[i] = new double[n];
c = new double*[n];
for (int i = 0; i < n; i++)
 c[i] = new double[n];
}

The function call completes but it does not initialize the pointers I give as function arguments. For example if I call this function in the main

double **x,**y,**z;
initArrays(3,x,y,z);

I cannot access

x[0][0]

what am I doing wrong here?

• Cory Kramer: possible duplicate of How do I declare a 2d array in C++ using new?
• AlphaWolf: This works if the init is done in the main which as explained in the question above. This failed when I moved the init to a function which is being called in the main
• Fytch: Don't use new in C++, we have std::vector and std::unique_ptr. Don't use arrays of arrays as a 2D array, rather use a 1D array with the size x * y and map the accesses.

Answer 1

The pointers stay uninitialized because you never assign them a value. The pointers in your init function are copies of the ones that you passed. You can pass the pointers by reference:

void initArrays(int n,double **&a,double **&b,double **&c)

That said, you'll probably be better off with std::vector.