220 19975 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Obtaining the number of arguments of a function Date: Sun, 23 Aug 2015 16:33:58 +0800 Lines: 97 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2098\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_55489AA9-C2EC-4E58-B15A-5FB98C7E1DF0" X-Trace: ger.gmane.org 1440318853 9356 80.91.229.3 (23 Aug 2015 08:34:13 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 23 Aug 2015 08:34:13 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBAEL42XAKGQE2AQFCQQ@isocpp.org Sun Aug 23 10:34:12 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f199.google.com ([209.85.214.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZTQjC-0002kV-5X for gclcip-std-proposals@m.gmane.org; Sun, 23 Aug 2015 10:34:10 +0200 Original-Received: by obkg7 with SMTP id g7sf152705877obk.0 for ; Sun, 23 Aug 2015 01:34:09 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:message-id:mime-version :subject:date:references:to: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=v26khkSE0ueQIgAlnYfwAK+RZWA8C61ksbUkilUoCsc=; b=Ap7+KHG/JwKE0E6YVJPmL1r8nFMll8IwgQBN+4ymfZjOzoqkdery7Tl2IOGepq0Nlx nAu6HCLkaDrbjYVV2jVVLX44szxiw35m/SXIFKDq5llMmxUYs5ZF9AImSZQBIDpJefbg QDOlboBrPTUocAbCKIVoSOUIyG5kpRXfmGU2N1fTD6KSaEo7UA30lLRabqcSQbHwgHxq JQyKMn/BbM2SCLOWAiDaRy3hck/vcGbgD7t6grELj9xQ1Sfy1uAky604HC2cx2HIlXhV tj4idpinntvIIfUfWoIgXsDkj0Yw1RQ1lA+i4duSp+J2IBCjOabcefvF1d25K9doOXFv Bn5w== X-Gm-Message-State: ALoCoQml1jsPFZq/6q1VEJztCIaAkAG3ZmZNoOwuqIlj8znGQU92aSC5s/uLPGRLm3q6UBT1q6G0 X-Received: by 10.50.25.233 with SMTP id f9mr12591500igg.8.1440318848947; Sun, 23 Aug 2015 01:34:08 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.32.75 with SMTP id g72ls1174729iog.3.gmail; Sun, 23 Aug 2015 01:34:08 -0700 (PDT) X-Received: by 10.66.102.71 with SMTP id fm7mr33790327pab.5.1440318848326; Sun, 23 Aug 2015 01:34:08 -0700 (PDT) Original-Received: from mail-pa0-x22f.google.com (mail-pa0-x22f.google.com. [2607:f8b0:400e:c03::22f]) by mx.google.com with ESMTPS id oj5si21638902pab.238.2015.08.23.01.34.08 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 23 Aug 2015 01:34:08 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22f as permitted sender) client-ip=2607:f8b0:400e:c03::22f; Original-Received: by pacti10 with SMTP id ti10so767257pac.0 for ; Sun, 23 Aug 2015 01:34:08 -0700 (PDT) X-Received: by 10.66.193.132 with SMTP id ho4mr8947598pac.154.1440318847925; Sun, 23 Aug 2015 01:34:07 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.94]) by smtp.gmail.com with ESMTPSA id qp5sm13326112pbc.0.2015.08.23.01.34.06 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 23 Aug 2015 01:34:07 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.2098) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22f as permitted sender) smtp.mailfrom=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:19975 Archived-At: --Apple-Mail=_55489AA9-C2EC-4E58-B15A-5FB98C7E1DF0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2015=E2=80=9308=E2=80=9323, at 4:03 PM, NDos Dannyu wrote: >=20 > Why this function isn't in the stantard library? It's too easy to impleme= nt! Is the standard library supposed to do everything that=E2=80=99s easy? > template > constexpr size_t count_args(R (*)(Args...)) noexcept { > return sizeof...(Args); > } This doesn=E2=80=99t handle member function signatures with cv-qualifiers o= r ref-qualifiers. Separately, it doesn=E2=80=99t handle member functions (P= TMFs). It doesn=E2=80=99t handle C-style varargs signatures, or Callable ob= jects besides function pointers. It doesn=E2=80=99t tell you about default = arguments. This is not easy. I don=E2=80=99t see how it=E2=80=99s useful, either. Usually it=E2=80=99s better to avoid trying to inspect function parameter t= ypes. Use perfect forwarding instead, with expression SFINAE or std::result= _of which also does SFINAE. Discrimination should happen at the call site. = If you need to discriminate against a type before any object gets called, t= hen fabricate a call in an appropriate context such as static_assert or SFI= NAE. --=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/. --Apple-Mail=_55489AA9-C2EC-4E58-B15A-5FB98C7E1DF0 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2015=E2=80=9308= =E2=80=9323, at 4:03 PM, NDos Dannyu <ndospark320@naver.com> wrote:

Why this function isn't in the stantard library? It's too easy to= implement!

Is= the standard library supposed to do everything that=E2=80=99s easy?
<= br class=3D"">
template <class R= , class ...Args>
conste= xpr size_t count_args(R (*)(Args...)) noexcept {
    return sizeof...(Args);
}

This doesn=E2=80=99t handle member functi= on signatures with cv-qualifiers or ref-qualifiers. Separately, it doesn=E2= =80=99t handle member functions (PTMFs). It doesn=E2=80=99t handle C-style = varargs signatures, or Callable objects besides function pointers. It doesn= =E2=80=99t tell you about default arguments.

This is not easy. I don=E2=80=99t see how it= =E2=80=99s useful, either.

Usually it= =E2=80=99s better to avoid trying to inspect function parameter types. Use = perfect forwarding instead, with expression SFINAE or std::result_of which also does SFINAE. Discriminat= ion should happen at the call site. If you need to discriminate against a t= ype before any object gets called, then fabricate a call in an appropriate = context such as static_assert or S= FINAE.

--

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