220 30210 <0aa20bca-079b-4ca8-b744-08bdaeeb8d44@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: enum_cast proposal Date: Tue, 3 Jan 2017 19:52:18 -0800 (PST) Lines: 313 Approved: news@gmane.org Message-ID: <0aa20bca-079b-4ca8-b744-08bdaeeb8d44@isocpp.org> References: <119a6ed9-e6b1-4c8c-b9b1-5152f7c7a3d6@isocpp.org> <53189e5e-b1cf-0411-c87e-542822dbd4c0@wanadoo.fr> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2482_1134974176.1483501938664" X-Trace: blaine.gmane.org 1483501954 24300 195.159.176.226 (4 Jan 2017 03:52:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 4 Jan 2017 03:52:34 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBB5HCWHBQKGQEOGXPJWQ@isocpp.org Wed Jan 04 04:52:25 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f69.google.com ([209.85.214.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cOcca-0004Lq-Mo for gclcip-std-proposals@m.gmane.org; Wed, 04 Jan 2017 04:52:17 +0100 Original-Received: by mail-it0-f69.google.com with SMTP id g187sf451337468itc.2 for ; Tue, 03 Jan 2017 19:52:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to: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=e/dBrS6ppPIZnwBIIzxB3OnFXiMp7SgQLfWE961ulWI=; b=ImpI3GrfSYlkRNka/d71H8LZe2jNGXs3bjCbMWOnLJwxfJBU9uzxIMPDZxcpP5bsqp vk+dOmKFcgvNEkaaV++JeTMDre+9tUsUvgwXKNOTrPgxdfM7hmnFiS+ZcMhyg+7Vpfkw nlKrqiCkMmbD5MNtntguRxbZ1ZnoLnTA/9HJkEQEUOhswAD1KMsrB8DEfeTCQMm+Dl+1 GCsVimv0Btf/WTizPVtTqLQnXk22+APLKOPSJuIFSJYW+W9xrHdHMkc+ESr6kHtKN+zL 4U7X6k59VPYTe2y0x3oZNO7HMtpaGHSvxG+bzDYSNHMLrN6OD37c2Qc2QqTznPY7mABV Elqw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to: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=e/dBrS6ppPIZnwBIIzxB3OnFXiMp7SgQLfWE961ulWI=; b=M9Ty5kEPPxFFWEAj42oEgil2bpBQOzczAam1PYzc/mYwGD0R460QmQ5JK8wtjMwMej OfsRgww+NpgzHiEC3B1xkPC9Oh92sn6DzmBAYOJL3UlOpYEoNQEMfmqSxF01nQWHJxQw koJJrR5vxKU4FA+U64LHujAbususdiC4FH1s7nNgQTqoP4PmNDOno/Gx66aMqbWYKbcF Y7ylD2AYtePUJrWk0c+Dtw6zvjsavrqpbnbSkLc3JBEMONTbBLvWUd43sxuu6JxGHQck f1s/t70My+i7c7Yi7SNGmmAuJiYgR5cmgy/pTQfKQrK/btnECNWKYXhkt6zpK+Q5YCBL Rwmw== 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: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=e/dBrS6ppPIZnwBIIzxB3OnFXiMp7SgQLfWE961ulWI=; b=A/WtcUwzXQFkkVTN+2bMZtVQ4dtGG2dEI49ZbupCYvw7eWfbLxL5TZn68gjo1RMWtK TawZZeLVzcQuCeKo/dnESWljF54bTnfrvWHv9GEIqaXAH0vDlRX9zlcZ5cHjTOM2fpPK vRFptc+DOgi8HeC0NXOlxzRpg373K6eQug6ltJU5Mbn09MMIEdgQHM3UT309goM6+ZDD OvOmdXGgQBR+R1ZfqCjlcl+QNok2RMEfMc8rkY21SylZ4RC/9GWk68oBMn7y3mVlfdL4 ThoJHQXeBrfaZt04Wla7T4CMm69+osUs4HerSPKgEBE8kjfgQXqjtWXoky8V+a2ysXZD YhWQ== X-Gm-Message-State: AIkVDXIayGZPHAZgSOBqvYSnI+icxx/3j+zrZzrVsw8rTDQdZuNHDsJrd3+J+QTDl1MGeg== X-Received: by 10.36.95.209 with SMTP id r200mr14266694itb.16.1483501940507; Tue, 03 Jan 2017 19:52:20 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.17.154 with SMTP id v26ls160007otf.0.gmail; Tue, 03 Jan 2017 19:52:19 -0800 (PST) X-Received: by 10.157.42.16 with SMTP id t16mr2030261ota.18.1483501939421; Tue, 03 Jan 2017 19:52:19 -0800 (PST) In-Reply-To: <53189e5e-b1cf-0411-c87e-542822dbd4c0@wanadoo.fr> 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:30210 Archived-At: ------=_Part_2482_1134974176.1483501938664 Content-Type: multipart/alternative; boundary="----=_Part_2483_1262497948.1483501938664" ------=_Part_2483_1262497948.1483501938664 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Tuesday, January 3, 2017 at 5:04:22 PM UTC-5, Vicente J. Botet Escriba= =20 wrote: > > Le 03/01/2017 =C3=A0 19:43, Nicol Bolas a =C3=A9crit : > > > > On Tuesday, January 3, 2017 at 1:25:20 PM UTC-5, Vicente J. Botet Escriba= =20 > wrote:=20 >> >> Le 03/01/2017 =C3=A0 10:56, m.ce...