SudokuBoard-RecursionUpdates.cpp

// adjust to deque for pushing back on front
using cellQueue = deque<shared_ptr<SudokuCell>>;

class SudokuBoard {
    cellQueue cells;

public
  /*
   * .....
   */
  
    // fill the board with valid solution
    void fillCells(){/* ... */}
    // recursive step for filling the board
    bool doFillCells(cellQueue &remainingCells) {/* ... */}
    
  /*
   * .....
   */
};