尝试使某些 gobal 变量链接器命令失败

这是我的主文件，调用处理游戏保存文件的函数

#include <iostream>
#include <fstream>
#include "Menu.cpp"
using namespace std;
extern int level;
int main() {
    extern int level;
    MM_class MM_obj;
    MM_obj.MM_func();
    cout << level;
}

我希望我的MM_class是全局的，这样我就不必调用很多函数，这是MM_class：

#include "Menu.h"
#include <iostream>
#include <fstream>
using namespace std;
int level, weapon, secondWeapon;
int itemslot1, itemslot2;
class MM_class{
public:
void MM_func() {
    string saveName, saveInfo;
    int rechoice = 1;
    char gamename[] = "TBA";
    cout << "Welcome to " << gamename << "nnDo you want to load an old save or a new save?nn1 = new save      2 = old savenn";
    int saveChoice;
    cin >> saveChoice;
    if (saveChoice == 1) {
        ofstream file;
        cout << "what would you like to name the new file? (one word please)nn";
        cin >> saveName;
        file.open(saveName);
        file << "11100";
        level, weapon, secondWeapon = 1;
        itemslot1, itemslot2 = 0;
        file.close();
    }
    else {
        if (saveChoice == 2) {
            while (rechoice == 1) {
            ifstream file;
            cout << "please enter the save namenn";
            cin >> saveName;
            file.open(saveName);
            if  (file.is_open()) {
                cout << "save successfully opened!n";
                int getInfo = 0;
                int saveInfo[5];
                extern int level, weapon, secondWeapon;
                extern int itemslot1, itemslot2;
                while (getInfo != 4) {
                    file >> saveInfo[getInfo];
                    getInfo++;
                }
                level = saveInfo[0];
                weapon = saveInfo[1];
                secondWeapon = saveInfo[2];
                itemslot1 = saveInfo[3];
                itemslot2 = saveInfo[4];
                rechoice = 0;
                file.close();
            }
            else {
                if (!file.is_open()) {
                    cout << "file could not be opened... nnattempt to open a different save file?nn";
                    cin >> rechoice;
                    file.close();
                    }
                }
            }
        }
    }
}
};

它会创建一个新的文本文件，或者在选择时保存文件，但如果选择这样做，它也会读取一个，所以我希望我的变量：级别、武器、第二武器、itemslot1 和 itemslot2 可以被整个程序访问，并且可以编辑，以便如果玩家升级，它可以更改