//#include <iostream.h> MODIFIED: Modern library headers do not have extension.
    #include <iostream>
 
    using namespace std; 
 
    class demo {
        string a;
        // string *p; DELETED: Not sure what was the purpose of this.
 
        public: 
 
        demo() 
        {
         //a=0;         DELETED: std::string already have a sane initialization.
         //                      and assigning it to 0 looks like a bad idea to me.
         //                      (it will be taken as a char * nullptr)
         //p = new int; DELETED: We removed p. Also p was a pointer to string
         //*p = NULL;   DELETED: we removed p (modern C++ will use nullptr)
        } 
 
        demo ( const string *q ): a(*q) // MODIFIED: use initializers
        {
            //p= new int; DELETED: we don't have p.
            //*p=q;       DELETED: we don't have p.
        }
 
 
        demo (demo &r):a(r.a) { 
          //a= r.a;      MOVED as initializer.
          //p= new int;  DELETED: we don't have a p.
          //*p= *(r.p);  DELETED: we don't have a p.
        }
 
        demo (const std::string &_a):a(_a) { 
        	// NEW: to support the constructor from string
        }        
 
        ~demo ()  {
           //delete p;  //No need
         }
 
        void show () {
           cout << a;  
        } 
        void change (const std::string &_a) { // MODIFIED: added signature.
           //s3.a=s2.a; MODIFIED: properly assign input value to member
           a = _a;
        }
 
    };
 
    int main () {
 
            demo s1;
            demo s2("Hello");
            demo s3(s2);
            s1.show();
            s2.show();
            s3.show();
            s2.change("Java");
            s2.show();
            s3.show();
     }

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