invalid use of non-static data member 'Board::N' - c++

Question

This is the header:
class Board {
public:
friend class Game;
Board() = default;
Board(int n) : N(n) { }
Board& SetType(int, int, char);
void GetType(int, int);
Board& CreateEmptyBoard();
void BoardDisplay();
private:
int N = 0;// dimension
char Maze[15][15];
const static int MaxSize = 15;
};
class Game {
public:
Game() = default;
Game(int x, int y) : PosX(x), PosY(y) { }
void BuildGame();
void GameDisplay();
void MoveUp();
void MoveDown();
void MoveLeft();
void MoveRight();
private:
int PosX = 0;
int PosY = 0;
};
void Game::BuildGame() {
srand(time(NULL));
for (int i = 0; i < Board::N; i++) {
for (int j = 0; j < Board::N; j++) {
if (i == rand() % (Board::N) && j == rand() % (Board::N))
Board::Board& SetType(i, j, 'W');
}
}
}
In class Game's member function void BuildGame,I want to call member functionBoard& SetType(int,int,char) in class Board.I define this function in a header file and not show here. Then I build the project, I got invalid use of non-static data member 'Board::N' and 'SetType' was not declared in this scope. Like this
Where I wrong? I can't find it.

Answer 1

The compiler is letting you know that you are using an instance variable as a static. A static variable is associated with an entire class and not a single object, so it is called through the class name and not an object of the class. but it would need to be marked as static like so
class Board
{
public:
static Board& setType(int, int, char);
...
private:
static int N;
...
}
my instinct however tells me that you want to use it at an instance level, so you would write your void Game::buildGame() method using a Board that it creates (possibly making it an attribute of the Game class:
void Game::BuildGame() {
//make your board here. alternatively make an instance of the game
Board myBoard();
srand(time(NULL));
//in the following, use myBoard as the instance of a board.
for (int i = 0; i < myBoard.N; i++) {
for (int j = 0; j < myBoard.N; j++) {
if (i == rand() % (myBoard.N) && j == rand() % (Board::N))
myBoard.setType(i, j, 'W');
}
}
}
And a Board class that looks something like this. You will probably want your setType method to modify the instance and return void instead of returning another board reference.
class Board
{
public:
//this one will change this particular Board instance.
void setType(int, int, char);
//this one may make sense to be static if it is a factory method
//but why not use a constructor instead?
static Board& createEmptyBoard();
//maybe you meant something to reset the board to empty state.
void resetBoardToEmpty();
...
private:
int N;
...
}
while you're at it you might make it a struct (which has members public by default) as it seems to be a "hidden" holder class for the game, and this would alleviate the need to use a friend class (these are to be used judiciously as they can get messy really fast). using a struct would also allow you to make a ChessGame class that reuses the Board struct.

Answer 2

N isn't a static member of class Board, hence you'll need an instance of board to access it.
Your Game class actually needs to have a Board member variable to achieve that above mentioned instance.