On 2 dimensional array of C++

I was asked about this today. In practice, I rarely use 2-dimensional array, instead I use vector of vectors.

To allocate a 2-d array on the stack, a C-style array is

int d[2][3];

Then to refer to an element it is like

d[i][j];

To make a dynamical allocation, one can NOT write his code like this

int **wrong_d = new int[2][3];

since the d in int d[2][3]; is not a int**, instead it is of the type

int (*)[3]

or in your evil human words, it is a int[3] pointer.

It is a little tricky to declare a 2-d array dynamically.

int (*d)[3] = new int[2][3];

Or

int v1 = 2;
int (*d)[3] = new int[v1][3];

The number 3 here can NOT be replace by a non-constant. My understanding is that since this value is associated with the type of d, in a strong type language like C/C++ which should be known by the compiler.

We can check the size of variables to verify this interpretation.

 1 #include 
  2 
  3 using namespace std;
  4 
  5 int main(int argc, char **argv)
  6 {
  7     int v1 = 2;
  8     int (*d)[3] = new int[v1][3];
  9     cout << "sizeof(d): " << sizeof(d) << endl;
 10     cout << "sizeof(d[0]): " << sizeof(d[0]) << endl;
 11     cout << "sizeof(d[1]): " << sizeof(d[1]) << endl;
 12     cout << "sizeof(d[0][0]): " << sizeof(d[0][0]) << endl;
 13     return 0;
 14 }

The output is:

$./test 
sizeof(d): 8        //< 2 pointers, &d[0] and &d[1]
sizeof(d[0]): 12    //< int[3], d[0][0] d[0][1] d[0][2]
sizeof(d[1]): 12    //< int[3], d[1][0] d[1][1] d[1][2]
sizeof(d[0][0]): 4  //< an integer

To save your life, I would recommend to use vectors.

int v1 = 2;
int v2 = 3;
vector > d(v1, vector(v2, 0));