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From: lijewski@theory.tn.cornell.edu (Mike Lijewski)
Newsgroups: comp.std.c++
Subject: Re: Default copy constructor not making a copy
Keywords: cfront g++ copy constructor
Message-ID: <1991Feb27.141337.23826@batcomputer.tn.cornell.edu>
Date: 27 Feb 91 14:13:37 GMT
References: <253@blsouth.UUCP>
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In article <253@blsouth.UUCP> klein@blsouth.UUCP (Michael Klein) writes:
>
>Can someone explain why the two classes defined below exhibit different
>behavior?  The class without the default 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; };
>};

Replacing the line

   NoCopy  operator + (int a) { return NoCopy(*this) += a; };

with

   NoCopy  operator + (int a) { NoCopy tmp = *this; return tmp += a; };

you will get what you expected.  This could be taken to be a bug, but
I'm inclined to think that it's just another grey area in the
language which is best avoided.  No where in E&S is there any example
or text that I can find which states that one can explicitely call a
copy constructor which the compiler generates for you.  My
understanding is that if you make use of a construct which requires a
copy constructor, the "right thing" will be done, where the "right
thing" is a bitwise copy.

It is interesting to note that when I tried the original code with
g++, I got an error to the effect that the argument in
`NoCopy(*this)' is wrong - g++ was expecting an int since the only
constructor which had been defined was NoCopy::Nocopy(int).
Seemingly g++ doesn't allow one to explicitely call a copy
constructor which you haven't explicitely defined.  I would suggest
that this is the more correct behavior.  Certainly it is better than
silently allowing the construct and then doing the wrong thing.

>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


-- 
Mike Lijewski  (H)607/272-0238 (W)607/254-8686
Cornell National Supercomputer Facility
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