Accessing static member function from friend function - c++

Question

I am using a linked list to implement a set class. In order to hide my struct Node from the users, I put the struct Node declaration into private. Furthermore, I overloaded the operator +, which denotes union operation between two sets. Because I want to implement it by recursion, I overloaded the operator + in the private field called unionMerge. However, it produced an error "error: ‘unionMerge’ was not declared in this scope", but I did put the unionMerge declaration above the operator+ in the Set.cpp file. Could someone help me out?
Set.h is an interface file.
Set.cpp is the implementation for the interface.
Here is the interface file, Set.h:
#ifndef SET_H
#define SET_H
#include <iostream>
using namespace std;
class Set{
public:
Set();
friend const Set operator+(const Set& a, const Set& b);
private:
struct Node{ //private Node
int value;
Node* next;
};
Set(Node *p); //private constructor
Node* list;
//overload the public operator+ above
static Node* unionMerge(const Node* p, const Node *q);
};
#endif
Here is the implementation file, Set.cpp:
#include <iostream>
#include "Set.h"
Set::Set():list(nullptr){
}
Set::Set(Node* p){
list = p;
}
Set::Node* Set::unionMerge(const Node *p, const Node *q){
// to debug, I return nullptr
return nullptr;
}
const Set operator+(const Set& a, const Set& b){
//error: ‘unionMerge’ was not declared in this scope
return Set(unionMerge(a.list,b.list));
Set::Node *p = unionMerge(a.list,b.list);
Set s;
s.list = p;
return s;
}

Answer 1

operator+() is not a member of Set (friends are not members), so if you want to access a static member of Set you have to qualify it:
return Set(Set::unionMerge(a.list,b.list));