How to allocate a 2D array of pointers in C++

Question

I'm trying to make a pointer point to a 2D array of pointers. What is the syntax and how would I access elements?

Answer 1

By the letter of the law, here's how to do it:

// Create 2D array of pointers:
int*** array2d = new (int**)[rows];
for (int i = 0; i < rows; ++i) {
  array2d[i] = new (int*)[cols];
}

// Null out the pointers contained in the array:
for (int i = 0; i < rows; ++i) {
  for (int j = 0; j < cols; ++j) {
    array2d[i][j] = NULL;
  }
}

Be careful to delete the contained pointers, the row arrays, and the column array all separately and in the correct order.

However, more frequently in C++ you'd create a class that internally managed a 1D array of pointers and overload the function call operator to provide 2D indexing. That way you're really have a contiguous array of pointers, rather than an array of arrays of pointers.

Answer 2

It depends. It can be as simple as:

int main()
{
    int*   data[10][20]; // Fixed size known at compile time

    data[2][3] = new int(4);
}

If you want dynamic sizes at runtime you need to do some work.
But Boost has you covered:

int main()
{
   int  x;
   int  y;
   getWidthAndHeight(x,y);

   // declare a 2D array of int*
   boost::multi_array<int*,2>   data(boost::extents[x][y]);

   data[2][3] = new int(6);
}

If you are fine with jagged arrays that can grow dynamically:

int main()
{
    std::vector<std::vector<int*> >   data;

    data.push_back(std::vector<int*>(10,NULL));
    data[0][3] = new int(7);
}

Note: In all the above. I assume that the array does not own the pointer. Thus it has not been doing any management on the pointers it contains (though for brevity I have been using new int() in the examples). To do memory management correctly you need to do some more work.

Answer 3

int *pointerArray[X][Y];
int **ptrToPointerArray = pointerArray;

That's how you make a true (contiguous in memory) multidimensional array.

But realize that once you cast a multidimensional array to a pointer like that, you lose the ability to index it automatically. You would have to do the multidimensional part of the indexing manually:

int *pointerArray[8][6]; // declare array of pointers
int **ptrToPointerArray = &pointerArray[0][0]; // make a pointer to the array
int *foo = pointerArray[3][1]; // access one element in the array
int *bar = *(ptrToPointerArray + 3*8 + 1); // manually perform row-major indexing for 2d array

foo == bar; // true
int *baz = ptrToPointerArray[3][1]; // syntax error