Question

I’m getting the error “expected class-name before ‘{‘ token” in my C++ Qt project. After googling it, it seems like its a problem of circular includes. I have pawn.h that includes piece.h, which includes board.h, which completes the circle by including pawn.h. I’ve read that this can be fixed with forward declarations, but I’ve tried forward declaring a few of problem classes, and it doesn’t work.

#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

Answer 1

Instead of randomly trying things, try changing board.h to include forward declarations for all the pieces:

board.h

class Pawn;
class Knight;
class Bishop;
class Queen;
class King;

And remove the corresponding #include statements. (You’ll probably need to put those #include statements in board.cpp, when you decide you need to see inside the various piece classes.)