220 2807 <41bc2377-7654-43f4-83f1-133edeca18d3@isocpp.org> article Path: news.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Improvements for string literals Date: Mon, 11 Feb 2013 03:01:41 -0800 (PST) Lines: 197 Approved: news@gmane.org Message-ID: <41bc2377-7654-43f4-83f1-133edeca18d3@isocpp.org> References: <913f9603-aff2-4abf-a330-d4d0f8a2d1e0@isocpp.org> <5a76aa73-1940-4247-b40d-c7c1ba972246@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_949_33418972.1360580502048" X-Trace: ger.gmane.org 1360580503 26055 80.91.229.3 (11 Feb 2013 11:01:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 11 Feb 2013 11:01:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCEKFTV6ZUMBBF474OEAKGQEP76PJCQ@isocpp.org Mon Feb 11 12:02:04 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f198.google.com ([209.85.223.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1U4r99-00080B-Sr for gclcip-std-proposals@m.gmane.org; Mon, 11 Feb 2013 12:02:04 +0100 Original-Received: by mail-ie0-f198.google.com with SMTP id 17sf26131926iea.1 for ; Mon, 11 Feb 2013 03:01:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:date:from:to:message-id :in-reply-to:references:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=7eFkeIb8MMejDTfXVBVfDjZ9ZwvopsvhEengK3fySN8=; b=rCh/kl3MlvAjY4r+RwdjsB2It84u6lDbV09VZ3wVDiw9TwW5WF2nWrzzXUjwHbt8ZL 5HBFPHjR+GZPCf0iJyb86zoK3bKunmLLbiGGym2P+t9HdyIVPBX5Ls+z3h/L9gvCTW83 Rh6JQhXcpUsZdtIlHuHsYcbVqw0uhB5c1YPXnPlFZZ7yB7LJhxbQjxO/OqJXbcaqYfPo hnXlVSGUVCNzYVBUSqcm8E2HtR4W0edafHttxk4VDRBw/XwSxnDbZqFjkUUOnFT/4RLI /tZFfqYgm5zokx3nZpnYom1xZMJIySEFqTpb6uGqYImtKiju3PdNnbxfnb6mDSqHXoas ZZgQ== X-Received: by 10.50.159.193 with SMTP id xe1mr8295342igb.0.1360580503710; Mon, 11 Feb 2013 03:01:43 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.5.175 with SMTP id t15ls1591089igt.33.canary; Mon, 11 Feb 2013 03:01:43 -0800 (PST) X-Received: by 10.50.149.132 with SMTP id ua4mr272379igb.8.1360580503015; Mon, 11 Feb 2013 03:01:43 -0800 (PST) In-Reply-To: <5a76aa73-1940-4247-b40d-c7c1ba972246@isocpp.org> X-Original-Sender: jmckesson@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:2807 Archived-At: ------=_Part_949_33418972.1360580502048 Content-Type: text/plain; charset=ISO-8859-1 On Monday, February 11, 2013 1:46:18 AM UTC-8, Nikolay Ivchenkov wrote: > > On Monday, February 11, 2013 6:52:27 AM UTC+4, Nicol Bolas wrote: >> >> >> 2: We can use template metaprogramming to differentiate between someone >> passing any old `const charT*` and an actual string literal. This could be >> useful for std::string implementations, where they could store an allocated >> string or a string_literal, depending on how it was initialized. Obviously >> non-const member accesses will cause a copy of the string, similar to COW, >> but without the general dangers of that construct. >> > > std::basic_string cannot be implemented so. Expression str[0] (where str > is an instance of std::string) shall not throw exceptions and the equality > str.c_str() == &str[0] shall always hold: > > void f(std::string &str) > { > char const *p1 = str.c_str(); > char *p2 = &str[0]; > if (!str.empty()) > *p2 = '?'; > assert(p1 == p2); > } > Fair enough. > > It would be possible to improve classes for immutable strings if > parameters could be declared with 'static constexpr' and the overload > resolution could select appropriate function depending on whether the > argument is known to be a constexpr object with static storage duration: > > template > class basic_immutable_string > { > public: > template > basic_immutable_string(static constexpr CharT (&s)[N]); // #1 > basic_immutable_string(CharT const *s); // #2 > .... > }; > > using immutable_string = basic_immutable_string; > > template > void f(static constexpr char (&x)[N]) > { > static constexpr char a1[] = "text"; > static char a2[] = "text"; > auto &ra1 = a1; > > immutable_string s0 = x; // calls #1 > immutable_string s1 = a1; // calls #1 > immutable_string s2 = a2; // calls #2 > immutable_string s3 = ra1; // calls #2 > immutable_string s4 = "text"; // calls #1 > } > > Not quite: static constexpr ar3[] = {'n', 'o', 't', ' ', 'l', 'i', 't', 'e', 'r', 'a', 'l'}; That would work just as well with your way. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. ------=_Part_949_33418972.1360580502048 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable

