220 37316 article Path: news.gmane.org!.POSTED!not-for-mail From: Miguel Ojeda Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Pointer overload for || Date: Mon, 12 Mar 2018 12:18:27 +0100 Lines: 55 Approved: news@gmane.org Message-ID: References: <1fea07df-46a6-4df2-865a-849be8f8d129@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1520853413 28108 195.159.176.226 (12 Mar 2018 11:16:53 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 12 Mar 2018 11:16:53 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRZHGH43YJRBHWETHKQKGQESXMOUNA@isocpp.org Mon Mar 12 12:16:49 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f198.google.com ([209.85.220.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1evLRg-0007B2-EZ for gclcip-std-proposals@m.gmane.org; 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[209.85.220.65]) by mx.google.com with SMTPS id q20sor1541211qtq.145.2018.03.12.04.18.53 for (Google Transport Security); Mon, 12 Mar 2018 04:18:53 -0700 (PDT) Received-SPF: pass (google.com: domain of miguel.ojeda.sandonis@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; X-Received: by 10.237.36.167 with SMTP id t36mr2001794qtc.136.1520853527634; Mon, 12 Mar 2018 04:18:47 -0700 (PDT) Original-Received: by 10.200.52.247 with HTTP; Mon, 12 Mar 2018 04:18:27 -0700 (PDT) In-Reply-To: X-Original-Sender: miguel.ojeda.sandonis@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=AaWs7mvk; spf=pass (google.com: domain of miguel.ojeda.sandonis@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=miguel.ojeda.sandonis@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE 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:37316 Archived-At: On Mon, Mar 12, 2018 at 1:28 AM, wrote: > > > On Sunday, March 11, 2018 at 7:06:59 PM UTC+1, Miguel Ojeda wrote: >> >> Your first version is the best one, to be honest :-) In any case, if >> you really need something like you propose (a pseudo-DSL, generated >> code, ...?), then a macro with a descriptive name would be the >> simplest and most readable, e.g. for your second version: >> >> #define OR_ELSE(v, e) ((v) ? (v) : (e)) >> >> // ... >> x = OR_ELSE(x, new int); >> >> For your third version: >> >> #define ENSURE_INIT(v, e) do { if (not (v)) (v) = (e); } while(false) >> >> // ... >> ENSURE_INIT(x, new int); >> > > This solution is not perfect because it reuse `v` multiple times, if value > is not simply value it could be problematic. > > Better would be using lambda similar like in your next point: > > > #define OR_ELSE(v, e) ([&](auto MACRO_OR_ELSE_VALUE_X){ return > (MACRO_OR_ELSE_VALUE_X ? MACRO_OR_ELSE_VALUE_X : (e)); }(v)) > > > > This will allow evaluate `v` only once and can be inserted in other > expressions: > > auto z = h() ? OR_ELSE(OR_ELSE(x, foo()), bar()) : zz(); > Indeed! The usual problems (and workarounds) with macros apply -- which is why it would have been great if C++ would have had something similar to the "expression E" thingy I put above. :-) Thanks for pointing it out! Cheers, Miguel -- 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/CANiq72kDQVx6yaQQSBQCGvtZGtuYeVsSHNHFSanGXnDd82wa4A%40mail.gmail.com. .