Need help with 2d array o.O

hey folks, i need some help with a 2d array that i am using ->

i have a class with the member variables

protected:
//member variables
int m_iScreenWidth, m_iScreenHeight;
int m_iTilesAcross, m_iTilesDown;
int m_iTileWidth;

GameEngine* m_pGame;
Bitmap** m_pBitmapGrid;

then i have in the constructor for the class i have:

Grid::Grid(int iTileWidth, GameEngine*_pGame)
{
m_pGame = _pGame;
m_iTileWidth = iTileWidth;
m_iScreenWidth = m_pGame->GetWidth();
m_iScreenHeight = m_pGame->GetHeight();
m_iTilesAcross = m_iScreenWidth / m_iTileWidth;
m_iTilesDown = m_iScreenHeight / m_iTileWidth;
m_pBitmapGrid = new Bitmap*[m_iTilesAcross];
}

Its a very simple class atm, and i plan to expand on it later, but currently i know that the get height and getWidth functions being called work. And all the code above works properly, however the fault comes when i try to make the array into a 2D array:

m_pBitmapGrid = new Bitmap*[m_iTilesAcross][m_iTilesDown];

i get the following compile errors related to the line above:

error C2540: non-constant expression as array bound

error C2440: '=' : cannot convert from 'Bitmap *(*)[1]' to 'Bitmap **'

the two variables that define the size of the array are of the same type and are set to a specific value in the constructor. Even when i made the 2nd dimention of the array [m_iTilesDown] it still didnt compile and gave me the same errors.

But i know that m_iTilesAcross should count as a constant integer becuase it works fine when initializeing just a 1 dimensional array. So why is it that a single dimention array works and a 2d array dosnt. And is there any way around this?

Any help is much appriciated, thank you.

Code:

---

m_pBitmapGrid = new Bitmap*[m_iTilesAcross];
for(int i = 0; i < m_iTilesAcross; ++i)
  m_pBitmapGrid[i] = new Bitmap[m_iTilesAcross];

---

Try to use it.