220 39066 article Path: news.gmane.org!.POSTED!not-for-mail From: bastienpenava@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: expression based deduction guides Date: Tue, 10 Jul 2018 17:00:32 -0700 (PDT) Lines: 521 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3587_1773918286.1531267232166" X-Trace: blaine.gmane.org 1531267109 22744 195.159.176.226 (10 Jul 2018 23:58:29 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 10 Jul 2018 23:58:29 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCG3XHNI6IDRBIMRSXNAKGQEWC7UYJQ@isocpp.org Wed Jul 11 01:58:25 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f197.google.com ([209.85.213.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fd2WU-0005ot-Ul for gclcip-std-proposals@m.gmane.org; Wed, 11 Jul 2018 01:58:23 +0200 Original-Received: by mail-yb0-f197.google.com with SMTP id v1-v6sf23690174ybm.15 for ; Tue, 10 Jul 2018 17:00:34 -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:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=oDOOjHjQ7jBTUSbrfLPiz3Sz7ajuxgc6V0ZvDYV4y8A=; b=cUx1xtXTXV9CD+6OR7oFjEmSYMm+NkVlO0f/gOYPC/BHpdkpxA/I2QUIwcPmWgxrN6 Zpos388yYg6HP13CBXn/QJoDyMweLIeYCoQLw+HXFUWhZzqRzxApicjQIrNr/N6zgJEh BG52vcBhJPVC0He9814a3CmEm8k2ofmo1/edo96DH6g0IZib0OnhP4EX0+/5ewUIO81q fpUcWh3LBaxPF8rZSSucc0qw+0/DUnfJKFofENmzE3Tif/bdhQ+PLSCrneLz6OPPTq2b 1Mi9tNSdGxkPxtFw4j4Yhfph8DU31nShX+Zr+vRGzuapPqe29k2hKEEVPu+H36EPu4jr vitw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=oDOOjHjQ7jBTUSbrfLPiz3Sz7ajuxgc6V0ZvDYV4y8A=; b=XoUSZ1of/xaYkYxI5BmCS5PpoiIQl/82BOfZzw87VHA/nEe/pytZRV18sgXfvjezkl 80rB9BB2x7Ev/hrOJDUHElQHxtMpfCbK76PwhCUWVwodBFn+XSCujR+GSXuz9jgpr7En +yryAuGZGqmGp0LEux2gTKg0/JMI5F1Wtb27VBNDm9q7L+pIhZN35VIrKfO6CNss6tav 6tApVDW0yPgXNmJwp67rQEvuMCXP9pJYx6nXCzxgR2acXery0MCBS22j/hRh5UBP/ZUi CBRBquW76PWwjAtwmP9drrMw91kSJu5GajAqpCAzY8SZiL4IEarmH+46Xap4xCcrVofO tgwg== 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: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=oDOOjHjQ7jBTUSbrfLPiz3Sz7ajuxgc6V0ZvDYV4y8A=; b=DeXa59iVQExV06c9JDPnqTJgzRGkzoNHX/O3AfRTztTdYdHMoez6FBaVbjAG8t67AV Lie/y7fJLFkdI2jSAEUSPIPACjli6wh28KL3YCKoJNeo6rn9euzcMMfd/vY2yN1/JdxP eiuynuBuK9DuSE5veQuWFLrXUTy9uiHh5kRaFECu/u5LGM6RP5GYw2R4Dua3nPZChx1B Us0P4KieyVUDj0xCQBv/tUfGnCjhb/yvgw8/IBoKyE+CHmfo+54pbQeVQss1em4RwUhZ Fd+KH3CZiiiMEJMyfWIpdTv9Ni403ZoXtdysxScxo1r4NFn9EPGwWS1W5FqyVpUL7RUH qM9g== X-Gm-Message-State: APt69E0Z0oBdD/dw7yt30eyMn2vgEGSW6Mdf2sUoGvkCGtiQK+ytkqQI 0NFngsn3vl6Q+qHJQU2yGYeqmg== X-Google-Smtp-Source: AAOMgpfWX4Xy3X3kxWTvngvrZiYdv4xFwQeaSVEyDF4TC/+vOFofDrPnwcC/IqCgktd6LBcHvU13BA== X-Received: by 2002:a81:99c9:: with SMTP id q192-v6mr8291649ywg.88.1531267233904; Tue, 10 Jul 2018 17:00:33 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a81:b3c9:: with SMTP id r192-v6ls61887ywh.35.gmail; Tue, 10 Jul 2018 17:00:33 -0700 (PDT) X-Received: by 2002:a81:78c6:: with SMTP id t189-v6mr3565453ywc.7.1531267232808; Tue, 10 Jul 2018 17:00:32 -0700 (PDT) X-Original-Sender: BastienPenava@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:39066 Archived-At: ------=_Part_3587_1773918286.1531267232166 Content-Type: multipart/alternative; boundary="----=_Part_3588_1446964829.1531267232168" ------=_Part_3588_1446964829.1531267232168 Content-Type: text/plain; charset="UTF-8" Hi, Deduction guides are great but could be even better if we were able to better control the conversion and why not even produce custom conversions. For instance if you wanted this work: std::list y { ... }; std::vector x { y }; you could just write the following to make it work: template std::vector(Container&& t) { return std::vector{t.begin(), t.end()}; } //or "= expr".The block would be as restrictive as c++11's constexpr functions And if those expression where preparsed and injected instead of working like normal function returns: //Implementing lazy on the library side template struct Lazy : public T { Lazy(T&& f) : T(std::forward(f)) {} using T::operator(); }; template Lazy(T&& x) { return Lazy{[&]{ return x; }}; } //no recursion, uses given constructor as deduction guide template void func(Lazy x) { x(); x(); } int f() { std::cout << "hello" << std::endl; return 0; } int main() { func(f()); //func(Lazy{[&]{return f(); }}); } //Implementing overload set as arguments by allowing unresolved types to be passed to OverloadSet before error template struct OverloadSet : public T { Lazy(T&& f) : T(std::forward(f)) {} using T::operator(); }; template OverloadSet(T&& x) { return OverloadSet{ [](Args&&... args) noexcept(noexcept(x(std::forward(args))) -> decltype(x(std::forward(args)) { return x(std::forward(args)); } }; } int f() { return 0; } int f(int) { return 0; } template void func(T&& x) { x(0); x(); } int main() { func(f); /* func(OverloadSet{[](Args&&... args) noexcept(noexcept(x(std::forward(args))) -> decltype(x(std::forward(args)) { return x(std::forward(args)); }); */ } This would allow to have a real C++ alternative to long expression, type conversions macros. Just a thought, what do you think? Anyway thanks for the read. -- 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/c0bed6aa-9fd9-42fe-9ad6-ab17b0c1d590%40isocpp.org. ------=_Part_3588_1446964829.1531267232168 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi,

