220 34718 <03509464-1e39-4d2f-ba26-c192191e72e6@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Mingxin Wang Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Extensions for Disambiguation Tags Date: Mon, 9 Oct 2017 08:48:01 -0700 (PDT) Lines: 222 Approved: news@gmane.org Message-ID: <03509464-1e39-4d2f-ba26-c192191e72e6@isocpp.org> References: <527c448b-b8dd-4877-b63f-b399c167da4d@isocpp.org> <20171009040212.5161043.12800.37351@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2745_1648152961.1507564081963" X-Trace: blaine.gmane.org 1507564105 32499 195.159.176.226 (9 Oct 2017 15:48:25 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 9 Oct 2017 15:48:25 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDNMBNHJWIGBBMVU53HAKGQEKCOFSQQ@isocpp.org Mon Oct 09 17:48:18 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f197.google.com ([209.85.223.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e1aHm-0006PA-00 for gclcip-std-proposals@m.gmane.org; Mon, 09 Oct 2017 17:48:06 +0200 Original-Received: by mail-io0-f197.google.com with SMTP id b186sf6122362iof.21 for ; Mon, 09 Oct 2017 08:48:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=5OJWOdV1xuqEkY5SHNUSlh2oFqxCe4/epr5ljLB1r0s=; b=MfWXGTq1PZ5FnTHeCK37MdNIrgDB65le3KrPNR3RimtFGfSn/0YbSSV2TW4xdM8TCm t80/8iA5aPVhqtz74v1606EbUEoTi+t/wylBBTFlbcUPqSoKJAG480IutdqCdKZG/jwa ZSaPY6hKdjwAy7SFe9jUBks+cMbc5tmuIzpf/h6etBCKtJ8y3BQ4q3kI0NjA3eHOkEjK lTStpyYlMG5CJWNxbYzwC+ydUKT095mBfRisFouZbyymHgy5S7Fjiy+3/tne83OdhPyE srCopadmwqqecw/aICnVQAtACLSy6L+cXF2v0T137KFeavSzbTPfwZxr0mHfSuEnn4QF MhPg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version: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=5OJWOdV1xuqEkY5SHNUSlh2oFqxCe4/epr5ljLB1r0s=; b=LqY3fK/XKPZx8ulqcITB5aedN9NvoIlfidGrCq2oEwpjPe1gojBmvdDHSxysTKF1dm xKRznNzzk8JhtL5352z/3nA+e7VcB0eS2iXne+5qb56yYkUmi23H18OBNiBJS184IEa3 HOHcF4GbbYltzRh87MuSCTouj98XNIGBE8UI5Bi69w5hlApBDBuVcFcgdtb6B+7OGjSb KaOjlgVdVdJNIrSo06LrKVVczoheMQUy0simfZaryqtkB1jWwFIIDpLhGoRuUYrskX8Y jeWTe9/deKLQRgVINUUQNWe/jVZr5p+sXWIrDpZeVMlwNjTInOf+OwCBcpBfrvQdQO8+ 0pPg== X-Gm-Message-State: AMCzsaWt9BIU76uusxeFvSt6T/hCJWvTRUP2i2skQ8/3c820E96UCCFg W3L8t6YI2T2BsdNcyOVD0jPmUg== X-Google-Smtp-Source: AOwi7QApRYjldhNbZ96gItj/eWdzQc6XC1HiPEAi5xG3zGHAMUElchWOq8c2AJv+uoTZNwdRtdbQPg== X-Received: by 10.31.7.81 with SMTP id 78mr1445232vkh.42.1507564093212; Mon, 09 Oct 2017 08:48:13 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.60.206 with SMTP id j197ls1229802vka.8.gmail; Mon, 09 Oct 2017 08:48:02 -0700 (PDT) X-Received: by 10.31.153.200 with SMTP id b191mr471199vke.28.1507564082559; Mon, 09 Oct 2017 08:48:02 -0700 (PDT) In-Reply-To: <20171009040212.5161043.12800.37351@gmail.com> X-Original-Sender: wmx16835vv@163.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:34718 Archived-At: ------=_Part_2745_1648152961.1507564081963 Content-Type: multipart/alternative; boundary="----=_Part_2746_1907148332.1507564081963" ------=_Part_2746_1907148332.1507564081963 Content-Type: text/plain; charset="UTF-8" With the support of `in_place_arg`, it becomes easy to pass any constexpr value (providing its type is valid for a template non-type parameter) to a function template using a uniform disambiguation tag, especially with constructors. Providing there is a enum class defined as follows: enum class State { RUNNINE, AVAILABLE, OFFLINE }; And there is a constexpr function that could convert a `State` to its string expression: constexpr const char* make_state_str(State s) { switch (s) { case State::RUNNINE: return "Running State"; case State::AVAILABLE: return "Available State"; case State::OFFLINE: return "Offline State"; default: return "Unknown State"; } } It is relatively easy to design a class with `in_place_arg`, which is explicitly constructible from a `constexpr State` and stores its string expression without executing the constexpr function `make_state_str` at runtime: class Machine { public: template explicit Machine(std::in_place_arg_t) : state_str_(STATE_STR) {} const char* get_state_str() { return state_str_; } private: const char* state_str_; template static constexpr const char* STATE_STR = make_state_str(S); }; Machine machine(std::in_place_arg); puts(machine.get_state_str()); `in_place_resource` has a wider scope of application than `in_place_arg` does, because it could carry all sort of constexpr data if the data is prior declared. For example, providing there is a struct carries some configuration: struct Config { double EPS = 1e-8; double PI = acos(-1); int INF = 0x7f7f7f7f; long long INFL = 0x7f7f7f7f7f7f7f7fL; } constexpr MATH_CONFIG; It is allowed to initialize a type with the resource by templates with `in_place_resource`, even if we are not sure about the concrete type of the resource: class Calculator { public: template explicit Calculator(std::in_place_resource_t); }; Calculator calculator(std::in_place_resource); Actually, `in_place_resource` could also be used to improve the implementation in my earlier posts about polymorphism, which used to have bottleneck on the size (thanks to Nicol Bolas). With the support of `in_place_resource`, the bottleneck no longer exist, because it becomes possible to merge the virtual table with the lifetime management functions at compile time. Mingxin Wang On Monday, October 9, 2017 at 12:02:15 PM UTC+8, Tony V E wrote: > > How about some examples showing usage? > > Sent from my BlackBerry portable Babbage Device > -- 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 email 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/03509464-1e39-4d2f-ba26-c192191e72e6%40isocpp.org. ------=_Part_2746_1907148332.1507564081963 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
With the support of `in_place_arg`, it becomes easy t= o pass any constexpr value (providing its type is valid for a template non-= type parameter) to a function template using a uniform disambiguation tag, = especially with constructors.

