field has incomplete type error - forward declaration

Question

I think only the following headers are relevant here:

Game.h

#include "Player.h"
class Game
{
 private:
    Player White;
    Player Black;
    Board current_board; 
};

Player.h:

#include "Game.h"
#include "Piece.h"
class Player
{
private:
    Chessend end;
    std::string name;
    std::vector <Piece> pieces;
    Board* board_ptr;
    Game* game_ptr;

};

Piece.h:

   #include "Player.h"
   class Player; //forward declaration
   class Piece
  {
  private:
    Chesspiece type;
    bool moved, taken;
    Player *player;

  };

gives the following error

In file included from Player.h:11:0,
                 from Game.h:1,
                 from main.cpp:1:
Game.h:20:10: error: field 'White' has incomplete type 'Player'
   Player White;
          ^
In file included from Player.h:9:0,
                 from Game.h:1,
                 from main.cpp:1:
Piece.h:7:7: note: forward declaration of 'class Player'
 class Player;

I know there is a forward declaration in Piece.h but I'm not sure why this is a problem.

Answer 1

1) Add guards against double-inclusion in all your header files. Easiest way (supported by most compilers):

#pragma once

2) To break the circular dependency, in Player.h dont #include Game.h, but only do a "forward-declaration" of the Game class, since you only need to use it via pointers:

class Game;

3) Similarly, in Piece.h dont #include Player.h, but only forward-declare the class Player:

class Player;

As an easy and general rule, when you see in a header file that you reference another class but only via pointer, don't include the header of this other class, but only forward-declare it. Not only will this rule break circular dependencies, but it will even minimize the dependencies, which results in faster compilation and better maintainability.