220 32898 article Path: news.gmane.org!.POSTED!not-for-mail From: Mingxin Wang Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: safe integrals comparison Date: Mon, 26 Jun 2017 22:05:43 -0700 (PDT) Lines: 143 Approved: news@gmane.org Message-ID: References: <66f9bab2-7220-4bf1-afb7-77c5efa1bac3@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_711_1432126139.1498539944103" X-Trace: blaine.gmane.org 1498539948 15728 195.159.176.226 (27 Jun 2017 05:05:48 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 27 Jun 2017 05:05:48 +0000 (UTC) Cc: federico.kircheis@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDNMBNHJWIGBBKGPY7FAKGQEZTQKY6I@isocpp.org Tue Jun 27 07:05:44 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f200.google.com ([209.85.223.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dPih2-0003d1-QK for gclcip-std-proposals@m.gmane.org; Tue, 27 Jun 2017 07:05:41 +0200 Original-Received: by mail-io0-f200.google.com with SMTP id b204sf12558862ioe.11 for ; Mon, 26 Jun 2017 22:05:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc: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; bh=bI82zDiG8SV1KC+eGGL1hX9BPIQx0b5h0g0ON7kYOl8=; b=mm2J8n7v43y2kBS6TwJNQs7PZze8GjpCnv9F1LcaQ3XbAF3xeXx1yK3cuPk9JfLFIl jHKC+CLQ8p6/4FZ4l9EKIT8Dyi843TwYNEyzX+AzPd7DvrF+zzG8nXt4FPYLBe2mqeWj 3Yi3b/K/h/izsUaELrHNai/zyyVvQaCY3d4wveqUgGnLrTXLcu9WxwDAHMK3sVbFNRHv 95yyMJPLdXbxA3OAg9icXIfvtG51skN3TGpqkl7O2johyMOPS8Ys+QesFLVdY/n4Bh9D c9d8jBDuccdnLyntCpOWHJ1sPELXgf9A14ecmf8d4h9uql/UGLgiiCZW4M2b18O0jvA/ CpgA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc: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=bI82zDiG8SV1KC+eGGL1hX9BPIQx0b5h0g0ON7kYOl8=; b=S67b063yZhblt5mCh89TVMP4KcLQ9nAgO9W/pLhYMfeckmAJafJeRQsrzXrUuyn2uZ /yOUj22wNNWegyGTq2rd6PET7yzQhRDeYSY8FgjB11kRtEELKH2r/5ksMF9SDhSUq7na QvRkBR/JcWQXSiUqq9/Gu0QDJv6f9WHMX0Mq7/iccvYTKuTzEboZDVKNpRFHX9vbHbn7 NvvsJvVvslYluDKxrPUYnEiZusiSg0yw8ZTsb+b+p0ikRY6d6do6oXaou2xei99aUk3E BwiadULk1T16CMbv5HPVHk+jj3f/cfpKWKJCDcqRValg6xIfHTojDfSfGrqUbL6SBJKB CdtQ== X-Gm-Message-State: AKS2vOz2DeacmbsF7lUEvFVYSy3OrNw53fOohHjies82P2CKPhr9d2ac zcjOeDgism6S6+oL X-Received: by 10.107.58.87 with SMTP id h84mr1938174ioa.86.1498539945607; Mon, 26 Jun 2017 22:05:45 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.46.175 with SMTP id w44ls14227114ota.35.gmail; Mon, 26 Jun 2017 22:05:44 -0700 (PDT) X-Received: by 10.157.51.175 with SMTP id u47mr70001otc.4.1498539944647; Mon, 26 Jun 2017 22:05:44 -0700 (PDT) In-Reply-To: <66f9bab2-7220-4bf1-afb7-77c5efa1bac3@isocpp.org> X-Original-Sender: wmx16835vv@163.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:32898 Archived-At: ------=_Part_711_1432126139.1498539944103 Content-Type: multipart/alternative; boundary="----=_Part_712_416217759.1498539944104" ------=_Part_712_416217759.1498539944104 Content-Type: text/plain; charset="UTF-8" This is a good idea! However, I think maybe the following issues shall be considered. *This feature may introduce unnecessary extra overhead* Comparing to bare comparators between signed and unsigned integral types, this feature requires *one extra comparation* between the signed one and ZERO at runtime. Actually, it is not always necessary in some cases, e.g. std::vector v; for (int i = 0; i < v.size(); ++i) { // ... } Although variable i is a signed integer, the comparation is always executed correctly providing v.size() won't exceed std::numeric_limits::max(). Thus the extra comparation introduced in your solution is redundant as we can assert that i is always positive. Still, I think there is enough motivation for us to have this feature. *A uniform wrapper* Although the problem can be solved with a uniform wrapper, that would introduce even much overhead, especially when there are implementation defined integral types, e.g. __int128 in GCC. *Implementation with Concepts TS* I think the implementation could be much simpler with Concepts TS. For instance, we can define different overloads for the function template cmp_less with different constraints, as is shown below: template bool cmp_less(const T&, const U&); // undefined template bool cmp_less(const T& lhs, const U& rhs) requires std::is_signed_v == std::is_signed_v { return lhs < rhs; } template bool cmp_less(const T& lhs, const U& rhs) requires std::is_signed_v && !std::is_signed_v { return lhs < 0 ? true : lhs < rhs; } template bool cmp_less(const T& lhs, const U& rhs) requires !std::is_signed_v && std::is_signed_v { return rhs < 0 ? false : lhs < rhs; } I hope these would help. Mingxin Wang -- 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/aa356514-dd5d-4210-8882-ffcef3ae1449%40isocpp.org. ------=_Part_712_416217759.1498539944104 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
This is a good idea! However, I think maybe the following = issues shall be considered.

