Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!purdue!bu-cs!bloom-beacon!apple!kanner
From: kanner@Apple.COM (Herbert Kanner)
Newsgroups: comp.lang.c++
Subject: Re: use of new allocator with reference variables
Keywords: c++, new, reference
Message-ID: <16124@apple.Apple.COM>
Date: 26 Aug 88 01:44:09 GMT
References: <646@paris.ICS.UCI.EDU>
Reply-To: kanner@apple.com.UUCP (Herbert Kanner)
Distribution: na
Organization: Development Systems Group, Apple Computer
Lines: 47

In article <646@paris.ICS.UCI.EDU> schmidt@bonnie.ics.uci.edu (Douglas C. Schmidt) writes:
>Hi,
>
>   I've been browsing through Stroustrup's C++ book and came
>across the following puzzling construct on page 34:
>
>Vec& sum = *new Vec(s);
>
>I don't believe that this use of the new allocator (preceded by the
>'*') is described anywhere else in the book.  Could someone please
>explain the meaning of this, in the context of the function
>depicted below?
>
>Vec operator+(Vec a, Vec b) {
>  int s = a.size();        
>  if (s != b.size()) error("foo");
>  Vec& sum = *new Vec(s);
>  int *sp = sum.v;
>  int *ap = a.v;
>  int *bp = b.v;
>  while (s--) *sp++ = *ap++ + *bp++;
>  return sum;
>}
>

It is probably impossible to give a short explanation that will be
understandable.  If my explanation makes no sense to you, study
section 3.2.6 on the "new" operator and section 2.3.10 on references.
Also, I suggest you simply not worry about anything that is not
understood in chapter 1 until you have read the rest of the book three
times.

Having said that:

Vec& sum = *new Vec(s); is explained as follows. sum is declared as a
reference to Vec--that is what Vec& is.  The assigment is an
initialization.  Initialization of a reference to Vec must by
assigning a Vec.  The "new" operator returns a pointer to the requested
type, namely a pointer to Vec.  The "*" dereferences this pointer,
yielding the Vec that is required.



Herb Kanner
Apple Computer, Inc.
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