On Monday, February 11, 2013 1:46:18 AM UTC-8, Nikolay Ivchenkov wr= ote:
On Monday, February 11, 20= 13 6:52:27 AM UTC+4, Nicol Bolas wrote:

2: We can use template metaprogramming to differentiate between som= eone passing any old `const charT*` and an actual string literal. This coul= d be useful for std::string implementations, where they could store an allo= cated string or a string_literal, depending on how it was initialized. Obvi= ously non-const member accesses will cause a copy of the string, similar to= COW, but without the general dangers of that construct.

std::basic_string cannot be implemented so. Expression str[0] (where= str is an instance of std::string) shall not throw exceptions and the equa= lity str.c_str() =3D=3D &str[0] shall always hold:

  &= nbsp; void f(std::string &str)
    {
  &= nbsp;     char const *p1 =3D str.c_str();
 &nbs= p;      char *p2 =3D &str[0];
  &= nbsp;     if (!str.empty())
    =         *p2 =3D '?';
  &nbs= p;     assert(p1 =3D=3D p2);
    }

Fair enough.
 

It would be possible to improve classe= s for immutable strings if parameters could be declared with 'static conste= xpr' and the overload resolution could select appropriate function dependin= g on whether the argument is known to be a constexpr object with static sto= rage duration:

    template <class CharT>
&n= bsp;       class basic_immutable_string
&n= bsp;   {
    public:
    = ;    template <std::size_t N>
   &nb= sp;        basic_immutable_string(static= constexpr CharT (&s)[N]); // #1
      = ;  basic_immutable_string(CharT const *s); // #2
   =      ....
    };

  &= nbsp; using immutable_string =3D basic_immutable_string<char>;
    template <std::size_t N>
   &nb= sp;    void f(static constexpr char (&x)[N])
 &n= bsp;  {
        static constexpr= char a1[] =3D "text";
        static= char a2[] =3D "text";
        auto &= amp;ra1 =3D a1;

        immutable= _string s0 =3D x; // calls #1
       = immutable_string s1 =3D a1; // calls #1
     &= nbsp;  immutable_string s2 =3D a2; // calls #2
   &n= bsp;    immutable_string s3 =3D ra1; // calls #2
 &n= bsp;      immutable_string s4 =3D "text"; // calls= #1
    }


Not quite:
static= constexpr ar3[] =3D {'n'= , <= span style=3D"color: #080;" class=3D"styled-by-prettify">'o', 't', ' '= , <= span style=3D"color: #080;" class=3D"styled-by-prettify">'l', 'i', 't'= , <= span style=3D"color: #080;" class=3D"styled-by-prettify">'e', 'r', 'a'= , <= span style=3D"color: #080;" class=3D"styled-by-prettify">'l'};
That would work just as well with your way.

--
 
---
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-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-pro= posals/?hl=3Den.
 
 
------=_Part_949_33418972.1360580502048-- .