220 19329 <1dd8b6ba-584c-4c1d-8667-6976afb5d785@isocpp.org> article Path: news.gmane.org!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: A new is_specialized type trait Date: Mon, 27 Jul 2015 15:44:35 -0700 (PDT) Lines: 135 Approved: news@gmane.org Message-ID: <1dd8b6ba-584c-4c1d-8667-6976afb5d785@isocpp.org> References: <8947d0c5-ed96-47fb-ad13-16557605356f@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1411_1125784689.1438037075459" X-Trace: ger.gmane.org 1438037081 8396 80.91.229.3 (27 Jul 2015 22:44:41 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 27 Jul 2015 22:44:41 +0000 (UTC) Cc: morwenn29@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDLZJYWNDQINJ2G2VUCRUBC25LVBA@isocpp.org Tue Jul 28 00:44:38 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f198.google.com ([209.85.214.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZJr8Q-0005d5-Ay for gclcip-std-proposals@m.gmane.org; Tue, 28 Jul 2015 00:44:38 +0200 Original-Received: by obkg7 with SMTP id g7sf179454964obk.0 for ; Mon, 27 Jul 2015 15:44:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type: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=VY80WITWAWvB8MY/hEDuQGQghHoc4mr6/oodLRPq3ok=; b=BQ6Ajd+Dz9LMIm8WDQhWDDPqMS3QelRZBdrqMDuEIj7ISraHCe5CL1u6xfSiteKeSL M++n8JAJY/Eh5Or3Ua0QFzXzggLyH08TrscQIMQRYBffd/nYHuokAIKjnI0cr0QyD13R QRHmOxcE2ySh/uAcW+Vocu8BNcDwNSnNxkthVGBMiRd7PH8BBOppTqMHYFaaoYbC8t+2 F/LIABPQTqAZQjseZ9OsZxh1bEkM6UuJMYIVnA0qvdxp2c1HVh0C05I1u5dvkrXa6qsl Sqzk3Kz9ZmRWW2KEuci7KejT7SYHawuCenuNVJg67LvNKqbPShQRy43FRFm88tJYHy+A cFCA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type: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=VY80WITWAWvB8MY/hEDuQGQghHoc4mr6/oodLRPq3ok=; b=TskSzVbG+t9t/AbtDJY8pPSIyEAcWCOC4Bo9QkyX/rWm4NP+gezX0SK41R4fLsd6Q1 IcYcZiZMwZEc8lHgxENQxyoVMFtREW8qnWHZ0HGOCMHBPbnA+gvrleotTM1vAlaxMnqr CIM8durIAdJ6W3eWKCZv3/diDkOSNKK2hskS/xcG22twK5zyC6Ge1cmWij/07yGYhzYT NQ8E3Rwt6wCb77BAa36KJ+B3i8uWDokTAUL4rwEl9TR3ONV0JdvFkqRvuRwKtucDag/s uL+b1F2LvGie+0i6Q1cnXAWsTgjUeJWIlxck6aM5QtdXe4dZwQo+E6wofI/NdH6Pauq6 o3YQ== X-Gm-Message-State: ALoCoQknWkaGvRix7l9zr48p1gmi8NIbmxe0aAb4oal9koDx5+g5IlzEyPhcYe4epC9/yF5R+l/G X-Received: by 10.50.111.231 with SMTP id il7mr280335igb.12.1438037077364; Mon, 27 Jul 2015 15:44:37 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.30.35 with SMTP id p3ls1084323igh.8.canary; Mon, 27 Jul 2015 15:44:36 -0700 (PDT) X-Received: by 10.50.4.1 with SMTP id g1mr3396igg.7.1438037076566; Mon, 27 Jul 2015 15:44:36 -0700 (PDT) In-Reply-To: <8947d0c5-ed96-47fb-ad13-16557605356f@isocpp.org> X-Original-Sender: arthur.j.odwyer@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:19329 Archived-At: ------=_Part_1411_1125784689.1438037075459 Content-Type: multipart/alternative; boundary="----=_Part_1412_586445083.1438037075460" ------=_Part_1412_586445083.1438037075460 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Monday, July 27, 2015 at 4:11:46 AM UTC-7, Morwenn wrote: > > A some point in one of my projects, I needed a tait to check whether a=20 > template was specialized for a given type to use it if it was and fall ba= ck=20 > to another template if it wasn't. Therefore, I wrote a small=20 > is_specialized trait class to do the job in the vein of the detection=20 > toolkit: > > template< > template class, > typename, > typename=3Dvoid > struct is_specialized: > std::false_type {}; > template< > template class Template, > typename T > struct is_specialized{})>>: > std::true_type {}; > > Would such an addition to the standard header be useful for= general-purpose specialization detection? > > I'm glad you've found a better solution to your problem. But I don't=20 understand the worse solution! Surely the code above merely checks whether Template is well-formed (and= =20 constructible from an empty braced-initializer). I don't see how it does anything to check whether Template comes from a= =20 specialization (either full or partial). For example, I see static_assert(is_specialized::value); static_assert(is_specialized::value); Are you trying to find some construct that would return true for=20 vector and false for vector? And if so, why?? =E2=80=93Arthur --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. ------=_Part_1412_586445083.1438037075460 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Monday, July 27, 2015 at 4:11:46 AM UTC-7, Morwenn wrote:
A some point in one of my pr= ojects, I needed a tait to check whether a template was specialized for a g= iven type to use it if it was and fall back to another template if it wasn&= #39;t. Therefore, I wrote a small is_specialized trait class to do the job in the vein of the= detection toolkit:

    template<
        template<typename<=
span>...> class,
        typename,
        typename=3Dvoid
    >
    struct is_specialized:
        std::false_type
    {};

    template<
        template<typename<=
span>...> class Temp=
late,
        typename T
    >
    struct is_specialized<=
Template, T, std<=
/span>::void_t<decltype(Template<T=
>{})>>:
        std::true_type
    {};

Would such an addition to=
 the standard header <=
type_traits> be useful for general-purpose specialization detecti=
on?

I'= m glad you've found a better solution to your problem. But I don't = understand the worse solution!
Surely the code above merely check= s whether Template<T> is well-formed (and constructible from an empty= braced-initializer).
I don't see how it does anything to che= ck whether Template<T> comes from a specialization (either full or pa= rtial).

For example, I see

=C2=A0 =C2=A0 static_assert(is_specialized<std::vector, int>::value= );
=C2=A0 =C2=A0 static_assert(is_specialized<std::vector= , bool>::value);

Are you trying to find s= ome construct that would return true for vector<bool> and false for v= ector<int>?
And if so, why??

=E2= =80=93Arthur

--

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