Operator overloading+ add two objects - c++

Question

I'm trying to add two object that they are in the same class.
In the private section of the class I have two int variables
class One {
private:
int num1, num2;
public:
One operator+=(const One&); // - a member operator that adds another One object - to the current object and returns a copy of the current object
friend bool operator==(const One&, const One&); // - a friend operator that compares two One class objects for equality
};
One operator+(const One&, const One&);// - a non-friend helper operator that adds One objects without changing their values and returns a copy of the resulting One
I'm not sure I have a problem on the opeartor+ I guess
One operator+(const One &a, const One &b){
One c,d,r;
c = a;
d = b;
r += b;
r += a;
return r;
}
I think the above code is wrong, but I tried to use like b.num1 and I get compile error
error: 'int One::num1' is private
error: within this context
and I can't use b->num1 as well because the above function is not in the member function section.
error: base operand of '->' has non-pointer type 'const One'
This is how it calls in main
Result = LeftObject + RightObject;

Answer 1

If you have already implemented this member function:
One One::operator+=(const One&);
Then you may implement the non-member addition operator thus:
One operator+(const One& lhs, const One& rhs) {
One result = lhs;
result += rhs;
return result;
}
This can be simplified somewhat into the following:
One operator+(One lhs, const One& rhs) {
return lhs += rhs;
}
This pattern (which you can adapt for all operator/operator-assignment pairs) declares the operator-assignment version as a member -- it can access the private members. It declares the operator version as a non-friend non-member -- this allows type promotion on either side of the operator.
Aside: The += method should return a reference to *this, not a copy. So its declaration should be: One& operator+(const One&).
EDIT: A working sample program follows.
#include <iostream>
class One {
private:
int num1, num2;
public:
One(int num1, int num2) : num1(num1), num2(num2) {}
One& operator += (const One&);
friend bool operator==(const One&, const One&);
friend std::ostream& operator<<(std::ostream&, const One&);
};
std::ostream&
operator<<(std::ostream& os, const One& rhs) {
return os << "(" << rhs.num1 << "#" << rhs.num2 << ")";
}
One& One::operator+=(const One& rhs) {
num1 += rhs.num1;
num2 += rhs.num2;
return *this;
}
One operator+(One lhs, const One &rhs)
{
return lhs+=rhs;
}
int main () {
One x(1,2), z(3,4);
std::cout << x << " + " << z << " => " << (x+z) << "\n";
}

Answer 2

I can't see why the operator+ is wrong:
#include <stdio.h>
class One {
public:
One(int n1, int n2): num1(n1),num2(n2) {}
private:
int num1, num2;
public:
One operator+=(const One& o) {
num1 += o.num1;
num2 += o.num2;
return *this;
}
friend bool operator==(const One&, const One&); // - a friend operator that compares two One class objects for equality
void print() {
printf("%d,%d\n", num1, num2);
}
};
One operator+(const One& a, const One& b) {
One r(0,0);
r += b;
r += a;
return r;
}
int main() {
One a(1,2),b(3,4);
One r = a + b;
r.print();
}