220 10462 article Path: news.gmane.org!not-for-mail From: douglas.boffey@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Generated initializer_list Date: Thu, 1 May 2014 02:52:45 -0700 (PDT) Lines: 172 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 Content-Type: multipart/alternative; boundary="----=_Part_3726_31111225.1398937965208" X-Trace: ger.gmane.org 1398937977 31011 80.91.229.3 (1 May 2014 09:52:57 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 1 May 2014 09:52:57 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDSIZVXVVUFBB3VSRCNQKGQESLGJ6WY@isocpp.org Thu May 01 11:52:49 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Wfnfb-0002aC-RP for gclcip-std-proposals@m.gmane.org; Thu, 01 May 2014 11:52:48 +0200 Original-Received: by mail-ob0-f200.google.com with SMTP id vb8sf15300152obc.11 for ; Thu, 01 May 2014 02:52:46 -0700 (PDT) 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 :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=daPSCPM88RBhVD4Qg2ctFoEJp7oEEPp/OX9MKLQimoE=; b=NGN53ADF5xBCS5EcA7Ronayowkx2EbT8ncJR9n65BVtQisXEMBDWJdCLB8x0Ss4ICc nAb9hn/tg1ZwtVsB0VN7aRZWA0ggdAQS/XJQzJm+Grf3gjuHmaAfjuw0wurs+CSlnp6P PSQgVIBFTMTP9rfuz4+nv7XEMNrg3rE7bxhczXL0e/FbB2jfbK1pkpaIonrwGysV8xTE cjkRdEmZ8OZ/M0lPyYu5QsSjClgF2mi3PFmeLV+vymmrio2UssQzsHYwKV1gKKMw3WY5 1i2ah3TBQ94geUelXSqsmgMKgeE+HrTpxxVwxnXWIyAJ8SEOsgjxb3tg2u3LBpGBs4L3 1V/g== 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:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=daPSCPM88RBhVD4Qg2ctFoEJp7oEEPp/OX9MKLQimoE=; b=RuakS0RlizCVyqwg17sOl6BHEvuMwjwzurjk2hs0G9JQKMk2uefwohgTMnTxltNFWP 2iIOG2MQ7qV+dlVEOdjo47JT7J3t+aOy3vAjDamrFn297yZqiyHFoLWGn0eBDsDE+vET HrLpdM4EuXPuhqJX0aYetlrDDQxOr/1ULFXT78C9GsXa5dde4z8CNgWDllw5Z/cXOoPr Ckc5V9BwB44OTXO1UsB2wj6e7Ccrybr4Ns4b9VlmV2j7esuhfn68s+WwB0W1NW5jLQsh WgLxorqEvCXU+1zIkN6Hnkf+fp2vrFAFAjHaZwVW1SdWeAPEb3Os5/TpbUw6+Rtkn0eQ AsZw== X-Gm-Message-State: ALoCoQmdEUijgKLz2sPTrYbm6OtJ86Fgh8C/v4D2Ol5aw05SDW6wgRTExkEGNdLabLySBXc9IxzG X-Received: by 10.182.75.193 with SMTP id e1mr5151193obw.1.1398937966779; Thu, 01 May 2014 02:52:46 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.95.84 with SMTP id h78ls974515qge.54.gmail; Thu, 01 May 2014 02:52:45 -0700 (PDT) X-Received: by 10.140.27.244 with SMTP id 107mr1753qgx.18.1398937965899; Thu, 01 May 2014 02:52:45 -0700 (PDT) In-Reply-To: X-Original-Sender: douglasboffey@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:10462 Archived-At: ------=_Part_3726_31111225.1398937965208 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Tuesday, 29 April 2014 13:56:07 UTC+1, Andrew Tomazos wrote:=20 > > > 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 > > 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 > > constexpr std::array cache_primes()=20 > {=20 > std::array A =3D {0};=20 > > for (size_t i =3D 0; i < 5; i++)=20 > A[i] =3D get_nth_primes(i);=20 > > return A;=20 > }=20 > > constexpr std:array primes =3D cache_primes();=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 > > Is that what you mean?=20 > =20 This goes part way to resolving the issue, but there are still two major=20 problems with that solution: =20 a) the size of the array might not be known by the user=E2=80=94suppose, fo= r=20 example, that I wanted to code tables for a LALR(1) parser (without the=20 need for a source code generator like yacc/byacc/bison), it would be=20 unreasonable for the system to expect the programmer to know the array=20 sizes, and b) this constrains the user to use a std::array=E2=80=A6 what if it were mo= re=20 appropriate for a different data structure? =20 After thinking about it, it seems that the best way to resolve such=20 deficiencies would be to add the necessary ctors to std::initializer_list,= =20 e.g. =20 template std::initializer_list::initializer_list(size_t members,=20 ValueType (*fn)(size_t)); =20 which would be equivalent to {fn[0], fn[1], =E2=80=A6, fn[members =E2=80=93= 1]} =20 template std::initializer_list::initializer_list(ValueType=20 (*next_datum)(const Data data), Data (*iterate)(const Data data), bool=20 (*more)(const Data data), const Data &initial_data) =20 which would be equivalent to {next_datum(initial_data),=20 next_datum(iterate(initial_data)),=20 next_datum(iterate(iterate(initial_data))), =E2=80=A6}, continuing while=20 more(iterate(=E2=80=A6(initial_data)=E2=80=A6). =20 I cannot see a way of using a ctor to generate the initializer_list from an= =20 object, since it would need to be generated within the ctor to be mutable. --=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/. ------=_Part_3726_31111225.1398937965208 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On Tuesday, 29 April 2014 13:56:07 UTC+1, Andrew Tomaz= os wrote:=20
> I would like to type somethin= g along the lines of:
>
> constexpr auto primes[5] =3D get_nt= h_prime;
>
> given a function
>
> constexpr uns= igned int get_nth_prime(size_t n)
>
> which would be equivale= nt to
>
> constexpr unsigned int primes[] =3D {2, 3, 5, 7, 11= };

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

    constexpr std::array<int, 5> cache_= primes()
    {
        std::array<= int, 5> A =3D {0};

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

       return A;
&n= bsp;   }

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

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

Is that what you mea= n?
 
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?
 
After thinking about it, it seems that the best way to resolve such de= ficiencies would be to add the necessary ctors to std::initializer_list, e.= g.
 
template<typename ValueType>
std::initializer_list<ValueType>::initiali= zer_list(size_t members, ValueType (*fn)(size_t));
 
which would be equivalent to {fn[0], fn[1], =E2=80=A6, fn[members =E2= =80=93 1]}
 
template<typename ValueType, typename Data>= ;
std::initializer_list<ValueType>::initiali= zer_list(ValueType (*next_datum)(const Data data), Data (*iterate)(const Da= ta data), bool (*more)(const Data data), const Data &initial_data)
 
which would be equivalent to {next_datum(initial_data), next_datum(ite= rate(initial_data)), next_datum(iterate(iterate(initial_data))), =E2=80=A6}= , continuing while more(iterate(=E2=80=A6(initial_data)=E2=80=A6).
 
I cannot see a way of using a ctor to generate the initializer_list fr= om an object, since it would need to be generated within the ctor to be mut= able.

--

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