vc编译出错：error C2248: 'x' : cannot access private member declared in class 'Point'解决方法

vc编译出错：error C2248: 'x' : cannot access private member declared in class 'Point'
//=====================================
// f0807.cpp
// member & non-member operator
//=====================================
#include<iostream>
using namespace std;
//-------------------------------------
class Point{
  int x, y;
public:
  void set(int a, int b){ x=a, y=b; }
  Point operator+(const Point& d){
  Point s;
  s.set(x+d.x, y+d.y);
  return s;
  }//----------------------------------
  friend ostream& operator<<(ostream& o, const Point& d);
};//===================================
inline ostream& operator<<(ostream& o, const Point& d){
  return o<<"("<<d.x<<","<<d.y<<")\n";
}//====================================
int main(){
  Point s,t;
  s.set(2,5);
  t.set(3,1);
  cout<<s+t;
}//====================================


用VC编译的时候出现以下错误：
ompiling...
main.cpp
F:\C++ project\main\main.cpp(20) : error C2248: 'x' : cannot access private member declared in class 'Point'
  F:\C++ project\main\main.cpp(9) : see declaration of 'x'
F:\C++ project\main\main.cpp(20) : error C2248: 'y' : cannot access private member declared in class 'Point'
  F:\C++ project\main\main.cpp(9) : see declaration of 'y'
F:\C++ project\main\main.cpp(26) : error C2593: 'operator <<' is ambiguous
F:\C++ project\main\main.cpp(27) : warning C4508: 'main' : function should return a value; 'void' return type assumed
Error executing cl.exe.

main.exe - 3 error(s), 1 warning(s)

我在书上看过，用friend建立的函数是可以访问局部变量的，但是这里怎么不行呢？

------解决方案--------------------
friend当然可以访问似有成员，
其意义就在于此。
不要误导lz

这里应该先声明这个函数

探讨

跟编译器没有关系，类的默认定义是private
不能在非成员函数中访问,friend也不行
class Point{
  int x, y;