114. Operator Overloading-II

A developer needs to support int + Vec (not just Vec + int):

class Vec {
   int x;
public:
   Vec(int v) : x(v) {}
   // Vec + int works as member:
   Vec operator+(int n) const { return Vec(x+n); }
   // int + Vec needs friend:
   friend Vec operator+(int n, const Vec& v) {
       return Vec(n + v.x);
   }
};

Vec a(10);
Vec b = 5 + a;  // int + Vec

Why is the friend function needed?