This feature=C2=A0may int= roduce unnecessary extra overhead

Comparing to= bare comparators between signed and unsigned integral types, this feature = requires one extra comparation between the signed one and ZERO at ru= ntime. Actually, it is not always necessary in some cases, e.g.
<= br>
=
std::vector<int> v;
<= div class=3D"subprettyprint">for (int i =3D 0; i < v.size(); ++i) {
=C2=A0 // ...
}

Although variable i is a signed in= teger, the comparation is always executed=C2=A0correctly providing v.size()= won't exceed=C2=A0std::numeric_limits<int>::max(). Thus the extr= a comparation introduced in your solution is redundant as we can assert tha= t i is always positive. Still, I think there is enough motivation for us to= have this feature.

A uniform wrapper
=

Although the problem can be solved with a uniform wrapp= er, that would introduce even much overhead, especially when there are impl= ementation defined integral types, e.g.=C2=A0__int128 in GCC.
Implementation with Concepts TS

I think the implementation could be much simpler with Concepts TS. For ins= tance, we can define different overloads for the function template cmp_less= with different constraints, as is shown below:

template <class T, class U>
bool cmp_less(const T&, const U&); // undefined

temp= late <class T, class U>
bool cmp_l= ess(const T& lhs, const U& rhs) requires std::is_signed_v<T> = =3D=3D std::is_signed_v<U> {
=C2= =A0 return lhs < rhs;
}

template <= class T, class U>
bool cmp_less(const= T& lhs, const U& rhs) requires std::is_signed_v<T> &&= ; !std::is_signed_v<U> {
=C2=A0 re= turn lhs < 0 ? true : lhs < rhs;
}=

template <class T, class U>
bool = cmp_less(const T& lhs, const U& rhs) requires !std::is_signed_v<= T> && std::is_signed_v<U> {
=C2=A0 return rhs < 0 ? false : lhs < rhs;
}

I = hope these would help.

Mingxin Wang

--
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.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/aa356514-dd5d-4210-8882-ffcef3ae1449= %40isocpp.org.
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