220 9244 article Path: news.gmane.org!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: add "implicit" keyword? 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[80.91.229.3]) by mx.google.com with ESMTPS id t10si8324922lat.27.2014.02.10.08.30.42 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Mon, 10 Feb 2014 08:30:42 -0800 (PST) Received-SPF: pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) client-ip=80.91.229.3; Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1WCtkn-0005yV-Uv for std-proposals@isocpp.org; Mon, 10 Feb 2014 17:30:41 +0100 Original-Received: from tripoint.kitware.com ([66.194.253.20]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 10 Feb 2014 17:30:41 +0100 Original-Received: from mw_triad by tripoint.kitware.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 10 Feb 2014 17:30:41 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 58 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: tripoint.kitware.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 X-Original-Sender: mw_triad@users.sourceforge.net X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) smtp.mail=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:9244 Archived-At: Objective: ---------- Create a new keyword "implicit", having the meaning 'not "explicit"'. That is, this keyword would annotate that a constructor or conversion operator is allowed to be called in a manner that would be denied by 'explicit'. Rationale: ---------- "explicit" is a great tool for avoiding mistakes in the use of a class. Unfortunately, programmers must remember to use it, and there is not presently a mechanism to diagnose failure to do so. Adding an "implicit" keyword would permit compilers to diagnose constructors and/or conversion operators which are in fact implicit according to the standard, but are not marked as such. This would provide a mechanism to warn programmers that a constructor or conversion operator is potentially dangerous, while allowing them to clearly specify when this is the intent. Considerations: --------------- Since keywords are scarce and the introduction of new keywords carries the potential to break existing code, I suggest to make "implicit" a context sensitive keyword in a similar manner as e.g. "override": - If the compiler encounters the "implicit" token within a class definition scope, it shall perform a forward scan of following tokens. If the following token is the class name or equivalent (e.g. if we get 'auto' to declare constructors), or is followed by "operator" and then a type name, "implicit" shall be treated as a keyword with the above meaning. - For all other cases, "implicit" shall be treated as a potential type name or identifier (i.e. historic behavior, not a keyword). Alternatives: ------------- Rather than a keyword, an attribute "[[implicit]]" could be used for this purpose. Projects that prefer the appearance of a keyword can define a macro "implicit" that expands to this attribute for supporting compilers, or empty otherwise. Addendum: --------- The behavior of a constructor or conversion operator that is not specified "explicit" remains "implicit" (i.e. this would not change the meaning of existing code). References: ----------- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60135 ----------------------------------------------------------------------- What do folks think? Is this a reasonable wish? Is there a better way to achieve this goal? -- Matthew -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .