Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!jpl-devvax!seq
From: seq@jpl-devvax.jpl.nasa.gov (Sequence Folks)
Newsgroups: comp.lang.c++
Subject: The "new" Operator and the Copy Constructor
Message-ID: <1991Mar27.233549.1487@jpl-devvax.jpl.nasa.gov>
Date: 27 Mar 91 23:35:49 GMT
Reply-To: russb@sampson.JPL.NASA.GOV (Russ Brill)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 57

In spite of a previous confidence in my understanding of the copy
constructor, and with the ARM by my side, the following has me
baffled:

1) The compiler complains when I try to create an object from the heap
using the default copy constructor.

	class D {};

	D d;
	D* pd;
	
	pd = new D(d);



2) The compiler complains when I try to create an object from the heap
using the default copy constructor of a derived class when there is no
default copy constructor for the base class.

	class B {};
	class D : public B {};
	
	D d;
	D* pd;

	pd = new D(d);


3) But everything works great if the base class has an explicity
defined copy constructor!

	class B {
	public:
		B() {}
		B(B&) {}
	};
	class D : public B {};	

	D d;
	D* pd;

	pd = new D(d);


4) And considering case #1, how come the *following* works?!

	class B {
	public:
		virtual B* copy() {return new B(*this);}
	};




This is all done on Sun cfront 2.0.  I would appreciate an explanation
(and, if possible, an ARM reference).