The correct way to initialize a dynamic pointer to a multidimensional array?

Question

I've been having bad luck with with dynamic pointers when I range them to 2 dimensions and higher. For example I want a pointer to a 2D array. I know that:

int A[3][4];
int (*P)[4] = A;

Is completely legit (even if I don't completely understand why). Taking into consideration that:

int *P = new int[4];

works, I imagined that:

int **P = new int[5][7];

Would also work, but it's not. This code states the error:

Error: A value of type "(*)[7]" cannot be used to initialize an entity of
       type "int **"

By seeing this the new part becomes a pointer to an array of 7 integers I made:

int (*P)[4] = new int[7][4];

And this does work but it's not what I want to accomplish. By doing it like that I'm limited to at least using a constant value for any subsequent dimension, but I want it to be fully defined at run time and therefore "dynamic".

How could I go and make this multidimensional pointer work??

Answer 1

Let's start with some basic examples.

When you say int *P = new int[4];

1. new int[4]; calls operator new function()
2. allocates a memory for 4 integers.
3. returns a reference to this memory.

4. to bind this reference, you need to have same type of pointer as that of return reference so you do

   int *P = new int[4]; // As you created an array of integer
                        // you should assign it to a pointer-to-integer
   

For a multi-idimensional array, you need to allocate an array of pointers, then fill that array with pointers to arrays, like this:

int **p;
p = new int*[5]; // dynamic `array (size 5) of pointers to int`

for (int i = 0; i < 5; ++i) {
  p[i] = new int[10];
  // each i-th pointer is now pointing to dynamic array (size 10)
  // of actual int values
}

Here is what it looks like:

To free the memory

1. For one dimensional array,

    // need to use the delete[] operator because we used the new[] operator
   delete[] p; //free memory pointed by p;`
   

2. For 2d Array,

   // need to use the delete[] operator because we used the new[] operator
   for(int i = 0; i < 5; ++i){
       delete[] p[i];//deletes an inner array of integer;
   }
   
   delete[] p; //delete pointer holding array of pointers;
   

Avoid memory leakage and dangling pointers!

Answer 2

You want something like:

int **P = new int*[7];
p[0] = new int[5];
p[1] = new int[5];
...

Answer 3

Another approach would be to use a 1D array as an 2D array. This way you only have to allocate the memory once (one continous block);

int *array;
size_t row=5,col=5;
array = (int*)malloc(row*col*sizeof(int)) //or new int[row*col]

This would result in the same as "int array[5][5]".

to access the fields you just do:

array[1 //the row you want
 * col //the number of columns
+2//the column you want
] = 4;

This is equal to:

array[1][2];

Answer 4

This performs bounds checking on some debug compilers, uses dynamic size and deletes itself automatically. The only gotcha is x and y are the opposite way round.

std::vector<std::vector<int>> array2d(y_size, std::vector<int>(x_size));

for (int y = 0; y < y_size; y++)
{
    for (int x = 0; x < x_size; y++)
    {
        array2d[y][x] = 0;
    }
}