为什么类声明顺序仍然与原型相关？

我在代码中遇到字段 player类型不完整错误，我的类 game_session包含一个 player，我在相同的头文件中声明了它们，如下所示：

I was struggling with a 'field "player" has incomplete type" error with my code. My class "game_session" contains a single "player" and I declare them both in the same header file as shown below:

#ifndef HEADER_H
#define HEADER_H

#include <iostream>
#include <vector>

using std::vector;

class Player;
class GameSession;

class GameSession{
 private:
  ...
  Player player;
 public:
  GameSession();
  ~GameSession();
  ...
};

class Player {
 public:
  Player( int maxdim );
  ~Player();
  ...
};  

上面的代码无法编译，因为GameSession找不到Player类的声明当我切换两个类时，它起作用，如下所示：

The above code would not compile because the GameSession could not find a declaration for the Player class. It worked when I switched the two classes as shown below:

#ifndef HEADER_H
#define HEADER_H

#include <iostream>
#include <vector>

using std::vector;

class Player {
 public:
  Player( int maxdim );
  ~Player();
  ...
};

class GameSession{
 private:
  ...
  Player player;
 public:
  GameSession();
  ~GameSession();
  ...
};

我不再需要原型。我的问题是，为什么原型制作不能避免由于缺少有序声明而导致的错误？此外，将来如何避免存在许多类和依赖项？

I no longer needed the prototypes. My question is why did the prototyping not prevent the errors from a lack of ordered declaration? Also, how can this be avoided in the future for when there are many classes and dependencies?

Danke

（对于那些想知道的人，我在类实现中使用了一个初始化程序列表来处理Player没有默认构造函数的事实）。

(For those wondering, I used an initializer list in the class implementation to deal with the fact Player does not have a default constructor)