220 30188 article Path: news.gmane.org!.POSTED!not-for-mail From: "Vicente J. Botet Escriba" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: enum_cast proposal Date: Tue, 3 Jan 2017 19:25:18 +0100 Lines: 364 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------BB00DC1B62BA3E31F9F197A6" X-Trace: blaine.gmane.org 1483467927 30353 195.159.176.226 (3 Jan 2017 18:25:27 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 3 Jan 2017 18:25:27 +0000 (UTC) User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDH67CONY4PBBD6ZV7BQKGQEP2KV4WY@isocpp.org Tue Jan 03 19:25:22 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f69.google.com ([209.85.215.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cOTlr-0006bD-QO for gclcip-std-proposals@m.gmane.org; Tue, 03 Jan 2017 19:25:15 +0100 Original-Received: by mail-lf0-f69.google.com with SMTP id t196sf183396975lff.3 for ; Tue, 03 Jan 2017 10:25:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=BrzDQesA63lSehyAvGViZuFppZY3Jx6DmrLYi8KoRxo=; b=vvIA8N0iUUfDKCF71yaapcC4skVc7dWaR7fNh/qVokm+hLh2Kg7IkUj6Tu4gCI6mB1 5E43aWUtld21Y10rWh6UVd9eHjD1p7vnfo+hofaUQWEG6SI2UHN/DpDm64futyJby7Gz /4T9vtwl0QmGW9i/dsmFcsXTa8ESbR1T5DXX12owTMXel8N5khHtpw/aktFuEDSoLZYi e/zjk8qCjFogwkI8ZW+VCixZYzqS4trum6ukwcaFJ6syqKUumdSCDmV9wuKzNqZZQTSl MoyZmQVuqLeQ2d2fLLR9aY4T1lDBGB70unax3jfFwbMr6aZHMXk0v6BQeMPnXzosiBQ2 UOcg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=BrzDQesA63lSehyAvGViZuFppZY3Jx6DmrLYi8KoRxo=; b=cElFNQMZpJTLidsS1QxKVcY+2IQ0NSRGnWdtr1C2ZZQgfx6r+e4LBaatjEkMJdSKiC 0isk/lQhYwYMgH4vcGkLuXbygk17ltG/yNKBtCwfdVx5zqBwKXEBPISe2f9vadVRqynw l2PlVFd+Qo1EvwdAwfN0GYOAjhqY6zgsLPPeFOFBvCosNPSrOhhUIOdE000eTKwpoeKl KZe7uUUIC3n1YVJKv3u13hC57rUFlaoYwfe1Fl8M764ZS4TudyHYBpMFyuQRyUteE7fx R+f3ZoN5mIco+hAijGr7kBIvK8ls+JhiGfpfpX7pJR1C8vlRo9R+YORU1rtG4MCK/Hwm 3rjA== X-Gm-Message-State: AIkVDXL3OLmAtFFonG3mhtKAbJFv6OLWS7hN+eYndz9mB1d0opzcNMl6lm2kp+ET0En7cw== X-Received: by 10.46.21.6 with SMTP id s6mr5918816ljd.23.1483467919966; Tue, 03 Jan 2017 10:25:19 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.166.66 with SMTP id p63ls5857719wme.5.gmail; Tue, 03 Jan 2017 10:25:18 -0800 (PST) X-Received: by 10.28.98.130 with SMTP id w124mr57341798wmb.125.1483467918897; Tue, 03 Jan 2017 10:25:18 -0800 (PST) Original-Received: from smtp.smtpout.orange.fr (smtp06.smtpout.orange.fr. [80.12.242.128]) by mx.google.com with ESMTPS id qo1si78058822wjc.294.2017.01.03.10.25.18 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 03 Jan 2017 10:25:18 -0800 (PST) Received-SPF: neutral (google.com: 80.12.242.128 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) client-ip=80.12.242.128; Original-Received: from imac-de-vicente-botet-escriba.home ([92.139.156.24]) by mwinf5d12 with ME id TuRJ1u0030Xrduc03uRJQC; Tue, 03 Jan 2017 19:25:18 +0100 X-ME-Helo: imac-de-vicente-botet-escriba.home X-ME-Auth: dmljZW50ZS5ib3RldEB3YW5hZG9vLmZy X-ME-Date: Tue, 03 Jan 2017 19:25:18 +0100 X-ME-IP: 92.139.156.24 In-Reply-To: X-Original-Sender: vicente.botet@wanadoo.fr X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 80.12.242.128 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) smtp.mailfrom=vicente.botet@wanadoo.fr 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:30188 Archived-At: This is a multi-part message in MIME format. --------------BB00DC1B62BA3E31F9F197A6 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable Le 03/01/2017 =C3=A0 10:56, m.cencora@gmail.com a =C3=A9crit : > Hi, > > 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 > optionalenum_cast(Intvalue); > > requires:is_enum,is_integral > > enumcolor > { > red =3D1, > green =3D200, > blue =3D3 > }; > > static_assert(enum_cast(200u)=3D=3Dcolor::green); > static_assert(enum_cast(3)=3D=3Dcolor::blue); > static_assert(enum_cast(4)=3D=3Dnullopt); > > | > > 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,=20 > 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=20 > __builtin_* intrinsic). > Hi, I believe this could be useful. Library implementers could be free=20 to use builtins or reflection once it is there. Now, I believe that we need two kind of functions. One that is a cast=20 and that says just that we are casting. It would be the same as a=20 static_cast. Something like gsl::narrow_cast so that we express better=20 the intent. In addition we need the function you are proposing similar=20 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 * enum_cast ~static_cast * 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=20 considered also once adopted. Do we expect several error conditions? The fist is just a library solution. For the last two, we would need a=20 compiler builtin or reflection so that both can be easily and=20 efficiently implemented. My 2cts. Vicente P.S. I don't expect this to be available before reflection, so an=20 implementation using the current reflection (see github) interface=20 should be welcome. This could be important to balance the compiler effort. --=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/a19ed9c6-45c0-e3d7-e92a-06468bff63b7%40wanadoo.f= r. --------------BB00DC1B62BA3E31F9F197A6 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Le 03/01/2017 =C3=A0 10:56, m.cencora@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 <typenam= e Enum, typenam= e Int>
optional<Enum> enum_cast(Int value<= /span>);

requires: is_enum<Enum>, is_integral<Int>

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

static_assert(enum_ca= st<col= or>(200u) =3D=3D<= /span> color<= /span>::green);
static_assert(enum_ca= st<col= or>(3) =3D=3D<= /span> color<= /span>::blue);
static_assert(enum_ca= st<col= or>(4) =3D=3D<= /span> nullop= t);


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
* 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?

The fist is just a library solution. For the last two, we would need a compiler builtin or reflection so that both can be easily and efficiently implemented.

My 2cts.
Vicente

P.S. I don't expect this to be available before reflection, so an implementation using the current reflection (see github) interface should be welcome. This could be important to balance the compiler effort.

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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/a19ed9c6-45c0-e3d7-e92a-06468bff63b7= %40wanadoo.fr.
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