220 8673 <336403ba-e7da-4fca-99f9-de91492327b5@isocpp.org> article Path: news.gmane.org!not-for-mail From: Bengt Gustafsson Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 17:12:23 -0800 (PST) Lines: 190 Approved: news@gmane.org Message-ID: <336403ba-e7da-4fca-99f9-de91492327b5@isocpp.org> References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <186E0927-C962-4E34-AED0-E7509DD1F0A0@gmail.com> <7aad44d8-6b4e-438a-b309-bd898b994b34@isocpp.org> <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> <8E74B747-7450-41E0-8D1E-48C2D6F81535@gmail.com> <104254A6-C69A-4571-8455-89D7E466D493@gmail.com> <6a95bf9c-55c9-4ae2-966f-d1b1f4df6a37@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_13_21255250.1390007543036" X-Trace: ger.gmane.org 1390007539 2658 80.91.229.3 (18 Jan 2014 01:12:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 18 Jan 2014 01:12:19 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCRIRSPDTQIRB6FJ46LAKGQE2N25MKA@isocpp.org Sat Jan 18 02:12:27 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qe0-f71.google.com ([209.85.128.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4KSX-0001g8-Rj for gclcip-std-proposals@m.gmane.org; Sat, 18 Jan 2014 02:12:26 +0100 Original-Received: by mail-qe0-f71.google.com with SMTP id 8sf7770191qea.10 for ; Fri, 17 Jan 2014 17:12:25 -0800 (PST) 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=Z2CHDV+r7tdQAyEwoWQF5x7y2svjsyKWaW7UpvfZ2CQ=; b=Wm5k1CCzEyWrXrG3f+EDJ88MaE+shBR4JurV5+KcChM0AzCZxcNSLmT09HMlXidWak kFiJ+qBcfrfoPZxJgultQKk+VVtcc6VW8G7rxfIxKlJejj60sKTj7rKgMoO1YNtHwJyo Zx5PwUMiJT4omF7ywshhC0+sme30HVeB0KihzbM+r08aP7GX80hNOsfOkazkXSif4/+x 6zBFfv6V9D3V2zWM6BNISH+sqtGA+lbms9nQ21SSZkdJh0E5jal6uZUw8rZg3kU3/+m6 0MmWZTA8sNxAyC89xpxFC1lW4eb65imzE1+43OoT0p107fM0iKln45LBpjrUy9ZGNDyE UoQg== X-Gm-Message-State: ALoCoQku8XvQxXXXeBT3nYy94HFs1Wpmg/rJHS70EnkFt1d56dHBfjSSN1lb5m0c3FTUp29ENvu0 X-Received: by 10.58.134.15 with SMTP id pg15mr1908665veb.14.1390007545000; Fri, 17 Jan 2014 17:12:25 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.239.165 with SMTP id vt5ls751588igc.6.gmail; Fri, 17 Jan 2014 17:12:24 -0800 (PST) X-Received: by 10.50.253.195 with SMTP id ac3mr27723igd.15.1390007544240; Fri, 17 Jan 2014 17:12:24 -0800 (PST) In-Reply-To: <6a95bf9c-55c9-4ae2-966f-d1b1f4df6a37@isocpp.org> X-Original-Sender: bengt.gustafsson@beamways.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:8673 Archived-At: ------=_Part_13_21255250.1390007543036 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Someone said that constructing a string from (nullpltr, 0) is not allowed,= =20 and that is used as an argument here. How about looking into why this is=20 not allowed first? I can't see any practical reason for string nor for=20 string_view. Someone else claimed that if nullptr could be returned from data() then=20 you'd have to test for this in _addition_ to testing for size() =3D=3D 0. I= =20 can't understand ahy as if size() is 0 there is exactly 0 offsets at which= =20 it is ok to dereference the pointer returned from data. This is more=20 information you get from testing whether the returned pointer is nullptr,= =20 i.e. even if data() returned non-null there may be no elements to access,= =20 but if size() returns 0 there can be _no_ elements whether the pointer is= =20 null or not. Den l=C3=B6rdagen den 18:e januari 2014 kl. 02:00:12 UTC+1 skrev Bengt=20 Gustafsson: > > I have not followed this entire thread, instead I hacked up a little=20 > implementation which I used in implementing something for another thread = on=20 > an improved "printf" functionality. What I noticed was: > > - I had use for a C array of string_view to keep the parts of the origina= l=20 > string that fell between each data inserted. This means that I must be ab= le=20 > to default construct string_view as there is no other way to construct an= =20 > array element. > - It is logical to do this default construction by setting m_begin and=20 > m_end to nullptr, or m_begin to nullptr and m_size to 0. > - It must be allowed to take a default constructed string_view as the=20 > range of a range based for statement __or__ to check if the range is in= =20 > default constucted state using something like os_null(). > > - both of these methods are impossible if the presumptions stated above= =20 > "begin() should never return nullptr" and "we don't need a special=20 > is_null()". Well, not entirely. Of course you could create some bogus=20 > object and use its address instead of nullptr. > > Someone stated above that the \0 at the end of the string should be=20 > accessible even if the string_view is empty. This is clearly impossible a= s=20 > a string_view may be constructed as a part of a string which continues on= ..=20 > Thus there is no \0 at *end() anytime. > > This said I think that the very simple solution is to just set the=20 > pointer(s) to nullptr in the default ctor, don't implement any special=20 > is_null but return "null" iterators from begin and end in default=20 > constructed string_view, which of course results in zero length if they a= re=20 > subtracted. I don't really see a problem here, isn't this how all=20 > containers do it. For instance a newly default constructed vector? Is the= =20 > issue whether you can rely on this to know wheter the string_view (or=20 > vector) has been non-empty before? If so, follow the example of vector an= d=20 > give the same promises on the standard level! > > > Den l=C3=B6rdagen den 18:e januari 2014 kl. 01:06:18 UTC+1 skrev Jeffrey= =20 > Yasskin: >> >> On Fri, Jan 17, 2014 at 3:51 PM, Marshall Clow =20 >> wrote:=20 >> > On Jan 17, 2014, at 2:10 PM, Jeffrey Yasskin =20 >> wrote:=20 >> >=20 >> > Agreed. To mitigate this, if string_view::data() could return nullptr,= =20 >> I=20 >> > believe we could still create strings using=20 >> std::string(null_sv.begin(),=20 >> > null_sv.end()).=20 >> >=20 >> >=20 >> > I thought that=E2=80=99s why we had to_string().=20 >> >> We have to_string mostly for convenience, I think, although it is=20 >> another way to get from a null string_view to a std::string.=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/. ------=_Part_13_21255250.1390007543036 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Someone said that constructing a string from (nullpltr, 0)= is not allowed, and that is used as an argument here. How about looking in= to why this is not allowed first? I can't see any practical reason for stri= ng nor for string_view.

