问C++错误:对全局变量和对象数组使用未声明的标识符

EN

#ifndef List_hpp
#define List_hpp

#include <stdio.h>
#include <string>
#include <iostream>
#include "Item.hpp"

using namespace std;

class List
{
    private:
    static int itemcount;
    Item items[100];

    public :
    List(){
        this->itemcount = 0;
    };
    void addItem(string, string, int, double);
    void removeItem(string);
    void display();
}; //End List Class

#endif /* List_hpp */

#include "List.hpp"

//This method will add a new item
void addItem(string name, string unit, int quantity, double price){
    Item newItem(name, unit, quantity, price);
    itemcount++;
}

void removeItem(string name){
}

void display(){
}