赋值、相加、比大小、前++、后++
#include<iostream.h>
#include<string.h>
class String
{
private:
int length;
char *ptr;
public:
String(char *s);
~String();
String(String &s);
void display();
String operator =(String &s);
String operator +(String &s);
int operator >(String &s);
int operator <(String &s);
int operator <=(String &s);
int operator >=(String &s);
String operator++();
String operator++(int);
};