Один класс не видит другой класс

//header.h
#pragma once
#include <iostream>
#include <clocale>
 
using namespace std;
class knight;
class skeleton;
 
class skeleton {
    
public:
    int health = 100;
 
    void strike(knight &s) {
        s.health -= 20;
        cout << "Skeleton damaged you, your health is " << s.health << endl;
        if (s.health <= 0)
            cout << "You are dead!" << endl;
    }
 
    void heal(skeleton &s) {
        s.health += 20;
        cout << "Enemy healed himself, his health is " << s.health << endl;
    }
};
class knight {
 
public:
    int health = 100;
    void strike(skeleton &s) {
        s.health -= 20;
        cout << "You hit Skeleton, its health is " << s.health << endl;
        if (s.health <= 0)
            cout << "Enemy is killed!" << endl;
    }
    void combo(skeleton &s) {
        s.health -= 40;
        cout << "You hit Skeleton, its health is " << s.health << endl;
        if (s.health <= 0)
            cout << "Enemy is killed!" << endl;
    }
    void heal(knight &k) {
        k.health += 20;
        cout << "You healed yourself, your health is " << k.health << endl;
    }
};
 
//source.ccp
#include <iostream>
#include <clocale>
#include "Header.h"
 
using namespace std;
 
int main()
{
    setlocale(LC_CTYPE, "Russian");
 
    skeleton enemy;
    knight player;
 
    player.strike(enemy);
    player.combo(enemy);
    enemy.strike(player);
    enemy.heal(enemy);
    player.heal(player);
    
 
    
    system("pause");
    return 0;
}

//пример
// есть foo.h - заголовочный файл
void foo(int); // он имеет описание ф-и
 
//foo.cpp - реализация ф-и описаной в заголовке
#include"foo.h"
void foo(int i)
{
    cout << i << endl;
}

#include <iostream>
#include <clocale>
 
using namespace std;
 
struct skeleton {
    int health;
};
struct knight {
    int health;
};
 
void strike(knight &s) {
        s.health -= 20;
        cout << "Skeleton damaged you, your health is " << s.health << endl;
        if (s.health <= 0)
            cout << "You are dead!" << endl;
    }
 
    void heal(skeleton &s) {
        s.health += 20;
        cout << "Enemy healed himself, his health is " << s.health << endl;
    }
    void strike(skeleton &s) {
        s.health -= 20;
        cout << "You hit Skeleton, its health is " << s.health << endl;
        if (s.health <= 0)
            cout << "Enemy is killed!" << endl;
    }
    void combo(skeleton &s) {
        s.health -= 40;
        cout << "You hit Skeleton, its health is " << s.health << endl;
        if (s.health <= 0)
            cout << "Enemy is killed!" << endl;
    }
    void heal(knight &k) {
        k.health += 20;
        cout << "You healed yourself, your health is " << k.health << endl;
    }
 
 
int main()
{
    setlocale(LC_CTYPE, "Russian");
 
    skeleton enemy;
    knight player;
 
    strike(enemy);
    combo(enemy);
    strike(player);
    heal(enemy);
    heal(player);
    
    system("pause");
    return 0;
}

#pragma once
#include <iostream>
#include <clocale>
 
using namespace std;
 
class knight;
class skeleton;
 
class skeleton
{
 
public:
    int health = 100;
    void strike(knight &s);
    void heal(skeleton &s);
 
};
 
class knight
{
 
public:
    int health = 100;
    void strike(skeleton &s);
    void combo(skeleton &s);
    void heal(knight &k);
 
};
 
void skeleton::strike(knight &s){
    s.health -= 20;
    cout << "Skeleton damaged you, your health is " << s.health << endl;
    if (s.health <= 0)
        cout << "You are dead!" << endl;
}
 
void skeleton::heal(skeleton &s){
    s.health += 20;
    cout << "Enemy healed himself, his health is " << s.health << endl;
}
 
 
 
 
void knight::strike(skeleton &s){
    s.health -= 20;
    cout << "You hit Skeleton, its health is " << s.health << endl;
    if (s.health <= 0)
        cout << "Enemy is killed!" << endl;
}
 
void knight::combo(skeleton &s){
    s.health -= 40;
    cout << "You hit Skeleton, its health is " << s.health << endl;
    if (s.health <= 0)
        cout << "Enemy is killed!" << endl;
}
void knight::heal(knight &k){
    k.health += 20;
    cout << "You healed yourself, your health is " << k.health << endl;
}