220 41266 <4fe7ab83-be8f-4f62-a1a4-72b39fb540d3@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Alberto Barbati Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: constexpr also in switch() and even in for(). Date: Fri, 14 Dec 2018 01:20:28 -0800 (PST) Lines: 82 Approved: news@gmane.org Message-ID: <4fe7ab83-be8f-4f62-a1a4-72b39fb540d3@isocpp.org> References: <18750262-f0ab-4330-ab08-29004735c6bb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_684_675424228.1544779228798" X-Trace: blaine.gmane.org 1544779106 22725 195.159.176.226 (14 Dec 2018 09:18:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 14 Dec 2018 09:18:26 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCPY5DV6RIMBBX7LZXQAKGQEJ5Y3A7Q@isocpp.org Fri Dec 14 10:18:22 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw1-f71.google.com ([209.85.161.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1gXjbx-0005o1-UC for gclcip-std-proposals@m.gmane.org; Fri, 14 Dec 2018 10:18:22 +0100 Original-Received: by mail-yw1-f71.google.com with SMTP id l131sf2837670ywc.21 for ; Fri, 14 Dec 2018 01:20:32 -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=1JwY/h2t2owNnz5OGjiOdn5DEg1cD0xZ+eWRhCrqVOE=; b=fSD5JMGNVp9YS8kFM209r/EILa7s8MiyW0c4sDm+BEMdMQL1NWQ5f5XfuyGO5ghEg8 T/iStIJG2qQ1CohbwyKjwRhN/hU8OX89sPBlo9YwDAWq9WaGzrTPif7Rm6UmdjvolvnQ luHzLhcr9lBS2i6UHlcAQQc8EqEPKu3QfBaEKdsaSg1+yVtrQOe5uIc8kiQQ6HvPzgFi nFd+TZyTzUAgbApRvZOPZJb0AJmkp02ndyAPQ6ndOs01WHiELOo/FnI/RVMsLhCpdkP/ Boe6Kk9Qo0O5LPhdSN0JHfULwmirQfjlejG1IDSPYs44L08IwSDfKIfXlGsMuITeqG/B qrpQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; 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=1JwY/h2t2owNnz5OGjiOdn5DEg1cD0xZ+eWRhCrqVOE=; b=I4GLSAKnyuXuV0Jba91xiv6bQopIxqLVZb8gcu82v8O3BcODPI+l1m7tc0f4UZixGt 7tAGIbxiXGtrvhZA7M/nGUdwPZqHJjCj8BqAZT6HxsKP1V5kvDh0qFZpEYjh6XxHeD51 FNt1SObiENPKBQNtEApjb+xqYNgoWYDX5X6SzMlH+5848gAuYWh+k1PVpTgrrdrMyImK KRJO5hbBKEYTkbiX7HYBcIbq8DvnYyDClE3n4Wnsmt1tFjPABfxkgyM2SXQco8HTMtCR Q+vRUFRib594LXGY9aNsaysYVcyVVnJKhdUndx2o/1G3FcBHBvEVc22nuUUy7QdG7oKg ITEA== 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=1JwY/h2t2owNnz5OGjiOdn5DEg1cD0xZ+eWRhCrqVOE=; b=TbkXS8BvnVmubedtz+OqN0oqQ46d4VjGRcEAk3O+6bGSdoj8IVEOWVV56G+50Yw5Yt CsIw2Tbrhrim4SfelB2BnuEl76NfYW15pKUrVpZDaSW6BMWj1+Uv/lp86A0fIYwriUd/ hU5pT9oYyHD+mpNi8XlLk/WsIMRAaDDTnYCL8geT+pWappbqo3+Nfr/HbD9oTXnYgobl WLtY0uQYuFERRjNAO3z6u9RuFfh5bdIqizIyp+/sTuITTQXhP4Y4HuigbnG8DHJWPvFK 1tL+qFdMFE6sWzyQE2D5oLfalFT9WS/uXbIzZXFpRuQwn9hWIc2+4Xs0nYG2fnnBOoG4 bt8Q== X-Gm-Message-State: AA+aEWZR9fJLCqlHWTRj9t+Z5EBZSeT7FsLR/lhwDuEZR+QU841T+JAF 9yYfnrErcTZ42y8ge+dBP0ws0Q== X-Google-Smtp-Source: AFSGD/VmGSupF/edtoK9hrDrkZcDGMdmpJsTDijd/ts8+MJBBwPATgOFKGyPl2lWV9pyxIM7zldaGw== X-Received: by 2002:a25:9d0f:: with SMTP id i15-v6mr1100188ybp.23.1544779232223; Fri, 14 Dec 2018 01:20:32 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a81:7589:: with SMTP id q131ls678685ywc.1.gmail; Fri, 14 Dec 2018 01:20:30 -0800 (PST) X-Received: by 2002:a81:450:: with SMTP id 77mr19204ywe.6.1544779229599; Fri, 14 Dec 2018 01:20:29 -0800 (PST) In-Reply-To: <18750262-f0ab-4330-ab08-29004735c6bb@isocpp.org> X-Original-Sender: AlbertoBarbati@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:41266 Archived-At: ------=_Part_684_675424228.1544779228798 Content-Type: multipart/alternative; boundary="----=_Part_685_365109050.1544779228798" ------=_Part_685_365109050.1544779228798 Content-Type: text/plain; charset="UTF-8" Hi Frank, I believe you are missing the point of "if constexpr". The real usefulness of "if constexpr" is that it makes the branch that is not selected a "discarded statement" so that it doesn't trigger compilation errors if it is ill-formed after template parameters substitution. In the case of the for statement, that usefulness simply isn't there, because there's only one statement to consider, so either it is ill-formed for all iterations or none of them. The idea of having the iteration variable constexpr is intriguing, but I'm not totally sure that it has a real usefulness, especially after the introduction of consteval functions. The switch statement is a bit more complicated, as its structure is too "flexible", with jumps at labels that may not respect scopes. Because of that a switch is *not* equivalent to a cascade of if-statements at least in the general case. Anyway, your proposal indicates a possibile solution but doesn't describe a real problem that the proposal is trying to solve. I am afraid but "It would be nice to have this..." is not a satisfactory description of real problem. Alberto -- 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/4fe7ab83-be8f-4f62-a1a4-72b39fb540d3%40isocpp.org. ------=_Part_685_365109050.1544779228798 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi Frank,

I believe you are = missing the point of "if constexpr". The real usefulness of "= ;if constexpr" is that it makes the branch that is not selected a &quo= t;discarded statement" so that it doesn't trigger compilation erro= rs if it is ill-formed after template parameters substitution. In the case = of the for statement, that usefulness simply isn't there, because there= 's only one statement to consider, so either it is ill-formed for all i= terations or none of them. The idea of having the iteration variable conste= xpr is intriguing, but I'm not totally sure that it has a real usefulne= ss, especially after the introduction of consteval functions. The switch st= atement is a bit more complicated, as its structure is too "flexible&q= uot;, with jumps at labels that may not respect scopes. Because of that a s= witch is *not* equivalent to a cascade of if-statements at least in the gen= eral case.

Anyway, your proposal indicates a possi= bile solution but doesn't describe a real problem that the proposal is = trying to solve. I am afraid but "It would be nice to have this..." is not a satisfactory descr= iption of real problem.

Alberto


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