220 36210 <5088a543-efd6-4944-8a4c-0981cc33db55@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Mingxin Wang Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Make aliased templates first class citizens Date: Thu, 21 Dec 2017 06:14:15 -0800 (PST) Lines: 112 Approved: news@gmane.org Message-ID: <5088a543-efd6-4944-8a4c-0981cc33db55@isocpp.org> References: <8e4dd67b-e929-4f83-8f68-4a7ebd20a4e3@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_5292_382028347.1513865655504" X-Trace: blaine.gmane.org 1513865540 16472 195.159.176.226 (21 Dec 2017 14:12:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 21 Dec 2017 14:12:20 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDNMBNHJWIGBBOED57IQKGQEZTZNVFQ@isocpp.org Thu Dec 21 15:12:16 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eS1a3-0003vK-F3 for gclcip-std-proposals@m.gmane.org; Thu, 21 Dec 2017 15:12:15 +0100 Original-Received: by mail-ua0-f197.google.com with SMTP id i45sf15489396uaa.9 for ; Thu, 21 Dec 2017 06:14:18 -0800 (PST) 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=AarxQ+HmfC530ZUAOf+F0igyPIccIQuHZKXXbI078gk=; b=cc6olbzLsDlZnZdspWtAgHCGcwuH0kHGV9EQ2rTeAMSgTsQZSmCC0zS1ZWWefdO8Fd J0bV6SXuyFu4zHPBs+9LtR+344yghLEKyQDxbqIV1Ys3VGCp5Gl1L8WvyLOMT1aFeTUM S2YjuTf+9r/NAjGBX6dpWwmKK0NRPxECxNmlXGNuRuArq5n4CWWIfjxSAG4Irh7LOYP6 dQyUEfyryDG/T2QLcj3coIxUuQmdbOxfQCOwjX23Q7LKROfXh6puxRW5/Rit5b/pclbZ MDtGaRFFwW4qRlvkDn0G1m0bViLrZQwXeK/UlBbFY05k0+Huoi3yw5EaUVhBCWSFBrrh O/DQ== 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=AarxQ+HmfC530ZUAOf+F0igyPIccIQuHZKXXbI078gk=; b=pKlMtHBhMPXobbrqPjmtL4lGdZF2VvG+oG2Y5TMNX/kMEiDjyZuFTEvtGEqirqv85u G02tyoBvhQF9BMO8mu6N7Tgs49JRfMfjQ6GaE1uaffOSi6UW6Ug3H0JShcpnTpAGU2dD OGi6xxR1roO2Axnc16D2LcDNrxRJDhLIKxOGTkc/wloMLoxqr/DTSmb61zHJY+MLrOw3 9NGWjFFmvilizvd0kNAsBuGsNdX5fCSWZL1ITiBKOuKCgLSqfUsp4SO3iAVPoprhko7l lPbv7A4f3Jtdi59lj/5/Fsu2fngYW538YrVvmzRgXkFqNMtCxbcC3A7Cur9SHWSPyUZL tzfg== X-Gm-Message-State: AKGB3mL8ncK4k9+2VM4hi/bOC1hpeE0cDqkcsPpslIcJazSRPzy5/kQe k9/XfwrjkfUz3U+Yk1OStf8gyw== X-Google-Smtp-Source: ACJfBou0BCsiolAPrHmSm19DHdv6nT1ERo69+oQviR+5qA/JpDY7mK/xOe9BfumNq7AhT1VMlkCUow== X-Received: by 10.31.238.130 with SMTP id m124mr4944173vkh.80.1513865657852; Thu, 21 Dec 2017 06:14:17 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.174.76 with SMTP id x73ls90544vke.21.gmail; Thu, 21 Dec 2017 06:14:16 -0800 (PST) X-Received: by 10.31.192.135 with SMTP id q129mr998740vkf.0.1513865655995; Thu, 21 Dec 2017 06:14:15 -0800 (PST) In-Reply-To: <8e4dd67b-e929-4f83-8f68-4a7ebd20a4e3@isocpp.org> X-Original-Sender: wmx16835vv@163.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:36210 Archived-At: ------=_Part_5292_382028347.1513865655504 Content-Type: multipart/alternative; boundary="----=_Part_5293_588116073.1513865655504" ------=_Part_5293_588116073.1513865655504 Content-Type: text/plain; charset="UTF-8" As far as I am concerned, this feature could be mainly used on specifying template arguments in separated semantic levels. On the one hand, I think the motivation of this idea is probably not sufficient enough for a *core language feature*. After all, this is a pattern in compile-time programming and could be easily implemented with struct templates (as illustrated in the initial post), and I did not see the peculiarities of this pattern so that it need support at the language level. On the other hand, as other feedback reports, this feature introduces a new language-level concept - template of template, which is at the same level as the concepts of "types" and "type templates". If this concept is standardized, a number of clauses may be reappraised. For example, should "template of template" be a valid template argument type? Is it allowed to declare a type or function template with templates in multiple layer? Actually, I have handled some cases that required specifying a type template in multiple separated semantic levels, and I found it acceptable to design a helper struct (or class) template as a "*configurator*", e.g.: template class Aaa; // requires concept Bbb template struct AaaConfigurator { template using type = Aaa; }; template