220 30189 <119a6ed9-e6b1-4c8c-b9b1-5152f7c7a3d6@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 10:43:41 -0800 (PST) Lines: 273 Approved: news@gmane.org Message-ID: <119a6ed9-e6b1-4c8c-b9b1-5152f7c7a3d6@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1832_42876821.1483469021460" X-Trace: blaine.gmane.org 1483469026 29736 195.159.176.226 (3 Jan 2017 18:43:46 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 3 Jan 2017 18:43:46 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBXXBV7BQKGQEMUXXJ5I@isocpp.org Tue Jan 03 19:43:41 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f72.google.com ([209.85.214.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cOU3e-0006ji-Ng for gclcip-std-proposals@m.gmane.org; Tue, 03 Jan 2017 19:43:39 +0100 Original-Received: by mail-it0-f72.google.com with SMTP id n68sf441303432itn.4 for ; Tue, 03 Jan 2017 10:43:43 -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=bU4b6yJ61LsRolmyg/At6ij0yu7yJG0HCv1f7vIk59A=; b=WGPtCiwNCuIe69SX5R2KM83aRsUnS+DrN/O40bSyxvi7ZvqxoOfchFMy8nIaEL+REj XebLwrN3QDkU3iK28nEe5b1xSrYl+dkr5GBcqWy/IdfenIFxnMuPpyJKQWtH3a9DeSpD 1lEnyBB72ux95dnYGlOE+g1M+WKWmg4hIwXJd1GyKK6pupS8ehNzEH7qqEskIiCfA7iq hPK50ZJWoG2ySGaTdonVoyNf2I6wMxwS/WiOEUq1kYGzLfCml02zutYOT8ZrfSP4piT7 wEQ1bXteqxHxNQRJldjpHvfIxqEsJuNC256YOwcDLWX6B7qhV+xTktlvWuvlnyJ64eZX eD0g== 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=bU4b6yJ61LsRolmyg/At6ij0yu7yJG0HCv1f7vIk59A=; b=fxj49UoTIXKHV0yGXNXXGPbQkhLz1IzBN4rYYvfxuYAjjjCvIYVHgIUDYorYNNWblp GJYHOfQld4Ks3ulMTqomOSc/YaWoZQoTzQEjVHxbmkKQqJ5bNCynSETP55T86XVDyr8Z 5C99ALcG/Om9zhN6soBTujauMt+d9d2xYMX4PW9jXnuXo+BGBaHo0yVthIQXnp7Tlvkw HRdFFj0a9lTGqbJJVaZcbpc90FEG3cm0kXxa96/GOLACw5yguHRd/gTEowNLuEB7/HYG IKjk+TRYFV1K93HybpCuJvSFPJ44lT42eg2lovq9bWaWudO6x0QNLrp+xhKCyVRsIs4j zkDQ== 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=bU4b6yJ61LsRolmyg/At6ij0yu7yJG0HCv1f7vIk59A=; b=KAZnP/LOycYyWOyHY332xu1WdXp2AkaMl++JT4AsHDJRzxt7U5MlBdSbpmDW3RkwVR gbcUWKUUOkW4hbcD+XTxNnf28/No1wVQFyu0w7qJK3ENYv7akO98ekBFbp6IB3Oi/0qi wzO5SkECtVoTG4pdsnHLktGZxXeO5l3vQ2RKQsVtxtl3WPtSuEd3S62GFT3vW/J1IWnU bNpalTESakKRJSKElYQY2r2FQkQ5s3yXBbRu4/W6higuVK/3qVCXVxe2D+H9rf6A0Rl6 daNB0d/n6CNHMMc7SRwGSXskf3O6RUHo+HmshO9UNfHL/G4Jckz6lchQ9dNhrjU7YgVh EU0g== X-Gm-Message-State: AIkVDXLrVjyReNX0K7fBMDV81ufPEX4mJmVmN63+BlscjYOqpZCldSEafuwNhuss2AW2ew== X-Received: by 10.36.117.65 with SMTP id y62mr13500245itc.4.1483469022743; Tue, 03 Jan 2017 10:43:42 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.53.52 with SMTP id o49ls39689222otc.18.gmail; Tue, 03 Jan 2017 10:43:42 -0800 (PST) X-Received: by 10.157.17.3 with SMTP id g3mr2935014ote.8.1483469021990; Tue, 03 Jan 2017 10:43:41 -0800 (PST) In-Reply-To: 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:30189 Archived-At: ------=_Part_1832_42876821.1483469021460 Content-Type: multipart/alternative; boundary="----=_Part_1833_311645332.1483469021461" ------=_Part_1833_311645332.1483469021461 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Tuesday, January 3, 2017 at 1:25:20 PM UTC-5, Vicente J. Botet Escriba= =20 wrote: > > 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 > > enum color > { > red =3D 1, > green =3D 200, > blue =3D 3 > }; > > static_assert(enum_cast(200u) =3D=3D color::green); > static_assert(enum_cast(3) =3D=3D color::blue); > static_assert(enum_cast(4) =3D=3D nullopt); > > > This should work for both scoped and unscoped enums. > For opaque enums this would only check if value fits in underlying type. > > 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, but= =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 to= =20 > use builtins or reflection once it is there. > > Now, I believe that we need two kind of functions. One that is a cast and= =20 > that says just that we are casting. It would be the same as a static_cast= ..=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 the= =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. =20 > * to_enum : throw exception if error > * try_to_enum : returns optional or an interface based on error_cod= e=20 > (like from_chars). status_value and expected can be considered also once= =20 > 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. --=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/119a6ed9-e6b1-4c8c-b9b1-5152f7c7a3d6%40isocpp.or= g. ------=_Part_1833_311645332.1483469021461 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Tuesday, January 3, 2017 at 1:25:20 PM UTC-5, V= icente J. Botet Escriba wrote:
=20 =20 =20
Le 03/01/2017 =C3=A0 10:56, m.ce...@gmail.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 value);

requires: is_enum<Enum>, is_integral<Int>

enum color
{
=C2=A0 =C2=A0red =3D 1,
=C2=A0 =C2=A0green =3D 200,
=C2=A0 =C2=A0blue =3D 3
};

static_assert(enum_cast<color>= (200u) =3D=3D color::green);
static_assert(enum_cast<color>= (3) =3D=3D color::blue);
static_assert(enum_cast<color>= (4) =3D=3D nullopt);<= /span>


This should work for both scoped and unscoped enums.
For opaque enums this would only check if value fits in underlying type.

Why do we want this:
=C2=A0- validation of input data (e.g. coming from user 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=20 the given type being an actual enumeration? `static_cast` is the=20 standard way of saying "make this integer an enum without checking&quo= t;. I=20 don't think we need another way to spell that.
=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?

W= ell, there's only one failure state: the given value is not in the enum= eration. As such, I see no need for using complicated objects that store er= rors; `optional` should be sufficient.

--
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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/119a6ed9-e6b1-4c8c-b9b1-5152f7c7a3d6= %40isocpp.org.
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