标记之前'{'应具有类名

在我的c++ Qt项目中，我得到了错误"在'{'令牌之前预期的类名"。在谷歌上搜索之后，这似乎是一个循环包含的问题。我有pawn。h，它包含piece。h，它包含board。h，它通过包含pawn。h来完成这个圆。我读到这可以通过前向声明来解决，但我尝试前向声明一些问题类，但它不起作用。

#ifndef PAWN_H
#define PAWN_H
#include "piece.h"
class Pawn : public Piece
{
    Q_OBJECT
public:
    explicit Pawn(QWidget *parent = 0);
};
#endif // PAWN_H

.

#ifndef PIECE_H
#define PIECE_H
#include <QWidget>
#include "board.h"
class Board;
class Piece : public QWidget
{
    Q_OBJECT
public:
    explicit Piece(QWidget *parent = 0);
    void setPosition(int rank, int file);
    QPixmap pixmap;
protected:
    void paintEvent(QPaintEvent *);
private:
    int rank;
    int file;
    int x;
    int y;
};
#endif // PIECE_H

.

#ifndef BOARD_H
#define BOARD_H
#include <QWidget>
#include <QVector>
#include <QGridLayout>
#include "square.h"
#include "pawn.h"
#include "knight.h"
#include "bishop.h"
#include "queen.h"
#include "king.h"
class Board : public QWidget
{
public:
    explicit Board(QWidget *parent = 0);
    QVector < QVector<Square *> > sqrVector;
    Pawn *pawn[8];
    Knight *knight[2];
    Bishop *bishop[2];
    Queen *queen;
    King *king;
private:
    QGridLayout *layout;
};
#endif // BOARD_H