Someone else claimed that if nul= lptr could be returned from data() then you'd have to test for this in _add= ition_ to testing for size() =3D=3D 0. I can't understand ahy as if size() = is 0 there is exactly 0 offsets at which it is ok to dereference the pointe= r returned from data. This is more information you get from testing whether= the returned pointer is nullptr, i.e. even if data() returned non-null the= re may be no elements to access, but if size() returns 0 there can be _no_ = elements whether the pointer is null or not.


<= /div>
Den l=C3=B6rdagen den 18:e januari 2014 kl. 02:00:12 UTC+1 skrev = Bengt Gustafsson:
I have not followed this entire thread, instead I hacked up a little im= plementation which I used in implementing something for another thread on a= n improved "printf" functionality. What I noticed was:

-= I had use for a C array of string_view to keep the parts of the original s= tring that fell between each data inserted. This means that I must be able = to default construct string_view as there is no other way to construct an a= rray element.
- It is logical to do this default construction by = setting m_begin and m_end to nullptr, or m_begin to nullptr and m_size to 0= ..
- It must be allowed to take a default constructed string_view = as the range of a range based for statement __or__ to check if the range is= in default constucted state using something like os_null().

=
- both of these methods are impossible if the presumptions state= d above "begin() should never return nullptr" and "we don't need a special = is_null()". Well, not entirely. Of course you could create some bogus objec= t and use its address instead of nullptr.

Someone = stated above that the \0 at the end of the string should be accessible even= if the string_view is empty. This is clearly impossible as a string_view m= ay be constructed as a part of a string which continues on. Thus there is n= o \0 at *end() anytime.

This said I think that the= very simple solution is to just set the pointer(s) to nullptr in the defau= lt ctor, don't implement any special is_null but return "null" iterators fr= om begin and end in default constructed string_view, which of course result= s in zero length if they are subtracted. I don't really see a problem here,= isn't this how all containers do it. For instance a newly default construc= ted vector? Is the issue whether you can rely on this to know wheter the st= ring_view (or vector) has been non-empty before? If so, follow the example = of vector and give the same promises on the standard level!

<= /div>

Den l=C3=B6rdagen den 18:e januari 2014 kl. 01:06:18 UTC+1 sk= rev Jeffrey Yasskin:
On Fri, Jan 17,= 2014 at 3:51 PM, Marshall Clow <mclow...@gmail.com> wrote:
> On Jan 17, 2014, at 2:10 PM, Jeffrey Yasskin <jyas...@google= ..com> wrote:
>
> Agreed. To mitigate this, if string_view::data() could return null= ptr, I
> believe we could still create strings using std::string(null_sv.be= gin(),
> null_sv.end()).
>
>
> I thought that=E2=80=99s why we had to_string().

We have to_string mostly for convenience, I think, although it is
another way to get from a null string_view to a std::string.

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