Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!mailrus!umix!uunet!mcvax!unido!gmdka!fuchs
From: fuchs@gmdka.UUCP (Harald Fuchs)
Newsgroups: comp.lang.c++
Subject: Re: Following code bombs (possible assignment/initialization problem)?
Message-ID: <273@gmdka.UUCP>
Date: 8 Mar 88 12:18:41 GMT
References: <400@erc3ba.UUCP>
Organization: GMD Karlsruhe, W-Germany
Lines: 38
Posted: Tue Mar  8 13:18:41 1988

in article <400@erc3ba.UUCP>, rsc@erc3ba.UUCP (Rich Chomiczewski, AT&T - ERC, Princeton NJ) says:
> The following program blows the stack:
> ...
> class Position {
> 	Point off;	// offset is relative to (possibly null) ``to''.
> 	Position *to;
> public:
> 	Position(Point p)		{ off = p; to = 0; }
> 	Position(Point p, Position pos)	{ off = p; to = &pos; }
> 	Point position()		{ return (to) ? off+to->position() : off; }
> };

> main()
> {
> 	Position a = Position(Point(1,1));
> 	Position b = Position(Point(5,5), a);
> 	Position c = Position(Point(10,10), b);
> }

The bug in this program is
	Position(Point p, Position pos)	{ off = p; to = &pos; }
where you take the address of an object on the stack because pos is passed
by value. Use reference passing (Position& pos) and the problem disappears.

When you declare
 	Position a = Position(Point(1,1));
the following functions are called:
  Position::Position (Point);            // To make a temporary Position value
  Position::Position (Position&);        // Initialisation of a by temp
					 // See The Book p. 180
  Position::~Position ();                // To get rid of temp
Since you didn't declare Position::Position (Position&), Cfront picks the
default, namely bitwise copy. That happens to be ok in this context.

                               Harald Fuchs

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