Pass a dynamically allocated multidimensional array to a function

Question

#include<iostream>

using namespace std;

void seed(int* matrixAddr, int n, int m);

void main()
{
  int n, m;
  cin >> n >> m;

  int matrix[n][m]; // DevC++ does not throw an error here, where VS does instead 

  seed(matrix, n, m);
}

void seed(int* matrixAddr, int n, int m) {}

Accessing matrix directly means referencing a memory address - in this case, to the first element of the two-dimensional array.

Apparently, though, the address cannot be passed as-is to the seed function, which accepts a pointer as its first argument.

Why does this happen? Should this not be allowed?

The error DevC++ throws is the following: [Error] cannot convert 'int (*)[m]' to 'int*' for argument '1' to 'void seed(int*, int, int)'

Doesn't matter what error you get, in this context `int matrix[n][m]; ` is simply not valid C++. — , Oct 14 '17 at 19:05
Thank you, it makes sense as I thought. Our programming professor, however, suggests we use this initialisation method for multidimensional arrays. Is there any way to deal with this? — Johnny Bueti, Oct 14 '17 at 19:07
" however, suggests we use this initialisation method for multidimensional arrays" There are many ways to deal with this, but I suggest you start by getting a new so-called "professor". — , Oct 14 '17 at 19:10
I wish I could, Neil, trust me. May I ask how could I deal with that kind of initialisation and the question's problem? — Johnny Bueti, Oct 14 '17 at 19:11
Please try my code given below... It will work. Let u try in http://rextester.com/PULAXL63031 — Dr. X, Oct 14 '17 at 19:18

Dr. X · Answer 1 · 2017-10-14T19:21:12.360

1

#include <iostream>
using namespace std;

void seed(int** matrixAddr, int n, int m);

int main()
{
  int rows, cols;
  int **matrix = NULL;
  cin >> rows >> cols;

  matrix = new int*[rows]; 
  for(int i = 0; i < rows; i++)
     matrix[i] = new int[cols];

  seed(matrix, rows, cols);
  return 0;
}

void seed(int** matrixAddr, int rows, int cols) {
    cout << "It is OK" ;
}

You can check from here http://rextester.com/PULAXL63031

edited Oct 14 '17 at 19:21

answered Oct 14 '17 at 19:14

Dr. X

2,890
2
15
37

1

A 2d matrix may be easier to allocate by allocating a one dimensional array of size (rows * columns). – Thomas Matthews Oct 14 '17 at 19:21
2

A question has more than one answer as u know. – Dr. X Oct 14 '17 at 19:22
2

You are suggesting quite some things that are considered horrible practice – Passer By Oct 14 '17 at 19:41

Pass a dynamically allocated multidimensional array to a function

1 Answers1