220 31757 article Path: news.gmane.org!.POSTED!not-for-mail From: Avi Kivity Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allow constexpr static data members in a literal type. 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[2a00:1450:400c:c0c::22a]) by mx.google.com with ESMTPS id l9si1625117wre.335.2017.03.26.07.49.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 Mar 2017 07:49:20 -0700 (PDT) Received-SPF: pass (google.com: domain of avi@scylladb.com designates 2a00:1450:400c:c0c::22a as permitted sender) client-ip=2a00:1450:400c:c0c::22a; Original-Received: by mail-wr0-x22a.google.com with SMTP id l43so22492970wre.1 for ; Sun, 26 Mar 2017 07:49:20 -0700 (PDT) X-Received: by 10.28.135.21 with SMTP id j21mr5443962wmd.70.1490539759995; Sun, 26 Mar 2017 07:49:19 -0700 (PDT) Original-Received: from avi.cloudius-systems.com ([37.142.229.250]) by smtp.gmail.com with ESMTPSA id n80sm8374901wrb.24.2017.03.26.07.49.18 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 Mar 2017 07:49:19 -0700 (PDT) In-Reply-To: <470f86cc-36b7-4244-82dd-b795af107996@isocpp.org> X-Original-Sender: avi@scylladb.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@scylladb-com.20150623.gappssmtp.com; spf=pass (google.com: domain of avi@scylladb.com designates 2a00:1450:400c:c0c::22a as permitted sender) smtp.mailfrom=avi@scylladb.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:31757 Archived-At: This is a multi-part message in MIME format. --------------A9BDFBD26DF6F56B77F8A6FF Content-Type: text/plain; charset=UTF-8; format=flowed On 03/26/2017 05:35 PM, Nicol Bolas wrote: > On Sunday, March 26, 2017 at 10:30:29 AM UTC-4, Avi Kivity wrote: > > Consider this type-safe bool class: > > > template > > class bool_class { > > bool _value; > > public: > > explicit constexpr bool_class(bool v) noexcept : _value(v) {} > > // more emulation of bool > > static constexpr bool_class yes = bool_class{true}; > > static constexpr bool_class no = bool_class{true}; > > }; > > > Compilation fails, I believe rightly, on clang, because by the > time yes > and no are parsed, bool_class is an incomplete type, and therefore > not a > literal type. It could be worked around by making yes and no > functions. > > Should we make this legal? It seems reasonable for types to offer > constants scoped under their own name, and if they are literal types, > that these constants can be constexpr. > > > How does it seem reasonable to define variables of a type that has not > yet been defined? You can do that already, within an inline member function. > What's wrong with defining them outside of the scope of the class? > You have to invent a scope for them. Perhaps template class bool_class { bool _value; public: explicit constexpr bool_class(bool v) noexcept : _value(v) {} // more emulation of bool struct constants; }; template struct bool_class::constants { static constexpr bool_class yes = bool_class{true}; static constexpr bool_class no = bool_class{true}; }; But using them is awkward and artificial. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On 03/26/2017 05:35 PM, Nicol Bolas wrote:
On Sunday, March 26, 2017 at 10:30:29 AM UTC-4, Avi Kivity wrote:
Consider this type-safe bool class:


=C2=A0 =C2=A0template <typename Tag>

=C2=A0 =C2=A0class bool_class {

=C2=A0 =C2=A0 =C2=A0 =C2=A0bool _value;

=C2=A0 =C2=A0public:

=C2=A0 =C2=A0 =C2=A0 =C2=A0explicit constexpr bool_class(bool v) = noexcept : _value(v) {}

=C2=A0 =C2=A0 =C2=A0 =C2=A0// more emulation of bool

=C2=A0 =C2=A0 =C2=A0 =C2=A0static constexpr bool_class yes =3D bo= ol_class{true};

=C2=A0 =C2=A0 =C2=A0 =C2=A0static constexpr bool_class no =3D boo= l_class{true};

=C2=A0 =C2=A0};


Compilation fails, I believe rightly, on clang, because by the time yes
and no are parsed, bool_class is an incomplete type, and therefore not a
literal type. =C2=A0It could be worked around by making yes and n= o functions.

Should we make this legal? =C2=A0It seems reasonable for types to offer
constants scoped under their own name, and if they are literal types,
that these constants can be constexpr.

How does it seem reasonable to define variables of a type that has not yet been defined?


You can do that already, within an inline member function.

What's wrong with defining them outside of the scope of the class?


You have to invent a scope for them.

Perhaps

=C2=A0=C2=A0 template <typename Tag>

=C2=A0=C2=A0 class bool_class {

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bool _value;

=C2=A0=C2=A0 public:

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 explicit constexpr bool_class(bool= v) noexcept : _value(v) {}

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 // more emulation of bool

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct constants;

=C2=A0=C2=A0 };


=C2=A0=C2=A0 template <typenamer Tag>
=C2=A0=C2=A0 struct bool_class<Tag>::constants {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 static constexpr bool_class yes = =3D bool_class{true};

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 static constexpr bool_class no =3D= bool_class{true};
=C2=A0=C2=A0 };

But using them is awkward and artificial.

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