struct abc{
     String b;
   public:
       abc( String& xx){
         b=xx;  // <-- the xx variable will be copied to b;    
       }
       void fecha(){
          b="retornado";   // this not return: reference constructor is address (No, because you are not working on a reference. You are working on a local variable called "b", which will be destroyed with the struct, by the way.) 
       }
       ~abc(){}; // << - Will destroy b.
};

struct abc{
     String& b; // <-- Now b is a reference.
   public:
       abc( String& xx)
           : b(xx) // >  B will refer to xx;
       {
       }
       void fecha(){
          b="retornado";  // Will now set the referred object (xx).
       }
       ~abc(){}; // << - Will not destroy what b refers to (xx).
};