@gmail.com a =C3=A9crit : >> >> Hi,=20 >> >> I propose to add a enum_cast function that safely casts int value to a= =20 >> target enum type if int value represents a valid enumerator. >> >> template >> optional enum_cast(Int value); >> >> requires: is_enum, is_integral >> >> >> Why do we want this: >> - validation of input data (e.g. coming from user or deserialization). >> - other? >> >> I know this could be purely library extension if based on reflection, bu= t=20 >> since we don't know when we will get reflection, >> and implementation based on reflection may not be optimal, >> I think it should be implemented with compiler support (i.e. via=20 >> __builtin_* intrinsic). >> >> Hi, I believe this could be useful. Library implementers could be free t= o=20 >> use builtins or reflection once it is there. >> >> Now, I believe that we need two kind of functions. One that is a cast an= d=20 >> that says just that we are casting. It would be the same as a static_cas= t.=20 >> Something like gsl::narrow_cast so that we express better the intent. In= =20 >> addition we need the function you are proposing similar to gsl::narrow. >> The question is how the function reports errors. If we use exceptions, I= =20 >> agree with Nicol that a specific exception would be better. >> >> I will then propose=20 >> * enum_cast ~static_cast >> > > What's the point of `enum_cast`, save the fact that it would SFINAE on th= e=20 > given type being an actual enumeration? `static_cast` is the standard way= =20 > of saying "make this integer an enum without checking". I don't think we= =20 > need another way to spell that. > > What is the point of narrow_cast? > .... I don't know. But I'm pretty sure it's not part of the C++ standard, so= =20 why are you asking? =20 > I believe it is useful to know the kind of cast we are using. This helps= =20 > to inspect the code we (others) have written in a more efficient way. > It's a static_cast. That's the kind of cast you're using. I have a strong dislike of proliferating non-standard cast functions.=20 Pointer-based casts that are equivalent to standard library casts, those=20 are fine. But inventing a new cast with its own special behavior? Why?=20 > =20 > >> * to_enum : throw exception if error >> * try_to_enum : returns optional or an interface based on=20 >> error_code (like from_chars). status_value and expected can be considere= d=20 >> also once adopted. >> Do we expect several error conditions? >> > > Well, there's only one failure state: the given value is not in the=20 > enumeration. As such, I see no need for using complicated objects that=20 > store errors; `optional` should be sufficient. > > If there is only one error case, yes optional is a good candidate. > However it would be weird to have a different exception depending on=20 > whether the user uses to_enum or try_to_enum (or whatever names are more= =20 > appropriated) > The function returning an `optional` doesn't throw an exception. The=20 exception is thrown by the improper use of `optional`. Remember: the user who asked for an `optional` *wants the behavior* of an= =20 `optional`. Which almost certainly means that they're not going to call=20 `value` without actually checking its value. If they were happy with an=20 exception being thrown, they wouldn't be using the `optional` version. Let's not assume that the user has no idea how to use the API. --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/0aa20bca-079b-4ca8-b744-08bdaeeb8d44%40isocpp.or= g. ------=_Part_2483_1262497948.1483501938664 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Tuesday, January 3, 2017 at 5:04:22 PM UTC-5, V= icente J. Botet Escriba wrote:
=20 =20 =20
Le 03/01/2017 =C3=A0 19:43, Nicol Bolas a =C3=A9crit=C2=A0:


