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From: aed@netcom.COM (Andrew Davidson)
Newsgroups: comp.lang.c++
Subject: assignment operator for dervired classes
Message-ID: <1991Apr23.153754.4582@netcom.COM>
Date: 23 Apr 91 15:37:54 GMT
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 95

/*
Hi

I have been having trouble writing an assignment operator for a
derived
class. The problem is how do you initialize the the base class. The
reading
I have done to date suggest that that the assiginment operator for a
derived
class is written the same way the copy constructor is written.

The problem is that I can not call the base class constructor from
within the
derived class assignment operator. I get the following message from
the compiler

"assign.cc", line 49: warning: result of  constructor call expression
not used
*/


#include <s#ream.h>
class X
{
 public:

  X(int i = 0)
        {numX = i;}
  X(const X &other)
        {numX = other.numX;}
  X& operator = (const X &other)
        {
          numX = other.numX;
          return (*this);
        }

  void display() {cout << "numX = " << numX << " ";}

 private:
  int numX;
};

class Y : public X
{
 public:

  Y(int i = 1) : X(i)
        {numY = i;}
  Y(const Y &other) : X(other)
        {numY = other.numY;}

  Y& operator = (const Y &other)
        {
/* 49 */          X(other);
          numY = other.numY;
          return(*this);
        }

  void display() {X::display(); cout << "numY = " << numY << "\n";}
 private:
  int numY;
};

/*
 * out put is
 * numX = 2 numY = 2
 * numX = 3 numY = 3
 * numX = 2 numY = 3  this line should be 3,3
 * numX = 3 numY = 3
*/

main()
{
  Y y1(2);
  y1.display();

  Y y2(3);
  y2.display();

  y1 = y2;
  y1.display();

  Y y3(y2);
  y3.display();

}

-- 
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                  "bede-bede-bede Thats all Folks"
				Porky Pig
Andy Davidson
Woodside CA.
aed@netcom.COM
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