Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ames!elroy!jpl-devvax!rich
From: rich@jpl-devvax.JPL.NASA.GOV (Richard Pettit)
Newsgroups: comp.lang.c++
Subject: Re: use of new allocator with reference variables
Keywords: c++, new, reference
Message-ID: <2732@jpl-devvax.JPL.NASA.GOV>
Date: 26 Aug 88 18:04:23 GMT
References: <646@paris.ICS.UCI.EDU>
Reply-To: rich@jpl-devvax.JPL.NASA.GOV (Richard Pettit)
Distribution: na
Organization: Jet Propulsion Laboratory, Pasadena, CA.
Lines: 47

>
>Vec& sum = *new Vec(s);

Lets start from the top.
Reference is just generally confusing. Consider Pascal: (forgive the formatting)

program plugh (output);
  var i : integer;
  procedure xyzzy (var a : integer);
  begin a := 3; end;
begin xyzzy(i); end.

Note that in this example, the value "3" will be "copied back" into variable
"i" upon return from xyzzy.  This is because "a" is a REFERENCE variable.
Now look at the C++ version:

void xyzzy(int &a) { a = 3; }
main() { int i; xyzzy(i); }

This does the same thing. The C version:

void xyzzy(int *a) { *a = 3; }
main() { int i; xyzzy(&i); }

makes explicit use of the pointer.  The REFERENCE notation in C++ simply
eliminates the need for making pointers to everything.  Variables which are
made reference variables are treated like regular non-pointer variables
(like Pascal).  The C++ declaration:

Vec& sum = *new Vec(s);

is such because new "returns" a pointer.  The REFERENCE variable "sum" is
treated syntactically like a structure, and the value assigned to it
must also be a structure.  Hence the de-reference on the "new" allocator.
Just as the notation

Vec a;
Vec &b = a;

assigns "a reference to `a'" to "b", the previous statement assigns
"a reference to an anonymous heap space variable" to "sum".

I've probably confused the issue even worse.

Rich
-- 
           "Macrosolutions to Megaproblems" -- Voivod
                  rich@jpl-devvax.Jpl.Nasa.Gov