C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Q: Why has vector<vector<int>> been *kept* illegal?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Samee Zahur Guest Posted: Fri May 13, 2005 6:04 am    Post subject: Q: Why has vector<vector<int>> been *kept* illegal? We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! I know it is sometimes good coding practice to keep the space, but shouldn't coding style be there for the programmer to decide? What is the rationale behind this language design? Samee --- [ 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 Llewelly Guest Posted: Fri May 13, 2005 1:08 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? "Samee Zahur" <samee.zahur (AT) gmail (DOT) com> writes: Quote: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! [snip] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1757.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 David Abrahams Guest Posted: Fri May 13, 2005 1:42 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? "Samee Zahur" <samee.zahur (AT) gmail (DOT) com> writes: Quote: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! I know it is sometimes good coding practice to keep the space, but shouldn't coding style be there for the programmer to decide? What is the rationale behind this language design? No particular rationale; it's being fixed for the next version of the C++ standard. -- Dave Abrahams Boost Consulting www.boost-consulting.com --- [ 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 Pete Becker Guest Posted: Fri May 13, 2005 2:24 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? David Abrahams wrote: Quote: No particular rationale; it's being fixed for the next version of the C++ standard. One very good rationale: it's a fundamental change to the maximum munch rule that C and C++ lexing is explicitly based on. Which is why making this change requires careful analysis, as in the paper cited in another reply. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) --- [ 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 May 14, 2005 2:44 am    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? Llewelly wrote: Quote: "Samee Zahur" <samee.zahur (AT) gmail (DOT) com> writes: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! [snip] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1757.html That breaks existing code, silently. The committee usually rejects such suggestions. John Nagle --- [ 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 Jerry Coffin Guest Posted: Sat May 14, 2005 3:45 am    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? Samee Zahur wrote: Quote: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! I know it is sometimes good coding practice to keep the space, but shouldn't coding style be there for the programmer to decide? What is the rationale behind this language design? I don't believe there's a way of changing this without breaking or changing the meaning of at least some existing, legitimate code. This seems to be widely enough seen as a problem that it'll probably change, but when it does it will probably break or change the meaning of something else. It does seem likely that the "Something else" in question is obscure, while this is common, so the tradeoff may be reasonable, but it should at least be clear to those involved that it really is a tradeoff, not a simple situation of fixing something that has no side-effects. -- Later, Jerry. The universe is a figment of its own imagination. --- [ 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 Samee Zahur Guest Posted: Sun May 15, 2005 4:30 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? Thanks everyone for the replies and references. I read through n1757, but how far has it gone into actually being accepted? I feel the proposal there is fine, since I really doubt if there really is any existing code out in the real world that actually will break down. (except for tutorial codes meaning to show a possible source of programmer's confusion) For everyone's scrutiny, let me give another alternative here which will not break any existing code - parse it as it does now, and try to accept >> as a double angle bracket only if treating it like a right-shift op raises a syntax error.I believe that this approach will make it easier for compiler vendors to implement this (since all existing rules stay as-is), although I will admit that I've never been involved in any compiler writing before. Downsides I can think of: - humans may find it harder to figure out the real meaning of a '>>' token in certain contexts. - in case of multiple instances of '>>' in the same line, there can be more than one valid interpretation. In these particularly rare cases, the statement can be flagged an error due to ambiguity forcing the programmer to resort back to inserting a space wherever needed. Or, there can be priority rules which decide which one gets interpreted as what (but this can in turn lead to silent bugs since the compiler reads something the programmer didn't mean - not a good approach). Any opinions on this? Samee --- [ 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 Ron House Guest Posted: Mon May 16, 2005 4:20 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? Samee Zahur wrote: Quote: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! I know it is sometimes good coding practice to keep the space, but shouldn't coding style be there for the programmer to decide? What is the rationale behind this language design? I can think of one reason: it complicates the parser no end. A recursive algorithm looking for a type is primed to find vector<something>, and that something is recursively any type, and so might be vector<int>. But to detect that >> ends two different recursive levels involves a big redesign of such a parser - and for no real benefit; bung in the space and it's fixed. -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 P.J. Plauger Guest Posted: Mon May 16, 2005 4:21 pm    Post subject: Re: Why has vector<vector<int>> been *kept* illegal? "Samee Zahur" <samee.zahur (AT) gmail (DOT) com> wrote Quote: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! I know it is sometimes good coding practice to keep the space, but shouldn't coding style be there for the programmer to decide? What is the rationale behind this language design? The rationale stems from a basic principle of C++ (and C) parsing known as the "maximum munch" rule. It says that the longest sequence of characters that can make a valid token is always the next token. Thus ">>" always wins out over ">" ">". Having said that, I must report that the C++ committee seems to have solved this annoying problem last month in Lillehammer. The next revision of C++ will indeed accept "vector<vector" with the meaning we all want. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com --- [ 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: Mon May 16, 2005 4:22 pm    Post subject: Re: Q: Why has vector<vector<int>> been *kept* illegal? John Nagle wrote: Quote: Llewelly wrote: "Samee Zahur" <samee.zahur (AT) gmail (DOT) com> writes: We know that > can be a 'greater-than' operator, or it can be an ending-delimiter for template parameters. So why has the >> operator not been defined as being capable of ending two template paramenter lists in a row? That would make things like vector<vector legal without any extra space! [snip] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1757.html That breaks existing code, silently. The committee usually rejects such suggestions. John Nagle True, however this particular case seems to have significant support because of the perceived cost of sticking with the status quo, and the apparent scarcity of code which would be broken by the change. -- 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 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.