220 36011 article Path: news.gmane.org!.POSTED!not-for-mail From: David Brown Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto return proposal Date: Tue, 19 Dec 2017 14:27:58 +0100 Lines: 58 Approved: news@gmane.org Message-ID: References: <7a3b438a-ec7f-4827-a24c-5466fe57410c@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 1513689974 10672 195.159.176.226 (19 Dec 2017 13:26:14 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 19 Dec 2017 13:26:14 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDP6R2XHXIARB25H4TIQKGQENOYJS2Y@isocpp.org Tue Dec 19 14:26:10 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f70.google.com ([209.85.215.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eRHuM-0002Pb-7c for gclcip-std-proposals@m.gmane.org; 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Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1eRHuI-00026P-4j for std-proposals@isocpp.org; Tue, 19 Dec 2017 14:26:06 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 51 Original-X-Complaints-To: usenet@blaine.gmane.org In-Reply-To: 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-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:36011 Archived-At: On 19/12/17 13:54, Jan Wilmans wrote: > Semi-serious proposal: allow inference of auto variable types from > any use not just initialization: > > void foo(int& out); > > double bar() { > auto i; // i is int > auto j; //j is string > auto r; // r is double > foo(i); > j = "hello"s; > ... > return r; > } > > Semi-serious because full Hindley-Milner (or variants) inference is > probably not workable in C++, but some restricted variant still might. > > > Please provide a rationale why such a terrible looking idea would be > good? I think this would promote uninitialised variables, so effectively > drain auto of one of its best qualities. > > I would say it could be useful in cases like this: auto number(int x) { auto s; if (x == 0) { s = "zero"s; } else if (x == 1) { s = "one"s; } else { s = "not a nice number"s; } return s; } i.e., you don't have a suitable initial value to use. But you can eliminated cases where you have a simple constructor that the compiler can optimise away, cases that are easily expressed with conditional operator, cases where you could return the result directly without using a local variable, and cases where it's not particularly hard to give the type directly. I don't think the remaining use-cases are enough to warrant such a feature in the language. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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