C++ Operator Overload '+=' operator concatenates strings

#include <iostream>
#include <cstring>       //for strcpy(), strlen()
using namespace std;
#include <process.h>     //for exit()
class String             //user-defined string type
{
   private:/* w w w.j  a v  a  2  s .  com*/
   enum { SZ = 80 };              //size of String objects
   char str[SZ];                  //holds a C-string
   public:
   String()                       //no-arg constructor
   { strcpy(str, ""); }
   String( char s[] )             //1-arg constructor
   { strcpy(str, s); }
   void display()                 //display the String
   { cout << str; }
   String operator += (String ss) //add a String to this one
   {                           //result stays in this one
   if( strlen(str) + strlen(ss.str) >= SZ )
      { cout << "\nString overflow"; exit(1); }
   strcat(str, ss.str);        //add the argument string
   return String(str);         //return temp String
}
};
int main()
{
    String s1 = "Merry Christmas!  ";      //uses 1-arg ctor
    String s2 = "Happy new year!";         //uses 1-arg ctor
    String s3;                             //uses no-arg ctor
    s3 = s1 += s2;         //add s2 to s1, assign to s3
    cout << "\ns1="; s1.display();         //display s1
    cout << "\ns2="; s2.display();         //display s2
    cout << "\ns3="; s3.display();         //display s3
    cout << endl;
    return 0;
}