求八皇后问题C++程序设计

#ifndef _QUEENBOARD_H_
#define _QUEENBOARD_H_

const int BOARDSIZE = 8;

using namespace std;

class Queenboard {

private:
bool board[BOARDSIZE][BOARDSIZE];

public:
Queenboard();
bool is_space_under_attack(int, int) const;
void occupy_space(int, int);
void clear_column(int);

friend ostream& operator<<(ostream& out, const Queenboard& cb);
};

Queenboard::Queenboard() {
// Initialize the board to contain zero queens.
for (int row = 0; row < BOARDSIZE; row++) {
for (int col = 0; col < BOARDSIZE; col++) {
board[row][col] = false;
}
}
}

ostream& operator<<(ostream& out, const Queenboard& cb) {

// output the board
for (int row = 0; row < BOARDSIZE; row++) {

out << "---------------------------------" << endl;
for (int col =