请问下面的程序错在哪里?我在VC++上运行时说运算符重载函数调用不了私有成员，但是不是声明了友元函数了吗？到底怎么回事啊？请高手指点下
#include <iostream>
#include <string>
using namespace std;
class String
{
public:
    String(){p=NULL;}
    String(char *str);
    friend bool operator > (String &string1,String &string2);
    friend bool operator < (String &string1,String &string2);
    friend bool operator == (String &string1,String &string2);
    friend void compare(String &string1,String &string2);
    void display();
private:
    char * p;
};
String::String(char * str)
{
    p=str;
}
void String::display()
{
    cout<<p;
}
bool operator >(String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)>0)
        return true;
    else
        return false;
}
bool operator < (String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)<0)
        return true;
    else
        return false;
}
bool operator == (String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)==0)
        return true;
    else
        return false;
}
void compare(String &string1,String &string2)
{
    if((string1.p>string2.p)==1)
    {
        string1.display();cout<<">";string2.display();
    }
    else
        if((string1.p<string2.p)==1)
        {
            string1.display();cout"<";string2.display();
        }
        else
            if(string1.p==string2.p)==1)
            {
                string1.display();cout"=";string2.display();
            }
            cout<<endl;
}
int main()
{
    String string1("Hello"),string2("Book"),string3("Computer"),string4("Helle");
    compare(string1,string2);
    compare(string2,string3);
    compare(string3,string4);
    return 0;


}

谢谢高手指点，我是在VC++6.0上运行的，老是出错，我是菜鸟，怎么都搞不好！

这个程序是我在书上抄来的，做了下改动，高手帮忙看看原程序好在哪里可以吗？
#include <iostream>
#include <string>
using namespace std;
class String
{
public:
    String(){p=NULL;}
    String(char *str);
    friend bool operator > (String &string1,String &string2);
    friend bool operator < (String &string1,String &string2);
    friend bool operator == (String &string1,String &string2);
    void display();
private:
    char * p;
};
String::String(char * str)
{
    p=str;
}
void String::display()
{
    cout<<p;
}
bool operator >(String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)>0)
        return true;
    else
        return false;
}
bool operator < (String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)<0)
        return true;
    else
        return false;
}
bool operator == (String &string1,String &string2)
{
    if(strcmp(string1.p,string2.p)==0)
        return true;
    else
        return false;
}
void compare(String &string1,String &string2)
{
    if（operator > (string1,string2)==1)
    {
        string1.display();cout<<">";string2.display();
    }
    else
        if（operator < (string1,string2)==1)
        {
            string1.display();cout"<";string2.display();
        }
        else
            if（operator == (string1,string2)==1)
            {
                string1.display();cout"=";string2.display();
            }
            cout<<endl;
}
int main()
{
    String string1("Hello"),string2("Book"),string3("Computer"),string4("Helle");
    compare(string1,string2);
    compare(string2,string3);
    compare(string3,string4);
    return 0;


}

谢谢你的指点，但是我想问的是这个程序和上面问题上的函数有什么优劣之分吗？
两个程序的compare函数是不一样的