220 41412 article Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: David Brown Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Overloading `using` for temporary scopes Date: Tue, 29 Jan 2019 15:26:29 +0100 Approved: news@gmane.org Message-ID: References: <41a2bed6-6738-cd11-0037-9a471f8a75d5@gmail.com> Reply-To: std-proposals@isocpp.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="57655"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDP6R2XHXIARBHOFYHRAKGQES5CRYRQ@isocpp.org Tue Jan 29 15:26:39 2019 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lj1-f198.google.com ([209.85.208.198]) by blaine.gmane.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.89) (envelope-from ) id 1goULX-000EoF-3T for gclcip-std-proposals@m.gmane.org; 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Original-Received: from list by blaine.gmane.org with local (Exim 4.89) (envelope-from ) id 1goULS-000EkF-IW for std-proposals@isocpp.org; Tue, 29 Jan 2019 15:26:34 +0100 X-Injected-Via-Gmane: http://gmane.org/ In-Reply-To: <41a2bed6-6738-cd11-0037-9a471f8a75d5@gmail.com> Content-Language: en-GB X-Original-Sender: david@westcontrol.com X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) smtp.mailfrom=gclcip-std-proposals@m.gmane.org 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:41412 Archived-At: On 28/01/2019 17:20, Matthew Woehlke wrote: > I often wish C++ had the equivalent of Python's `with`: > > with (auto x = expr()) > { > // use 'x' here > } > // 'x' is out of scope > > With C++17 I can more or less achieve this: > > if (auto x = expr(); true) > { > // use 'x' here > } > // 'x' is out of scope > > ...and even: > > #define with(...) if (__VA_AGS__; true) > > ...but I still have to declare and name the variable, even if I don't > actually care (e.g. lock guards). While we will hopefully, eventually > solve the 'anonymous variable' problem, I wonder if there would be any > perceived value in having a dedicated syntax for this? I'm thinking either: > > with (auto x = expr()) > with (expr()) > > ...where in the latter case, the result of `expr()` is "live" until the > end of the `with` scope, e.g. like `if (auto&& ? = expr(); true)`. Or even: > > using (auto x = expr()) > using (expr()) > > The benefit of the latter, of course, is that it does not need a new > keyword. The benefit of *either* is that, as a language feature, it > could be made to work with either an expression or a declaration, which, > AFAICT, is not possible with any macro-based implementation. > > Does anyone think this would be useful? Or is it not worthwhile given > the work-arounds that exist? > Basically, what you want is to be able to give "expr()" a name so that it stays in scope, but you don't want to give it a name because it isn't something you are going to use. I don't know of any neat way to do this. #define anon anon##__counter__ { auto anon = expr(); auto anon = expr2(); T1 anon; // These are all still in scope } -- 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/q2pnql%241mvo%241%40blaine.gmane.org. .