220 10464 article Path: news.gmane.org!not-for-mail From: Nicola Gigante Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Generated initializer_list Date: Thu, 1 May 2014 12:30:06 +0200 Lines: 145 Approved: news@gmane.org Message-ID: References: <357dd72f-c749-4dc5-aa6d-5ac4528aba5a@isocpp.org> <2be92780-bf68-4634-a193-7461f71de3f7@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (1.0) Content-Type: multipart/alternative; boundary=Apple-Mail-B0131C36-B5B1-4B02-B2EE-F3E730ABE301 Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1398940219 29988 80.91.229.3 (1 May 2014 10:30:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 1 May 2014 10:30:19 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBDMMZOF5V4KBBM6ERCNQKGQEC4RQ2HI@isocpp.org Thu May 01 12:30:12 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wi0-f200.google.com ([209.85.212.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1WfoFo-0005v3-MK for gclcip-std-proposals@m.gmane.org; Thu, 01 May 2014 12:30:12 +0200 Original-Received: by mail-wi0-f200.google.com with SMTP id d1sf185146wiv.7 for ; Thu, 01 May 2014 03:30:12 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:references:from:in-reply-to:message-id :date:to:mime-version:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type:content-transfer-encoding; bh=bMapMZQ1mHTsfSrD2t7v57mdc3FR+UJhrVr61ATDB2Q=; b=YDyTQU6VgZsCsecesWoK/S1XVXYniLyYET7bSCOVIHy0asLlQAYrKXRAIzfmWK0IBK 2dr0wSyRCJg8R9KGzcjMuh6MPqrxbEth6IMQ83oDLjgNb/US3XZhjUM577xV2CrQjOJ2 8oHiF2RJJOrxresQ3J/OPTCLmDuSTl8C3MCe8IrN5eTpnEfEHMLudDdffwgyFr3a9C3O xSLuR60gls5Y7eajlqnb8eicmAM3k00pH3CAyZxZyH8GxK2z8yyzhWuYAW3cP2Qkwog4 zujBL7gBuSuV65y5GcYChgypxyHKvJPljLZOva8XfmWeI1LCzhW32j5P/kGkgq2O1WWf yBZg== X-Gm-Message-State: ALoCoQm9TduxC3HALgbGWBvMB9X3IcdKsCGEdSc2Ytg9XPTeD3cE72yHw3y+UN+K7cUg1HtRLcDZ X-Received: by 10.112.50.50 with SMTP id z18mr979378lbn.7.1398940212197; Thu, 01 May 2014 03:30:12 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.10.131 with SMTP id i3ls937962wib.31.canary; Thu, 01 May 2014 03:30:10 -0700 (PDT) X-Received: by 10.14.225.132 with SMTP id z4mr9254212eep.92.1398940210778; Thu, 01 May 2014 03:30:10 -0700 (PDT) Original-Received: from mail-ee0-x22e.google.com (mail-ee0-x22e.google.com [2a00:1450:4013:c00::22e]) by mx.google.com with ESMTPS id s46si33734291eeg.195.2014.05.01.03.30.10 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 01 May 2014 03:30:10 -0700 (PDT) Received-SPF: pass (google.com: domain of nicola.gigante@gmail.com designates 2a00:1450:4013:c00::22e as permitted sender) client-ip=2a00:1450:4013:c00::22e; Original-Received: by mail-ee0-f46.google.com with SMTP id t10so2135303eei.19 for ; Thu, 01 May 2014 03:30:10 -0700 (PDT) X-Received: by 10.15.73.134 with SMTP id h6mr9150717eey.3.1398940210598; Thu, 01 May 2014 03:30:10 -0700 (PDT) Original-Received: from [10.211.185.65] ([217.200.202.53]) by mx.google.com with ESMTPSA id q41sm75264524eez.7.2014.05.01.03.30.09 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 01 May 2014 03:30:09 -0700 (PDT) X-Mailer: iPhone Mail (11D201) In-Reply-To: X-Original-Sender: nicola.gigante@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of nicola.gigante@gmail.com designates 2a00:1450:4013:c00::22e as permitted sender) smtp.mail=nicola.gigante@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:10464 Archived-At: --Apple-Mail-B0131C36-B5B1-4B02-B2EE-F3E730ABE301 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Il giorno 01/mag/2014, alle ore 11:52, douglas.boffey@gmail.com ha scritto: >=20 >=20 >> On Tuesday, 29 April 2014 13:56:07 UTC+1, Andrew Tomazos wrote: >> > I would like to type something along the lines of:=20 >> >=20 >> > constexpr auto primes[5] =3D get_nth_prime;=20 >> >=20 >> > given a function=20 >> >=20 >> > constexpr unsigned int get_nth_prime(size_t n)=20 >> >=20 >> > which would be equivalent to=20 >> >=20 >> > constexpr unsigned int primes[] =3D {2, 3, 5, 7, 11};=20 >>=20 >> Let us ignore prime sieves for the moment, and assume you have an=20 >> expensive function get_nth_prime. You can write the following I=20 >> think:=20 >>=20 >> constexpr std::array cache_primes()=20 >> {=20 >> std::array A =3D {0};=20 >>=20 >> for (size_t i =3D 0; i < 5; i++)=20 >> A[i] =3D get_nth_primes(i);=20 >>=20 >> return A;=20 >> }=20 >>=20 >> constexpr std:array primes =3D cache_primes();=20 >>=20 >> cache_primes is called during translation (at compile-time), the=20 >> primes array is filed out and (assuming odr-used) is stored in the=20 >> object file like you want.=20 >>=20 >> Is that what you mean? > =20 > This goes part way to resolving the issue, but there are still two major = problems with that solution: > =20 > a) the size of the array might not be known by the user=E2=80=94suppose, = for example, that I wanted to code tables for a LALR(1) parser (without the= need for a source code generator like yacc/byacc/bison), it would be unrea= sonable for the system to expect the programmer to know the array sizes, an= d In this specific case, the size of the table is easily computed ahead of ti= me, so you can compute the template parameter with another constexpr functi= on, the user doesn't have to know anything.=20 > b) this constrains the user to use a std::array=E2=80=A6 what if it were = more appropriate for a different data structure? > =20 It can be any literal data type, actually. The problem is that you can't al= locate memory in a constexpr context, so your data structures are limited t= o something whose size you can compute in advance. Bye, Nicola=20 --=20 ---=20 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 e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail-B0131C36-B5B1-4B02-B2EE-F3E730ABE301 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Il giorno 01/mag/2014, alle ore 11= :52, douglas.boffey@gmail.com ha scritto:


