下面程序不知道错在何处，请高手指教


#include "stdafx.h"
#include <iostream>
#include <cstring>

using namespace std;

class String
{
public:
    String(){p=NULL;}
    String(char *str);
    void display();
    bool operator>(String &String2);
    bool operator<(String &String2);
    bool operator==(String &String2);
    void compare(String &,String &);
private:
    char *p;
};

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

bool String::operator>(String &String2)
{
    if(strcmp(this->p,String2.p)>0)
        return true;
    else return false;
}

bool String::operator<(String &String2)
{
    if(strcmp(this->p,String2.p)<0)
        return true;
    else return false;
}

bool String::operator==(String &string2)
{
    if(strcmp(this->p,string2.p)==0)
        return true;
    else return false;
}

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


void String::compare(String &string1,String &string2)
{
    if(operator>(string2)==1)
    {string1.display();cout<<">";string2.display();}
    else
        if(operator<(string2)==1)
        {string1.display();cout<<"<";string2.display();}
        else
            if(operator==(string2)==1)
            {string1.display();cout<<"=";string2.display();}
            cout<<endl;
}

void main()
{
    String string1("hello"),string2("book"),string3("computer"),string4("hello");
    compare(string1,string2);
    compare(string2,string3);
    compare(string3,string4);
   
}