C++ string class based on char array

#include <iostream>
#include <cstring>       // for strcpy(), strcat()
using namespace std;
class String/*from www.  j  a  v a  2 s.com*/
{
   private:
       enum { SZ = 80 };                  //max size of Strings
       char str[SZ];                      //array
   public:
       String()                           //constructor, no args
       { str[0] = '\0'; }
       String( char s[] )                 //constructor, one arg
       { strcpy(str, s); }
       void display()                     //display string
       { cout << str; }
       void concat(String s2)             //add arg string to
       {                               //this string
           if( strlen(str)+strlen(s2.str) < SZ )
              strcat(str, s2.str);
           else
              cout << "\nString too long";
       }
};
int main()
{
   String s1("Merry Christmas!  ");      //uses constructor 2
   String s2 = "Season's Greetings!";    //alternate form of 2
   String s3;                            //uses constructor 1
   cout << "\ns1="; s1.display();        //display them all
   cout << "\ns2="; s2.display();
   cout << "\ns3="; s3.display();
   s3 = s1;                              //assignment
   cout << "\ns3="; s3.display();        //display s3
   s3.concat(s2);                        //concatenation
   cout << "\ns3="; s3.display();        //display s3
   cout << endl;
   return 0;
}