To do what you want to do, you'd need something like this

struct GroupData {
int Rows, Cols;
char board[0]; // a placeholder for the board data
};

then when you allocate the board make a call like this:

GroupData *pData = (GroupData *)new BYTE[sizeof(GroupData)+Rows*Cols];

and when you call SetGroupData
pDP->SetGroupData(idGroup, pData, sizeof(GroupData)+Rows*Cols, dwFlags);

This will send the whole board.

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-vince