C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Comma within square brackets - not found in Google Code Sear          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message John Nagle Guest Posted: Wed Oct 25, 2006 11:24 pm    Post subject: Comma within square brackets - not found in Google Code Sear One of the old issues that's come up a few times is whether "operator[]" should have the same argument syntax as "operator()" or regular function call syntax. Currently, foo(a,b) is a two-argument function call, while foo[a,b] is an invocation of the comma operator which ignores a and is equivalent to foo[b]. As a result, "operator[]" can only take one argument. This prohibits the creation of multidimensional array classes using standard bracket syntax. The justification for this is protection of legacy code which might conceivably use the comma operator within square brackets. But now we have Google Code Search, and can easily look for such usages. A search key to use is: ^[^\"/]*\[[^\[\]\(\),]*,[^\[\]\(\),]*\] lang:c++ This isn't perfect, but comes as close as we can get with regular expressions. (The main problem is throwing out comments and quotes; there are about 12,000 occurences of commas inside square brackets inside quotes or comments.) This gets us about 100 hits. They're all in multline comments, Microsoft "@deftypefn" statements, char constants, template type arguments or embedded assembler. Try the search yourself. Commas inside subscript expressions just aren't being used in the visible universe of C++ code. At last, we have the tools to dispose of this question. John Nagle Animats --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Ron Natalie Guest Posted: Wed Oct 25, 2006 11:24 pm    Post subject: Re: Comma within square brackets - not found in Google Code John Nagle wrote: Quote: foo[a,b] is an invocation of the comma operator which ignores a and is equivalent to It doesn't ignore it, it evaluates it and then uses the right hand operand to determine the value of the expression. Quote: As a result, "operator[]" can only take one argument. This prohibits the creation of multidimensional array classes using standard bracket syntax. There's no such thing as a multidimensional array in C++. Just arrays of arrays. Quote: The justification for this is protection of legacy code which might conceivably use the comma operator within square brackets. Or how about the fact that C and C++ have never had that syntax. --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Me Guest Posted: Thu Oct 26, 2006 6:24 am    Post subject: Re: Comma within square brackets - not found in Google Code John Nagle wrote: Quote: One of the old issues that's come up a few times is whether "operator[]" should have the same argument syntax as "operator()" or regular function call syntax. As a result, "operator[]" can only take one argument. This prohibits the creation of multidimensional array classes using standard bracket syntax. The justification for this is protection of legacy code which might conceivably use the comma operator within square brackets. But now we have Google Code Search, and can easily look for such usages. A search key to use is: ^[^\"/]*\[[^\[\]\(\),]*,[^\[\]\(\),]*\] lang:c++ This isn't perfect, but comes as close as we can get with regular expressions. (The main problem is throwing out comments and quotes; there are about 12,000 occurences of commas inside square brackets inside quotes or comments.) This gets us about 100 hits. They're all in multline comments, Microsoft "@deftypefn" statements, char constants, template type arguments or embedded assembler. Try the search yourself. Commas inside subscript expressions just aren't being used in the visible universe of C++ code. At last, we have the tools to dispose of this question. There are lambda and expression template libraries that abuse the array index operator because it relies on the fact that it doesn't behave like the function call operator. The immediate example that comes to mind: http://spirit.sourceforge.net/distrib/spirit_1_6_1/libs/spirit/phoenix/doc/statements.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.comeaucomputing.com/csc/faq.html ] Back to top Aleksey Loginov Guest Posted: Thu Oct 26, 2006 3:45 pm    Post subject: Re: Comma within square brackets - not found in Google Code John Nagle wrote: Quote: One of the old issues that's come up a few times is whether "operator[]" should have the same argument syntax as "operator()" or regular function call syntax. Currently, foo(a,b) is a two-argument function call, while foo[a,b] is an invocation of the comma operator which ignores a and is equivalent to foo[b]. Don't forget you can overload comma operator. This can give interesting advantages. Real life example (http://code.google.com/p/night-lambda/): namespace pvt { struct arg {} _1; template<typename L, typename R> typename append<L,R>::type oprerator , ( L &, R & ); template<typename L, typename R> typename append<L const,R>::type oprerator , ( L const &, R & ); template<typename