From -5893515993485714981
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: f78e5,8eebfdfe25d22b37
X-Google-Attributes: gidf78e5,public
X-Google-ArrivalTime: 2001-12-22 01:20:02 PST
Return-Path: <devnull@stump.algebra.com>
X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f
Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!comp-std-cpp-robomod!not-for-mail
X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov)
From: "Richard Smith" <richard@ex-parrot.com>
Newsgroups: comp.std.c++
Subject: Re: C++0x: vector::pack()
Message-ID: <1008936696.14508.0.nnrp-10.3e31d362@news.demon.co.uk>
References: <3BD40077.4AB9D0A2@divalsim.it> <3bd464b5$0$8508$ed9e5944@reading.news.pipex.net> <3BD4B660.F3C7606@wizard.net> <3bd539f4$0$230$cc9e4d1f@news.dial.pipex.com> <3BD5FF27.55AFFDFC@wizard.net> <9vqs6m$58p$2@news.tuwien.ac.at> <3C228128.B4CC8CE3@wizard.net>
X-NNTP-Posting-Host: mailgate.transversal.co.uk:62.49.211.98
X-Trace: news.demon.co.uk 1008936696 nnrp-10:14508 NO-IDENT mailgate.transversal.co.uk:62.49.211.98
X-Complaints-To: abuse@demon.net
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000
Approved: Fergus Henderson <fjh@cs.mu.oz.au>, moderator of comp.std.c++
Date: Sat, 22 Dec 2001 03:16:56 CST
Lines: 69
Xref: archiver1.google.com comp.std.c++:8699


"James Kuyper Jr." <kuyper@wizard.net> wrote in message
news:3C228128.B4CC8CE3@wizard.net...
> Christopher Eltschka wrote:
> ...
> > It would not have to be undefined behaviour, if the (non-)existance of
> > the function is detectable by code, say through an allocator_traits.
>
> As a general rule, I distrust solutions which depend upon traits
> classes. A traits template will give you the correct information only if
> someone remembers to specialize the template for each new class it might
> be applied to. You can usually choose a "safe" default when it isn't
> specialized, but that means that in general the traits template can't be
> counted on to correctly the question it's designed to answer.

It is possible to write a traits class that automagically detects whether a
member function of a known signature exists.  I think this code is legal and
does that, unfortunately I've yet to find a compiler that can actually cope
with it!  Even Comeau segfaults with it ;-(.

    namespace details
    {
      template <class T, class Type, Type>
      struct type_fwd
      {
        typedef T type;
      };

      template <class T, class = T>
      struct has_reallocate
      {
        enum { value = false };
      };

      template <class T>
      struct has_reallocate
      <
        T,
        typename type_fwd
        <
          T,
          void *(T::*)(void *, size_t, size_t),
          &T::reallocate
        >::type
      >
      {
        enum { value = true };
      };
    }

    template <class T>
    struct allocator_traits
    {
      enum { has_reallocate = details::has_reallocate<T>::value };
    };

I admit the code isn't pleasant, but it does avoid having to remember to
specialise allocator_traits for every new allocator.

--
Richard Smith


---
[ 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.research.att.com/~austern/csc/faq.html                ]