c++运算符重载报错，如何解决？

Question

#include <iostream>
using namespace std;
struct box
{
  long num;
  int operator+(const box &b1,const box &b2) const
  {
    return b1.num + b2.num;
}
box b_one,b_two;
int main()
{
  b_one.num = 2000;
  b_two.num = 2000;
  cout << b_one + b_two;
  return 0;

操作系统：Windows 10
初步解决方法：删除const，方法没有效果

Answer 1

#include <iostream>
using namespace std;
struct box
{
  long num;
  int operator+(const box &b1) const
  {
    return num + b1.num;
  }
};
struct box b_one,b_two;
int main()
{
  b_one.num = 2000;
  b_two.num = 2000;
  cout << b_one + b_two;
  return 0;
}