Deduction guides are gre= at but could be even better if we were able to better control the conversio= n and why not even produce custom conversions.
For instance if yo= u wanted this work:

std::list<int> y { ... };
std::vector<int> x { y };

you could just write the = following to make it work:

template<Container T>
std::vector(Container&& t) { return std::vector{<= /span>t.begin(), =C2=A0t.end()}; } //or "=3D e= xpr".The block would be as restrictive as c++11's constexpr functi= ons

And if those expression wh= ere preparsed and injected instead of working like normal function returns:=

//Implementing lazy on the library side
template<class T>
s= truct Lazy : public T
{
=C2=A0 =C2=A0 Lazy(T&= ;& f) : T(std::forward<T>(= f)) {} =C2=A0 =C2=A0
=C2=A0 =C2=A0 using T::operator();
};

template&= lt;class T>
Lazy(T&& x) { r= eturn Lazy{[&]{ return x; }}; } = //no recur= sion, uses given constructor as deduction guide

template<class T>
void f= unc(Lazy<T> x)
{=
=C2=A0 =C2=A0 =C2=A0x();
=C2=A0 =C2=A0 =C2=A0x();
}

= int f()
{
=C2=A0 =C2=A0std::cout << "hello" << std::endl;
= =C2=A0 =C2=A0return = 0;
}

int main()
{
= =C2=A0 =C2=A0func(f<= /span>());= //func(Lazy{[&]{return= f(); }});=
}

//Implementing overload set as arguments by allowin= g unresolved types to be passed to OverloadSet before error
template<class T>
struct O= verloadSet= :<= span style=3D"color: #000;" class=3D"styled-by-prettify"> public T
{
=C2=A0Lazy(T&& f)= : T(std::forward<T&g= t;(f)) {}
=C2=A0using T::operator();
};
template<= /span><= class T>
OverloadSet(T&& x)
{
=C2=A0return<= span style=3D"color: #000;" class=3D"styled-by-prettify"> OverloadSet{
=C2=A0 []<class... Args>(Args&= amp;&... args)
=C2=A0 = =C2=A0 =C2=A0noexcept(noexcept= (x<= span style=3D"color: #660;" class=3D"styled-by-prettify">(std::forward<Args...>(args)))
=C2=A0 =C2=A0 =C2=A0-> decltype(x= (std::forward<Args...>(args))
=C2=A0 =C2=A0{
=C2=A0 =C2=A0 =C2=A0return x(std::forward<Args...>(args));
=C2=A0 =C2=A0}
=C2=A0};
}

i= nt f() { return 0; }
int f(int) { return 0; }

template&= lt;class T>
void func(T&& x){=
=C2=A0x(0);
=C2=A0x();
}

int main()=
{
=C2=A0func(f); /* func(OverloadSet{[]<class... Args>(Args&&= amp;... args)
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 no= except(noexcept(x(std::forward<Args...>(args)))
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 -> decltype(x(std::forward<Arg= s...>(args))
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0{
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return x(std::forward&= lt;Args...>(args));
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0}= ); */
<= /span>}

This would allow to have a real C++ alt= ernative to long expression, type conversions macros.
Just a thou= ght, what do you think?
Anyway thanks for the read.

--
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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/c0bed6aa-9fd9-42fe-9ad6-ab17b0c1d590= %40isocpp.org.
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