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From: jlk@pravda.gatech.edu (Janet Kolodner)
Newsgroups: comp.lang.c++
Subject: Default copy constructor not making a copy.
Message-ID: <22023@hydra.gatech.EDU>
Date: 14 Feb 91 14:55:10 GMT
Sender: news@prism.gatech.EDU
Reply-To: gatech.edu!blsouth!klein (Michael Klein)
Organization: BellSouth Services, Atlanta, GA
Lines: 49

Can someone explain why the two classes defined below exhibit different
behavior?  The class without the explicit copy constructor does not make
a copy of the *this argument for the + operator.  There is a related
comment in Ellis & Stroustrup (12.6.1): "Had no copy constructor been
declared...all would have happened exactly as before because a copy
constructor would have been generated.  Again, a good compiler would
eliminate the use of the generated copy constructor."  I'm not convinced
that this statement applies in this case.  Any enlightenment on this
subject would be welcome.  I'm using ATT C++ 2.0 on a Sparc.


#include <iostream.h>
#include <iomanip.h>

class NoCopy {
public:
  int x;
  NoCopy (int a) { x = a; };
  NoCopy &operator +=(int a) { x += a; return *this; };
  NoCopy  operator + (int a) { return NoCopy(*this) += a; };
};

class Copy {
public:
  int x;
  Copy(const Copy &c) : x(c.x) {};
  Copy (int a) { x = a; };
  Copy &operator =(int a) { x = a; return *this; };
  Copy &operator +=(int a) { x += a; return *this; };
  Copy  operator + (int a) { return Copy(*this) += a; };
};

void main()
{
  NoCopy xNo(1), xNo2(-1);
  Copy xYes(2), xYes2(-2);
  cout << "No=" << xNo.x << " Yes=" << xYes.x << endl;
  cout << "No2=" << xNo2.x << " Yes=" << xYes2.x << endl;
  xNo2 = xNo + 2;
  xYes2 = xYes + 2;
  cout << "No=" << xNo.x << " Yes=" << xYes.x << endl;
  cout << "No2=" << xNo2.x << " Yes=" << xYes2.x << endl;
}

Output of the program:
No=1 Yes=2
No2=-1 Yes=-2
No=3 Yes=2
No2=3 Yes=4