Providing there is a= enum class defined as follows:

enum class State {
=C2= =A0 RUNNINE, AVAILABLE, OFFLINE
};
=

And there is a constexpr function that could convert= a `State` to its string expression:

constexpr const char* make_state_str(State s) {
=C2=A0 switch (s) {
=C2=A0 =C2=A0 case State::RUNNINE: return "Running State= ";
=C2=A0 =C2=A0 case State::AVAILA= BLE: return "Available State";
=C2=A0 =C2=A0 case State::OFFLINE: return "Offline State";
=
=C2=A0 =C2=A0 default: return "Unknown S= tate";
=C2=A0 }
}

It is relatively e= asy to design a class with `in_place_arg`, which is explicitly constructibl= e from a `constexpr State` and stores its string expression without executi= ng the constexpr function `make_state_str` at runtime:

class Machine {
=C2=A0public:
=C2=A0 template <State S>
= =C2=A0 explicit Machine(std::in_place_arg_t<State, S>) : state_str_(S= TATE_STR<S>) {}

=C2=A0 const char* get_state_str() { return state_str= _; }

=C2=A0private:
=C2=A0 const char* s= tate_str_;

=C2=A0 template <State S>
=C2=A0 static constexpr const char* STATE_STR =3D make_state_str(S);
};

<= /div>
Machine machine(std::in_place_arg<Sta= te, State::AVAILABLE>);
puts(machine.= get_state_str());

`in_place_resourc= e` has a wider scope of application than `in_place_arg` does, because it co= uld carry all sort of constexpr data if the data is prior declared.

For example, providing there is a struct carries some con= figuration:

struct Config {
=C2=A0 double EPS =3D= 1e-8;
=C2=A0 double PI =3D acos(-1);
=C2=A0 int INF =3D 0x7f7f7f7f;
=C2=A0 long long INFL =3D 0x7f7f7f7f7f7f7f7fL;
} constexpr MATH_CONFIG;
<= /div>
It is allowed to initialize a type with the resource by templates = with `in_place_resource`, even if we are not sure about the concrete type o= f the resource:

<= div class=3D"subprettyprint">class Calculator {
=C2=A0public:
=C2=A0 template &l= t;class T, const T& CONFIG>
=C2= =A0 explicit Calculator(std::in_place_resource_t<T, CONFIG>);
};

Calculator calculator(std::in_place_resource= <Config, MATH_CONFIG>);

Actually, `in_place_resource` could also be used to improve the imple= mentation in my earlier posts about polymorphism, which used to have bottle= neck on the size (thanks to Nicol Bolas). With the support of `in_place_res= ource`, the bottleneck no longer exist, because it becomes possible to merg= e the virtual table with the lifetime management functions at compile time.=

Mingxin Wang


On Mon= day, October 9, 2017 at 12:02:15 PM UTC+8, Tony V E wrote:
=
How about some examples showing usage?
= =

= =
Sent=C2=A0from=C2=A0my=C2=A0BlackBerry=C2=A0portable=C2=A0Ba= bbage=C2=A0Device

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/03509464-1e39-4d2f-ba26-c192191e72e6= %40isocpp.org.
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