C++ 友元函数无法访问私有成员

#ifndef STRING_H

#define STRING_H

#include<iostream>



using namespace std;

class String

{

private:

    char *str;

    int len;

    static int num_strings;

    static const int CINLIM=80;



public:

    String(const char *s);

    String();

    String(const String &);

    ~String();

    int length()const{return len;}



    //overloaded operator methods

    String & operator=(const String &);

    String & operator=(const char *);

    char & operator[](int i);

    const char & operator[](int i)const;



    //overloaded operator friend

    friend bool operator<(const String &st,const String &st2);

    friend bool operator>(const String &st,const String &st2);

    friend bool operator==(const String &st,const String &st2);

    friend ostream & operator<<(ostream &os,const String &st);

    friend istream & operator>>(istream &is,String & st);

    //static function

    static int HowMany();

};

#endif // STRING_H



istream & operator>>(istream & is,const String &st)

{

    char temp[String::CINLIM];

    is.get(temp,String::CINLIM);

    if(is)

        st=temp;

    while(is&&is.get()!='\n')

        continue;

    return is;

}