定义一个字符串类，重载运算符，进一步修饰完善

#define NULL 0
#include <iostream>
#include <string.h>
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
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(int argc, char** argv) {
    String string1("Hello"),string2("Book"),string3("Computer"),string4("Hello");
    compare(string1,string2);
    compare(string2,string3);
    compare(string1,string4);
    return 0;
}