On Tuesday, January 3, 2017 at 1:25:20 PM UTC-5, Vicente J. Botet Escriba wrote:
Le 03/01/2017 =C3=A0 10:56, m.ce...@gm= ail.com a =C3=A9crit=C2=A0:
Hi,

I propose to add a enum_cast function that safely casts int value to a target enum type if int value represents a valid enumerator.

template <typename Enum, typename Int>
optional<Enum> enum_cast(= Int val= ue);
requires:<= span style=3D"color:#000"> is_enum<Enum>, is_integra= l<In= t>

Why do we want this:
=C2=A0- validation of input data (e.g. coming from use= r or deserialization).
=C2=A0- other?

I know this could be purely library extension if based on reflection, but since we don't know when we will get reflection,
and implementation based on reflection may not be optimal,
I think it should be implemented with compiler support (i.e. via __builtin_* intrinsic).

Hi, I believe this could be useful. Library implementers could be free to use builtins or reflection once it is there.

Now, I believe that we need two kind of functions. One that is a cast and that says just that we are casting. It would be the same as a static_cast. Something like gsl::narrow_cast so that we express better the intent. In addition we need the function you are proposing similar to gsl::narrow.
The question is how the function reports errors. If we use exceptions, I agree with Nicol that a specific exception would be better.

I will then propose
* enum_cast ~static_cast

What's the point of `enum_cast`, save the fact that it would SFINAE on the given type being an actual enumeration? `static_cast` is the standard way of saying "make this integ= er an enum without checking". I don't think we need another= way to spell that.
What is the point of narrow_cast?

... I don&= #39;t know. But I'm pretty sure it's not part of the C++ standard, = so why are you asking?
=C2=A0
I believe it is useful to= know the kind of cast we are using. This helps to inspect the code we (others) have written in a more efficient way.
<= br>It's a static_cast. That's the kind of cast you're using.
I have a strong dislike of proliferating non-standard cast functions. = Pointer-based casts that are equivalent to standard library casts, those ar= e fine. But inventing a new cast with its own special behavior? Why?
=C2=A0
* to_enum : throw exception if error
* try_to_enum : returns optional<Enum> or an interface based on error_code (like from_chars). status_value and expected can be considered also once adopted.
Do we expect several error conditions?

Well, there's only one failure state: the given value is not in the enumeration. As such, I see no need for using complicated objects that store errors; `optional` should be sufficient.
If there is only one error case, yes optional is a good candidate.
However it would be weird to have a different exception depending on whether the user uses to_enum or try_to_enum (or whatever names are more appropriated)

The function returnin= g an `optional` doesn't throw an exception. The exception is thrown by = the improper use of `optional`.

Remember: the user who asked for an = `optional` wants the behavior of an `optional`. Which almost certain= ly means that they're not going to call `value` without actually checki= ng its value. If they were happy with an exception being thrown, they would= n't be using the `optional` version.

Let's not assume that t= he user has no idea how to use the API.

--
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/0aa20bca-079b-4ca8-b744-08bdaeeb8d44= %40isocpp.org.
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