C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  The issue of const maps and operator[]          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Ney André de Mello Zunino Guest Posted: Thu Dec 02, 2004 5:38 am    Post subject: The issue of const maps and operator[] Hello. Searching around before posting this message revealed that the issue has been brought up a significant number of times (here are two related threads [1] [2]). So /std::map<>/ does not provide a const version of its /operator[]/ member function because that function is supposed to modify the map in case the passed key is not present. The question which follows is: why not provide a const overload which would throw when the given key would not exist? It seems like a natural and easy-to-implement solution which would contribute to map's associative array role. In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. Could anyone provide some information (comments, links, etc.) on this? [1] http://groups.google.com/groups?hl=en&lr=&th=47b92be5c9ed558c&rnum=1 [2] http://groups.google.com/groups?hl=en&lr=&th=698e93dddf8b22d0&rnum=5 Thank you very much, -- Ney André de Mello Zunino --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top James Dennett Guest Posted: Thu Dec 02, 2004 6:51 am    Post subject: Re: The issue of const maps and operator[] Ney André de Mello Zunino wrote: Quote: Hello. Searching around before posting this message revealed that the issue has been brought up a significant number of times (here are two related threads [1] [2]). So /std::map<>/ does not provide a const version of its /operator[]/ member function because that function is supposed to modify the map in case the passed key is not present. The question which follows is: why not provide a const overload which would throw when the given key would not exist? Because it would violate the important principle that adding const to code should not change its behaviour (except for propogation of constness to other code), although it may cause it to no longer compile. The language doesn't enforce that in isolation; good library design is also necessary. Quote: It seems like a natural and easy-to-implement solution which would contribute to map's associative array role. But it's too fragile, for the reason above. Quote: In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. I find it enormously unlikely that this would be changed in the next revision of C++. -- James --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Thorsten Ottosen Guest Posted: Fri Dec 03, 2004 2:54 am    Post subject: Re: The issue of const maps and operator[] "James Dennett" <jdennett (AT) acm (DOT) org> wrote Quote: In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. I find it enormously unlikely that this would be changed in the next revision of C++. yes true. but we are already looking into adding T& map<K,T>::at( const Key& ); const T& map<K,T>::at( const Key& ) const; which should throw if nothing is found. Alternatively, I guess we could use operator()(). -Thorsten --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top James Dennett Guest Posted: Fri Dec 03, 2004 4:19 pm    Post subject: Re: The issue of const maps and operator[] Thorsten Ottosen wrote: Quote: "James Dennett" <jdennett (AT) acm (DOT) org> wrote in message news:YWxrd.191852$hj.44035 (AT) fed1read07 (DOT) .. In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. |I find it enormously unlikely that this would be changed in the |next revision of C++. yes true. but we are already looking into adding T& map<K,T>::at( const Key& ); const T& map<K,T>::at( const Key& ) const; This is reasonable and somewhat consistent with the at member in other standard library class templates. Quote: which should throw if nothing is found. Alternatively, I guess we could use operator()(). I wouldn't like that: mymap[1] = 3; and mymap(1) = 3; would then both be valid, with different semantics in the case mymap.find(1) == mymap.end(); and identical semantics otherwise, but they look too similar. Let's be consistent, and use "at" when we mean range-checked access which throws if an element is absent. -- James --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Tom Widmer Guest Posted: Sat Dec 04, 2004 2:26 am    Post subject: Re: The issue of const maps and operator[] On Fri, 3 Dec 2004 02:54:24 GMT, [email]nesotto (AT) cs (DOT) auc.dk[/email] ("Thorsten Ottosen") wrote: Quote: "James Dennett" <jdennett (AT) acm (DOT) org> wrote in message news:YWxrd.191852$hj.44035 (AT) fed1read07 (DOT) .. In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. |I find it enormously unlikely that this would be changed in the |next revision of C++. yes true. but we are already looking into adding T& map<K,T>::at( const Key& ); const T& map<K,T>::at( const Key& ) const; which should throw if nothing is found. I like the symmetry with vector::at. Quote: Alternatively, I guess we could use operator()(). I don't like that, since operator() hasn't been used like that elsewhere and is usually used for functors. Tom --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top John Nagle Guest Posted: Sat Dec 04, 2004 5:26 am    Post subject: Re: The issue of const maps and operator[] If anything is done to "operator[]", it should be given the same syntax as "operator()", so we can have classes that support multiple subscripts. Inconsistently, foo(a,b) is a two-argument call, but foo[a,b] is an invocation of the comma operator followed by a one-argument call. One of the more obscure features of the language. John Nagle Animats Tom Widmer wrote: Quote: On Fri, 3 Dec 2004 02:54:24 GMT, [email]nesotto (AT) cs (DOT) auc.dk[/email] ("Thorsten Ottosen") wrote: "James Dennett" <jdennett (AT) acm (DOT) org> wrote in message news:YWxrd.191852$hj.44035 (AT) fed1read07 (DOT) .. In spite of having found several discussions on this topic, I could not find mention of whether this issue is going to be addressed when C++ gets its next major revision. |I find it enormously unlikely that this would be changed in the |next revision of C++. yes true. but we are already looking into adding T& map<K,T>::at( const Key& ); const T& map<K,T>::at( const Key& ) const; which should throw if nothing is found. I like the symmetry with vector::at. Alternatively, I guess we could use operator()(). I don't like that, since operator() hasn't been used like that elsewhere and is usually used for functors. Tom --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language, library and standards All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.