String类运算符重载函数

#include
    
     #include
     
      using namespace std;class String{public:    String( ){p=NULL;}    String(char *str);    friend bool operator>(String &s1, String &s2);    friend bool operator<(String &s1, String &s2);    friend bool operator==(String &s1, String &s2);        friend bool operator>=(String &s1, String &s2);    friend bool operator<=(String &s1, String &s2);    friend bool operator!=(String &s1, String &s2);    void display( );private:    char *p;};String::String(char *str){    p = new char[strlen(str)+1];    strcpy(p, str);} void String::display( ){    cout << p;} bool operator>(String &s1,String &s2){    if(strcmp(s1.p,s2.p)>0)        return true;    else        return false;} bool operator<(String &s1,String &s2){    if(strcmp(s1.p,s2.p)<0)        return true;    else        return false;} bool operator==(String &s1,String &s2){    return !((s1>s2)||(s1
      
       =(String &s1,String &s2){    return !(s1
       
        s2);} bool operator!=(String &s1,String &s2){    return ((s1>s2)||(s1
        
         s2)    {        s1.display( );        cout<<" > ";        s2.display( );    }    else if(s1