On Tuesday, 29 April 2014 13:56:07 UTC+1, Andrew Tomazos wrote= :=20
> I would like to type someth= ing along the lines of:
>
> constexpr auto primes[5] =3D get_= nth_prime;
>
> given a function
>
> constexpr u= nsigned int get_nth_prime(size_t n)
>
> which would be equiva= lent to
>
> constexpr unsigned int primes[] =3D {2, 3, 5, 7, = 11};

Let us ignore prime sieves for the moment, and assume you have= an
expensive function get_nth_prime.  You can write the following= I
think:

    constexpr std::array<int, 5> cach= e_primes()
    {
        std::array&l= t;int, 5> A =3D {0};

        for (size_t i = =3D 0; i < 5; i++)
             A= [i] =3D get_nth_primes(i);

       return A;     }

    constexpr std:array<int, 5> pr= imes =3D cache_primes();

cache_primes is called during translation = (at compile-time), the
primes array is filed out and (assuming odr-used= ) is stored in the
object file like you want.

Is that what you = mean?
 
This goes part way to resolving the issue, but there are still two maj= or problems with that solution:
 
a) the size of the array might not be known by the user=E2=80=94suppos= e, for example, that I wanted to code tables for a LALR(1) parser (without = the need for a source code generator like yacc/byacc/bison), it would be un= reasonable for the system to expect the programmer to know the array sizes,= and


b) this constrains the user to use a std::array=E2=80=A6 what if it we= re more appropriate for a different data structure?
 

It can be any literal data type, act= ually. The problem is that you can't allocate memory in a constexpr context= , so your data structures are limited to something whose size you can compu= te in advance.

Bye,
Nicola 

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/<= /a>.
--Apple-Mail-B0131C36-B5B1-4B02-B2EE-F3E730ABE301-- .