C++ 来自声明为另一个类的朋友的类的方法引发错误

我有以下两个职业

Player

和

FriendOfPlayer

：

Player.hpp

#ifndef PLAYER
#define PLAYER

#include"FriendOfPlayer.hpp"
#include<string>


using namespace std;

//class FriendOfPlayer;

class Player{

    public:

           // getters and setters ...

        static int getNbObj(){
            return nbObj;
        };

        friend void FriendOfPlayer::displayPlayerPrivateMember(Player &p);
        

        // constructor
        Player();
        Player(string name= "None", int health = 0, int xp = 0);
        Player(const Player &source);


        // Destructor
        ~Player(){
            --nbObj;
        };


    private:
        //private members not displayed
        string privMember = "private member";
        static int nbObj;

       
};

#endif

#ifndef FRIEND_OF_PLAYER
#define FRIEND_OF_PLAYER

#include"Player.hpp"

class FriendOfPlayer {

    public:

        void displayPlayerPrivateMember(Player &p);


};

#endif

FriendOfPlayer.cpp

#include "Player.hpp"
#include<string>

using namespace std;


Player::Player(){;};

Player::Player(string name, int health, int xp)
    : name{name}, health{health}, xp{xp}{
        ++nbObj;
    }

Player::Player(const Player &source)
        : name{"I am a copy"}, health{source.health}, xp{source.xp}{
            ++nbObj;
        }


int Player::nbObj = 0;

#include "Player.hpp"
#include "FriendOfPlayer.hpp"


#include<iostream>
#include<stdio.h>


void FriendOfPlayer::displayPlayerPrivateMember(Player &p){

    cout << p.privMember << endl;
}

---

我做错了什么？原型

void FriendOfPlayer:：displayerprivatember（int&）

从何而来？

我想知道为什么没有标题包含问题。你不能像以前那样做圆形包含。我建议从Friendofplayer.hpp中删除

#include“player.hpp

。而只需在Friendofplayer.hpp中向前声明player类

---

编辑：您收到的错误消息令人困惑，但我只是自己尝试过，循环依赖性是导致错误的原因。转发声明解决了该问题。请尝试最简单的示例。

我是否应该始终遵循以下准则：包含在cpp中，并在标题中转发声明？@FLOFL29:仅在需要时，请参阅此处的错误#7:。

g++ Player.cpp FriendOfPlayer.cpp
FriendOfPlayer.cpp:9:6: error: prototype for ‘void FriendOfPlayer::displayPlayerPrivateMember(Player&)’ does not match any in class ‘FriendOfPlayer’
 void FriendOfPlayer::displayPlayerPrivateMember(Player &p){
      ^~~~~~~~~~~~~~
In file included from Player.hpp:5:0:
FriendOfPlayer.hpp:10:14: error: candidate is: void FriendOfPlayer::displayPlayerPrivateMember(int&)
         void displayPlayerPrivateMember(Player::Player &p);