220 29627 <569dee8e-b869-40e0-9b41-1d1f39fecb12@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Myriachan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: "A Proposal to Relax Constexpr Restrictions for some Reinterpret Casts" ? Date: Fri, 2 Dec 2016 17:32:34 -0800 (PST) Lines: 79 Approved: news@gmane.org Message-ID: <569dee8e-b869-40e0-9b41-1d1f39fecb12@isocpp.org> References: <14dc989b-7a2f-478b-b16e-b9297fb1782f@isocpp.org> <591447c7-38cd-4ad4-83aa-fc1326c1829e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2085_10541132.1480728754340" X-Trace: blaine.gmane.org 1480728757 19143 195.159.176.226 (3 Dec 2016 01:32:37 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 3 Dec 2016 01:32:37 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDKLT4PURQHRBM6BRDBAKGQEWPCEZSY@isocpp.org Sat Dec 03 02:32:33 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f199.google.com ([209.85.213.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cCzBn-0003we-Uq for gclcip-std-proposals@m.gmane.org; Sat, 03 Dec 2016 02:32:32 +0100 Original-Received: by mail-yb0-f199.google.com with SMTP id 126sf267059910ybo.4 for ; Fri, 02 Dec 2016 17:32:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=+13YOKuo0uHlHLFUvoqAzwJrBYQtlumKFjldqclXpxs=; b=LFRBt+kc2wM+iXMIikAEdK4tZ9Ze1ZWVgqmtVG8uTzJ+quujhRvx39MuWDPDHhvfr2 za0RaXywYkaqjYWM556R0IcTWjdbrwQHFbLhW+9ou9Jx4m4+OgK06yJ49A+RZabikhsw 6Oz25HtFfeqCoLNjF0kAV5qbOc9uipK+t3ZBwbKw35bIfSwBHLzANq+okqf8/m1lFsEa tI0C4WoqAZLYsuuuxw+vdzW8MkKsAuiR3OGas5p5b63F+O0tp0/+RaUwdGlYpsw7Wqgs CMyC4W6SXQMTGNdcuD5A6iHj2MIV7ILWkcXo6pjkJG+rlayqxUI+A0z71xOgPkECu+An 7R5w== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=+13YOKuo0uHlHLFUvoqAzwJrBYQtlumKFjldqclXpxs=; b=sY4djwegkDrRu2Su2gv1CRB+qZsxNZ+nywgjXhyazEBPgigv/mupyO5AmHFphcAlNZ DXI33Gv5sm4mYjtXaTse4Ex0N2FzppwaRD6Uitnmv6z+SAcnVNZ6yM/ZFqYMzeMnT6Vm 56x08yYcFJ0aJ9OHl1pTRD+0+931/XhKYH87SynrshqQjhc1nSRtfAarNcPn1pG8s9ka LL67ZxRqKG0Pgza3NYW84QqcD5MfJmF9CpqUtRWzfuVjwR/wsi1eL0w7onm9tRa9nyZ3 rdxyq33vfnFRjFDr2R5rZB1CzMvSmgfBzxS4/mCh9Gfv3Nm/2ZVlFkOOz6J/53AgoLYc qTiQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=+13YOKuo0uHlHLFUvoqAzwJrBYQtlumKFjldqclXpxs=; b=LKey8qY9JKHlehjAoc8JbFRpVIlM+XDLkXAJROUKKxINf9ebV0Jf5f4251KI5fkEXn 0oR3baHFBPI5erCD/RXVy97/XSkWslx6k/uyIaZ061XM4OzJujEqEaqRBImb41JG8a9L KpuWM+B0gc9N5mTJHl2TD0auEF63yuyVaDqxZ7BaRX7EJ0GycMqATBw1Mef+5l8jDLsl G1aNK2jGUJSdMty43bAxH8m21A8gfbO9dY9f1KSe7km8QWbXLGlHWwP6UgCcKtI/JPZe y1+tVeE7CT7SQFJqcHaGYOOZzjIwYF5LXFwUoVRlqvCU/miQGzsJUiCRlAquSs+IC09l sPQA== X-Gm-Message-State: AKaTC01SrrNtEK4SYiVtIFCFypgaYtoQ9Tpbb7qL4aJPzul26U4zZB7LexqrpIgKOvYUyQ== X-Received: by 10.129.168.68 with SMTP id f65mr11727381ywh.49.1480728755705; Fri, 02 Dec 2016 17:32:35 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.52.220 with SMTP id t28ls10424266otd.3.gmail; Fri, 02 Dec 2016 17:32:35 -0800 (PST) X-Received: by 10.157.18.211 with SMTP id g77mr3244131otg.14.1480728754982; Fri, 02 Dec 2016 17:32:34 -0800 (PST) In-Reply-To: <591447c7-38cd-4ad4-83aa-fc1326c1829e@isocpp.org> X-Original-Sender: myriachan@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29627 Archived-At: ------=_Part_2085_10541132.1480728754340 Content-Type: multipart/alternative; boundary="----=_Part_2086_1171094534.1480728754340" ------=_Part_2086_1171094534.1480728754340 Content-Type: text/plain; charset=UTF-8 On Monday, November 28, 2016 at 6:07:57 PM UTC-8, Nicol Bolas wrote: > > This is *precisely* why `bit_cast` is a good thing. Because it's a > function which knows both the source and destination types, we would be > able to say that it is `constexpr` when doing certain conversions and not > `constexpr` when doing others. For example, any conversion of a pointer to > anything other than a pointer would not be `constexpr`. This would include > within various types, so you couldn't have a `struct{int *p};` that you > `bit_cast` to a `struct{intptr_t q};`. > > The OP asked specifically about converting integers to their > bit-representation to do hashing and so forth. `bit_cast` could be > `constexpr` for such operations. > > So there's no problem. > As far as I can tell, constexpr bit_cast would be useful for implementing things that'd otherwise require direct compiler support, such as std::numeric_limits::infinity(). Am I right in this assumption? Melissa -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/569dee8e-b869-40e0-9b41-1d1f39fecb12%40isocpp.org. ------=_Part_2086_1171094534.1480728754340 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Monday, November 28, 2016 at 6:07:57 PM UTC-8, Nicol Bo= las wrote:
Thi= s is precisely why `bit_cast` is a good thing. Because it's a fu= nction which knows both the source and destination types, we would be able = to say that it is `constexpr` when doing=20 certain conversions and not `constexpr` when doing others. For example,=20 any conversion of a pointer to anything other than a pointer would not=20 be `constexpr`. This would include within various types, so you couldn'= t have a `struct{int *p};` that you `bit_cast` to a `struct{intptr_t=20 q};`.

The OP asked specifically about converting integers to=20 their bit-representation to do hashing and so forth. `bit_cast` could be `constexpr` for such operations.

So there's no problem.

As far as I can tell, constexpr bit_cast woul= d be useful for implementing things that'd otherwise require direct com= piler support, such as std::numeric_limits<float>::infinity().=C2=A0 = Am I right in this assumption?

Melissa

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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/569dee8e-b869-40e0-9b41-1d1f39fecb12= %40isocpp.org.
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