From 5448726611484239400 X-Google-Thread: f78e5,574518e5c7a60feb X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news3.google.com!news.glorb.com!koehntopp.de!newsfeed01.sul.t-online.de!t-online.de!newsfeed.vmunix.org!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: phalpern@halpernwightsoftware.com (Pablo Halpern) Newsgroups: comp.std.c++ Subject: Re: Revised Allocator Proposal Date: Thu, 1 Sep 2005 05:44:07 GMT Organization: EarthLink Inc. -- http://www.EarthLink.net Lines: 44 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <7ibbe1hu4omoqsfibanas46idf9u6i5hjo@4ax.com> NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: news.demon.co.uk 1125553454 9288 158.152.254.254 (1 Sep 2005 05:44:14 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Thu, 1 Sep 2005 05:44:14 +0000 (UTC) X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Virus-Scanned: amavisd-new at cs.mu.OZ.AU X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id j815i7Dn015172; Thu, 1 Sep 2005 15:44:07 +1000 (EST) X-NNTP-Posting-Date: Wed, 31 Aug 2005 06:02:15 PDT X-Delivered-To: std-c++@ucar.edu X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Newsreader: Forte Agent 1.93/32.576 English (American) X-Newsgroups: comp.std.c++ Xref: g2news1.google.com comp.std.c++:1986 public@marlowa.plus.com (Andrew Marlow) wrote: >It is not clear to me exactly why wherever you have an allocator parameter >it tends to be a pointer rather than a reference. Why is that please? Will >the proposal specify what the behaviour is when the pointer is null? In general, it is easiest to construct an allocator object from a class derived from allocator_implementation. The automatic conversion from allocator_implementation* to allocator does the rest. For example, given a region allocator declaration: class region_allocator : public std::allocator_implementation {...}; It is simpler to write: region_allocator myAlloc; vector myVector(&myAlloc); than it is to write the equivalent: region_allocator myAllocImp; std::allocator myAlloc(&myAllocImp); vector myVector(myAlloc); Being pedantic, region_allocator should really be named region_allocator_implementation. However, once you start using this style of allocator, you start thinking of the implementation class as the actual allocator, with std::allocator being just a wrapper for backwards compatibility. By passing in a pointer to the implementation class object, we also make it explicit that only the pointer is being copied, not the entire implementation object. Thanks for asking. Pablo Halpern phalpern@halpernwightsoftware.com Author: The C++ Standard Library from Scratch http://www.halpernwightsoftware.com/stdlib-scratch --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ]