From 8039313446397529066 X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news3.google.com!news2.google.com!news.glorb.com!news.alt.net!frodo.cs.rpi.edu!not-for-mail From: Scott Meyers Newsgroups: comp.std.c++ Subject: Immutability of set elements Date: Sat, 19 Sep 2009 19:08:24 CST Organization: unknown Lines: 45 Sender: cppmods@cs.rpi.edu Approved: james.dennett@gmail.com Message-ID: <7.0.0.16.2.20090919120102.05e1e878@aristeia.com> NNTP-Posting-Host: netlab.cs.rpi.edu Content-Type: text/plain; charset="us-ascii" To: (Usenet) Return-Path: X-Original-Date: Sat, 19 Sep 2009 12:18:06 -0700 X-Submission-Address: std-c++@netlab.cs.rpi.edu Xref: g2news1.google.com comp.std.c++:1377 The word "immutable" appears exactly once in N2914. It's at the end of [associative.reqmts]/5: Keys in an associative container are immutable. Don't be fooled by the general-sounding nature of this edict: it really applies only to set. Keys in a map are already defined to be const (which presumably implies immutability, but it's hard to say, because "immutable" in this context is never defined), and the hashed containers aren't associative containers, they're unordered associative containers, a completely different thing. Ahem. But I digress. The sentence above comes straight from the resolution of Library Issue 103, which was opened in 1998 and, from what I can tell, resolved by 2001 ( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2001/n1291.html#103 ), yet was not incorporated into TC1. But again I digress. The commentary on Library Issue 103 says: >If users need to modify elements, it is possible to use mutable >members or const_cast. So here's my question: what does it mean for a set key to be immutable? The commentary on issue 103 suggests that modifying such keys via mutable members or a const_cast is okay, but presumably doing so in such a way that would affect the sortedness of the container is not okay. Does modifying a set key lead to undefined behavior? If so, in all cases, or only under some conditions, e.g., when the sortedness of the container is affected? All illumination appreciated. Thanks, Scott -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]