Realize "-" operator overloading as a friend function of class - c++

Question

I try to realize difference of sets with "-". For example:
Set a = 1 2 3 4;
Set b = 3 4 5 6;
Set c = a - b; // (1 2);
How i should overload operator? I tried to use frienldy function, but it can't work with variable "size". Visual Studio show error "c2597" at 28th line (size++;)
I can't understand, how to overloading "-" for using it with two sets. When I overload method of class, i can use only one argument. When i use friend-function, i can use twi arguments (Set a, Set b), but i can't work with variable "size".
#include <iostream>
#include <locale.h>
#include <conio.h>
#include <vector>
using namespace std;
class Set {
private:
int size;
vector <int> vect;
public:
Set() { size = 0; }
~Set() { vect.clear(); }
void Enter();
void Show();
friend Set operator-(Set a, Set b)
{
size = 0;
vect.clear();
int i, j, n = 0;
for (i = 0; i < a.size; i++) {
int cnt = 0;
for (j = 0; j < b.size; j++)
{
if (a.vect[i] == b.vect[j]) cnt++;
}
if (cnt == 0) {
size++;
vect.push_back(a.vect[i]);
}
}
return a;
}
void add()
{
int element;
cout << "Введите новый элемент " << endl;
cin >> element;
size = size + 1;
vect.push_back(element);
}
};
void Set::Enter() {
cout << "Введите размер " << endl;
cin >> size;
vect.resize(size);
cout << "Введите элементы :" << endl;
for (int i = 0; i < size; i++)
{
cin >> vect[i];
}
}
void Set::Show() {
cout << "Множество: " << endl;
for (int i = 0; i < size; i++)
cout << vect[i] << " ";
cout << endl;
}
int main() {
setlocale(LC_ALL, "RUS");
Set a;
a.Enter();
Set b;
b.Enter();
Set c;
c = a - b;
c.Show();
c.add();
c.Show();
_getch();
return 0;
}
UPD:
I made it through method:
Set operator-(const Set& b)
{
Set a = *this;
Set tmp;
tmp.size = 0;
vect.clear();
int i, j, n = 0;
for (i = 0; i < a.size; i++) {
int cnt = 0;
for (j = 0; j < b.size; j++)
{
if (a.vect[i] == b.vect[j]) cnt++;
}
if (cnt == 0) {
tmp.size++;
tmp.vect.push_back(a.vect[i]);
}
}
return tmp;
}

Answer 1

friend functions are not member methods, so you have to remove/replace usage of (implicit) this as size = 0, you might add extra object:
friend Set operator-(Set a, Set b)
{
Set res;
for (int i = 0; i < a.size; i++) {
int cnt = 0;
for (int j = 0; j < b.size; j++)
{
if (a.vect[i] == b.vect[j]) cnt++;
}
if (cnt == 0) {
res.size++;
res.vect.push_back(a.vect[i]);
}
}
return res;
}