C++ 2D dynamic array crash

Question

Hi I'm having some problem with 2D dynamic array.

int main()
{
   double **M;
   int M_dimension;
   int i;

   M_dimension = 10;

   M = new double *[M_dimension];
   for (i=0;i<M_dimension;i++)
   {
      M[i] = new double[M_dimension];
   }

   M[0][0] = 1.0;

...
}

Program works but I'd like to initialize 2D array using such a function:

void initialize2D(double **M,int M_dimension)
{
   int i;
   M = new double *[M_dimension];
   for (i=0;i<M_dimension;i++)
   {
      M[i] = new double[M_dimension];
   }
}

Finally the program looks like this:

int main()
{
   double **M;
   int M_dimension;
   int i;

   M_dimension = 10;

   initialize2D(M,M_dimension);        

   M[0][0] = 1.0; //crash

...
}

Unfortunately it crashes at M[0][0] = 1.0;

Thanks for any help or suggestions.

Answer 1

You are passing M by value, instead of by reference. initialize2D needs to change the value of the pointer-to-pointer M such that it points to the memory allocated

Try changing your function signature to this instead:

void initialize2D(double **&M,int M_dimension)

Or

void initialize2D(double ***M,int M_dimension) {
    ...
    *M = new double *[M_dimension];
    ...
}

Answer 2

You need to pass a reference to double** instead of double** to function, otherwise the modification done to a pointer after assigning M the reslut of new get lost on exit from a function.

Answer 3

what the problem might be is where you declaring an integer parameter such as int M_dimension void initialize2D(double **M,int M_dimension)

and then you initializing the dynamic array as: M[i] = new double[M_dimension];

where it comes to contradiction because you declared the variable M_dimension as an integer and then you using it as a double

try it this way: either change the data type of array or the M_dimension, so then both of them have the same data type.

hopefully this will help you out

Answer 4

Why don't you use std::vector or Boost.MultiArray

It would be quite easy exercise to define 2-dimension array as generic vector of vectors in C++