220 31773 article Path: news.gmane.org!.POSTED!not-for-mail From: Avi Kivity Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allow constexpr static data members in a literal type. 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[2a00:1450:400c:c0c::22c]) by mx.google.com with ESMTPS id b4si13254738wmb.98.2017.03.27.01.13.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 27 Mar 2017 01:13:51 -0700 (PDT) Received-SPF: pass (google.com: domain of avi@scylladb.com designates 2a00:1450:400c:c0c::22c as permitted sender) client-ip=2a00:1450:400c:c0c::22c; Original-Received: by mail-wr0-x22c.google.com with SMTP id l43so43934677wre.1 for ; Mon, 27 Mar 2017 01:13:51 -0700 (PDT) X-Received: by 10.28.225.69 with SMTP id y66mr3240160wmg.84.1490602430944; Mon, 27 Mar 2017 01:13:50 -0700 (PDT) Original-Received: from avi.cloudius-systems.com ([37.142.229.250]) by smtp.gmail.com with ESMTPSA id m186sm13318379wmd.21.2017.03.27.01.13.50 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 27 Mar 2017 01:13:50 -0700 (PDT) In-Reply-To: <5262b75e-8281-411d-2248-6be37fd52b85@wanadoo.fr> 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::22c 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:31773 Archived-At: On 03/26/2017 07:50 PM, Vicente J. Botet Escriba wrote: > Le 26/03/2017 =C3=A0 16:30, Avi Kivity a =C3=A9crit : >> 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 =3D bool_class{true}; >> >> static constexpr bool_class no =3D bool_class{true}; >> >> }; >> >> >> Compilation fails, I believe rightly, on clang, because by the time=20 >> yes and no are parsed, bool_class is an incomplete type, and=20 >> therefore not a literal type. It could be worked around by making=20 >> yes and no functions. >> >> Should we make this legal? It seems reasonable for types to offer=20 >> constants scoped under their own name, and if they are literal types,=20 >> that these constants can be constexpr. >> > It seems that you can do > > template > > class bool_class { > > bool _value; > > public: > > explicit constexpr bool_class(bool v) noexcept : _value(v) {} > > // more emulation of bool > > static const bool_class yes; > > static const bool_class no; > > }; > > constexpr bool_class bool_class::yes =3D bool_class{true}; > > constexpr bool_class bool_class::no =3D bool_class{true}; > > The compiler (clang) won't recognize yes and no as constexpr variables: template class bool_class { bool _value; public: explicit constexpr bool_class(bool v) noexcept : _value(v) {} // more emulation of bool constexpr bool operator=3D=3D(bool_class other) const { return _value =3D=3D other._value; } static const bool_class yes; static const bool_class no; }; template constexpr bool_class bool_class::yes =3D bool_class{true}; template constexpr bool_class bool_class::no =3D bool_class{true}; static_assert(bool_class::yes =3D=3D bool_class::no); bool_class2.cc:23:15: error: static_assert expression is not an integral=20 constant expression static_assert(bool_class::yes =3D=3D bool_class::no); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ bool_class2.cc:23:39: note: read of non-constexpr variable 'no' is not=20 allowed in a constant expression static_assert(bool_class::yes =3D=3D bool_class::no); ^ --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. 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