L, typename R> typename append<L,R const>::type oprerator , ( L &, R const & ); template<typename L, typename R> typename append<L const,R const>::type oprerator , ( L const &, R const & ); } struct array { result_type operator [] ( Tuple const & ); } a; Now you can white a [ _1, 10, _1 ] which equivalent to a [ tuple<arg, const int, const arg> (_1, 10, arg () ) ] Note that you save const qualifiers of original arguments. In this case it's not important, but sometimes you will need it. It addition it's possible to build list of n-args without n overloads. How can you archive it with operator ()? --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Alf P. Steinbach Guest Posted: Thu Oct 26, 2006 10:38 pm    Post subject: Re: Comma within square brackets - not found in Google Code * Ron Natalie: Quote: John Nagle wrote: foo[a,b] is an invocation of the comma operator which ignores a and is equivalent to It doesn't ignore it, it evaluates it and then uses the right hand operand to determine the value of the expression. Right, although I suspect John meant this as an example of inadvertently supplying two index values using syntax from some other language, in which case the first value (no side effects) has no side effects and is effectively ignored. Quote: As a result, "operator[]" can only take one argument. This prohibits the creation of multidimensional array classes using standard bracket syntax. There's no such thing as a multidimensional array in C++. Just arrays of arrays. Again, I think that's a misunderstanding. First, about the terminology: int a[3][4]; might be described as multidimensional array, by someone favoring that term. For example, <url: http://www.research.att.com/~bs/array34.c>. Or, for example, the standard's §8.3.4/4, «When several "array of" specifications are adjacent, a multidimensional array is created; ...". Second, whatever we call the built-in features of the language, surely that does not limit what we can call abstractions built upon those features. Quote: The justification for this is protection of legacy code which might conceivably use the comma operator within square brackets. Or how about the fact that C and C++ have never had that syntax. If I'm not very mistaken, that was the point. ;-) -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top John Nagle Guest Posted: Fri Oct 27, 2006 7:43 am    Post subject: Re: Comma within square brackets - not found in Google Code Me wrote: Quote: There are lambda and expression template libraries that abuse the array index operator because it relies on the fact that it doesn't behave like the function call operator. The immediate example that comes to mind: http://spirit.sourceforge.net/distrib/spirit_1_6_1/libs/spirit/phoenix/doc/statements.html That bit of wierdness dates from 2002. Can you find a production use of it in Google code search? Now that we have a big code archive available, we no longer have to speculate about what gets used and what doesn't. We can objectively answer the question. John Nagle Animats --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Mon Oct 30, 2006 3:38 pm    Post subject: Re: Comma within square brackets - not found in Google Code John Nagle wrote: Quote: Me wrote: There are lambda and expression template libraries that abuse the array index operator because it relies on the fact that it doesn't behave like the function call operator. The immediate example that comes to mind: http://spirit.sourceforge.net/distrib/spirit_1_6_1/libs/spirit/phoenix/doc/statements.html That bit of wierdness dates from 2002. Can you find a production use of it in Google code search? Now that we have a big code archive available, we no longer have to speculate about what gets used and what doesn't. We can objectively answer the question. That bit of wierdness is still in use today. boost::lambda borrowed the syntax (see http://tinyurl.com/zjafx search for "The BLL supports an alternative syntax for control expressions"). Then there's also Phoenix-2 (http://tinyurl.com/b25hr). Call it "abuse" or whatever, but the point is that you can't match the level of expressiveness with any other syntax. The whole point with DSEL is that we attempt to get as close as possible to the target syntax. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Pete Forman Guest Posted: Mon Oct 30, 2006 10:12 pm    Post subject: Re: Comma within square brackets - not found in Google Code nagle (AT) animats (DOT) com (John Nagle) writes: Quote: Now that we have a big code archive available, we no longer have to speculate about what gets used and what doesn't. We can objectively answer the question. At the risk of stating the obvious, that archive only holds public source code. -- Pete Forman -./\.- Disclaimer: This post is originated WesternGeco -./\.- by myself and does not represent pete.forman (AT) westerngeco (DOT) com -./\.- the opinion of Schlumberger or http://petef.port5.com -./\.- WesternGeco. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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