友元函数不能访问类的私有成员？

以下代码在编译时通不过，显示很多错误，我将错误信息放在代码的注示中了，请各位高手给珍断珍断，谢谢！
#include <iostream>
#include <string>
using namespace std;

class String
{public:
   String(){p=NULL;}
   String(char *str);
   friend bool operator>(String &st1,String &st2);
   void display();
private: char *p;//错误2：see declaration of 'p'
};

String::String(char *str)
{p=str;}

void String::display()
{cout<<p;}

bool operator>(String &st1,String &st2)
{if(strcmp(st1.p,st2.p)>0) //错误1：error C2248: 'p' : cannot access private
 // member declared in class 'String'
    return true;
  else return false;
 }

int main()
{String st1("Hello"),st2("Book")
  cout<<(st1>st2)<<endl;
  return 0;
}