Is it possible to define a friend function of a class with static members in C++?

I'm trying to declare a friend function of a class with static members. I compiled my program in Visual Studio 2017 and faced this compile-time error:

unresolved external symbol "private: static struct Number * user::Link" (?Link@user@@0PAUNumber@@A)

Here's my code:

#include<iostream>
using namespace std;

typedef struct Number
{
    int number;
    struct Number *Link;
}
num_t;


class user
{
private:
    static num_t *Link;
    static int Length;

public:
    static void Create()
    {
        cout << "You called a function." << endl;
        Link->number = 1;
    }

    friend void Show_menu();
};

void Show_menu()
{
    user::Create();
}

int user::Length = 1;
num_t user::*Link = nullptr;

int main()
{

     return 0;
}

Generally, Is it possible to define a friend function of a class with static members in C++? If so, how do I fix the problem above?