C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Legal C++ for loop? Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Gordon Marr Guest Posted: Fri Jan 23, 2004 9:04 pm    Post subject: Legal C++ for loop? This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? #include <iostream> #include <string> using namespace std; int main() { for({int i = 0; string dots("");} i < 10; ++i) { cout << "i = " << i << ": " << dots << endl; dots += '.'; } } The output is: i = 0: i = 1: . i = 2: .. i = 3: ... i = 4: .... i = 5: ..... i = 6: ...... i = 7: ....... i = 8: ........ i = 9: ......... The given email is invalid; please follow up to the group. Thank you! GM. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Sat Jan 24, 2004 1:22 pm    Post subject: Re: Legal C++ for loop? "Gordon Marr" <bitbucket (AT) www (DOT) pointless.com> wrote Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? Not legal. Some goofy figment of the GCC. The initial stuff in a for loop has to be either an expression-statement or a simple declaration. Compound statements are right out. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Erik Max Francis Guest Posted: Sat Jan 24, 2004 9:36 pm    Post subject: Re: Legal C++ for loop? Gordon Marr wrote: Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? No, it is a gcc extension. gcc does have an issue with creating their own extensions willy nilly, but at least it is pretty good about reporting them when in -ansi -pedantic mode: max@oxygen:~/tmp% g++ -W -Wall -ansi -pedantic loop.cc -o loop loop.cc: In function `int main()': loop.cc:8: warning: ISO C++ forbids compound statements inside for initializations -- __ Erik Max Francis && [email]max (AT) alcyone (DOT) com[/email] && http://www.alcyone.com/max/ / San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE __/ I go out with actresses because I'm not apt to marry one. -- Henry Kissinger [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Radu Grigore Guest Posted: Sat Jan 24, 2004 9:37 pm    Post subject: Re: Legal C++ for loop? Gordon Marr <bitbucket (AT) www (DOT) pointless.com> wrote Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? for({int i = 0; string dots("");} i < 10; ++i) It does not compile on VC7.1 and I strongly suspect it's a problem with gcc's grammar. But I can't be sure since I don't have a standard handy now. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bert Klaps Guest Posted: Sat Jan 24, 2004 9:44 pm    Post subject: Re: Legal C++ for loop? Gordon Marr <bitbucket (AT) www (DOT) pointless.com> wrote Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? #include #include using namespace std; int main() { for({int i = 0; string dots("");} i < 10; ++i) { cout << "i = " << i << ": " << dots << endl; dots += '.'; } } It's not legal: it's got a compound statement where an expression or declaration is expected. By the way, under G++ 3.3.1 this compiles too. E.g. Comeau online correctly says: line 8: error: expected an expression for({int i = 0; string dots("");} i < 10; ++i) ^ Regards, Bert Klaps [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sebastian Kapfer Guest Posted: Sat Jan 24, 2004 9:45 pm    Post subject: Re: Legal C++ for loop? On Fri, 23 Jan 2004 16:04:06 -0500, Gordon Marr wrote: Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? #include <iostream #include using namespace std; int main() { for({int i = 0; string dots("");} i < 10; ++i) { cout << "i = " << i << ": " << dots << endl; dots += '.'; } } g++ -pedantic -Wall -o test2 test2.cpp test2.cpp: In function `int main()': test2.cpp:9: error: ISO C++ forbids compound statements inside for initializations So I think the g++ programmers themselves think that it is an extension. (I think it's strange scoping in addition. Why would i and dots be visible in the loop body at all? They're declared in their own block.) -- Best Regards, | This signature is currently under construction. Sebastian | Please check back later! Quote: -------------------------------------------------------- mailbox in "From" silently drops any mail > 20k [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Robert Kindred Guest Posted: Mon Jan 26, 2004 8:30 pm    Post subject: Re: Legal C++ for loop? "Gordon Marr" <bitbucket (AT) www (DOT) pointless.com> wrote Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? #include #include using namespace std; int main() { for({int i = 0; string dots("");} i < 10; ++i) I believe I have seen commas used here, such as: for(int i = 0, string dots(""); i < 10; ++i) Quote: { cout << "i = " << i << ": " << dots << endl; dots += '.'; } } snip Thank you! GM. hth, Robert Kindred [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Tue Jan 27, 2004 10:45 am    Post subject: Re: Legal C++ for loop? Gordon Marr wrote: Quote: This compiles under G++ 3.2.2, but I hadn't come across the for loop syntax before, so I'm curious to know whether it's actually legal or not? for({int i = 0; string dots("");} i < 10; ++i) {...} for( pair {...} Just in case you didn't only wonder if it's legal but also if it's necessary. cheers Uli [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Tue Jan 27, 2004 11:07 am    Post subject: Re: Legal C++ for loop? Quote: Gordon Marr <bitbucket (AT) www (DOT) pointless.com> wrote for( {int i = 0; string dots("");} i < 10; ++i) { ... } [email]bertklaps (AT) yahoo (DOT) com[/email] (Bert Klaps) wrote Quote: It's not legal: it's got a compound statement where an expression or declaration is expected. Wouldn't it be great if this was legal? Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? The legal version uses a local block around the for: { string dots(""); for (int i=0; i<10; ++i) { ... } } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gordon Marr Guest Posted: Tue Jan 27, 2004 3:02 pm    Post subject: Re: Legal C++ for loop? Robert Kindred wrote: Quote: "Gordon Marr" <bitbucket (AT) www (DOT) pointless.com> wrote in message for({int i = 0; string dots("");} i < 10; ++i) I believe I have seen commas used here, such as: for(int i = 0, string dots(""); i < 10; ++i) { cout << "i = " << i << ": " << dots << endl; dots += '.'; } I tried that when I first saw this, and got: loop.C: In function `int main()': loop.C:8: error: parse error before `(' token loop.C:10: error: `dots' undeclared (first use this function) [the for(...) line is line 8] Basically, the compiler (G++ 3.3.2) thinks I'm trying to declare more integers. This works: for(int i = 0, dots = 0; i < 10; ++i) but obviously, no strings. GM. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue Jan 27, 2004 6:43 pm    Post subject: Re: Legal C++ for loop? In message <7f2735a5.0401261813.73151212 (AT) posting (DOT) google.com>, Allan W <allan_w (AT) my-dejanews (DOT) com> writes Quote: Gordon Marr <bitbucket (AT) www (DOT) pointless.com> wrote for( {int i = 0; string dots("");} i < 10; ++i) { ... } [email]bertklaps (AT) yahoo (DOT) com[/email] (Bert Klaps) wrote It's not legal: it's got a compound statement where an expression or declaration is expected. Wouldn't it be great if this was legal? Why? Objects declared within a block are destroyed at the end of a block so we would have to mess with far more of the language and introduce inconsistencies if we were to make that syntax useful. Quote: Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? Nonsense, we just declare them as a pair Quote: The legal version uses a local block around the for: { string dots(""); for (int i=0; i<10; ++i) { ... } } Well that is one but for(pair {...} is fine. For more than two you need a Boost tuple (I think). -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Richter Guest Posted: Tue Jan 27, 2004 6:48 pm    Post subject: Re: Legal C++ for loop? Hi, Quote: for( {int i = 0; string dots("");} i < 10; ++i) { ... } It's not legal: it's got a compound statement where an expression or declaration is expected. Wouldn't it be great if this was legal? No, I don't think so. It breaks with the rules of the language. By them, the lifetime of the variable "i" ends at the curly bracket in the head of the for(..) loop, thus the "i" in the body would/should not exist, or would refer to another global "i". Consider the following: foo(..) { int i,l; for( {int i = 0;short k = 1;l = 0 }; i < 10; ++i) { l += i + k; // which "i" is this? } } Quote: Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? The legal version uses a local block around the for: { string dots(""); for (int i=0; i<10; ++i) { ... } } Unless you insist on destroying "dots" immediately after the for() loop terminated, there's no need for the block at all. Sorry, I fail to see why this is an improvement. It messes with language rules and makes the code harder to read. So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Wed Jan 28, 2004 8:28 pm    Post subject: Re: Legal C++ for loop? Francis Glassborow <francis (AT) robinton (DOT) demon.co.uk> wrote Quote: In message <7f2735a5.0401261813.73151212 (AT) posting (DOT) google.com>, Allan W [email]allan_w (AT) my-dejanews (DOT) com[/email]> writes Gordon Marr <bitbucket (AT) www (DOT) pointless.com> wrote for( {int i = 0; string dots("");} i < 10; ++i) { ... } [email]bertklaps (AT) yahoo (DOT) com[/email] (Bert Klaps) wrote It's not legal: it's got a compound statement where an expression or declaration is expected. Wouldn't it be great if this was legal? Why? Objects declared within a block are destroyed at the end of a block so we would have to mess with far more of the language and introduce inconsistencies if we were to make that syntax useful. Well, I'm not sure that it's all that necessary either. But... Quote: Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? Nonsense, we just declare them as a pair Which is an excellent solution if the logical names for the two objects are "first" and "second", and totally unacceptable otherwise. Quote: The legal version uses a local block around the for: { string dots(""); for (int i=0; i<10; ++i) { ... } } Well that is one but for(pair {...} is fine. If you like obfuscation. I prefer using names that mean something, so that others can read my code. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Balog Pal Guest Posted: Thu Jan 29, 2004 9:48 am    Post subject: Re: Legal C++ for loop? "Francis Glassborow" <francis (AT) robinton (DOT) demon.co.uk> wrote Quote: Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? Nonsense, we just declare them as a pair<type1, type2>. That reminds me another issue. Why is const discarded in multiple onject declaration: for(int i=0, const maxI = 10; i < maxI; ++i) ... I get: warning C4228: nonstandard extension used : qualifiers after comma in declarator list are ignored while a * shall be repeated, and is accepted, why not const? Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gennaro Prota Guest Posted: Thu Jan 29, 2004 7:13 pm    Post subject: Re: Legal C++ for loop? On 29 Jan 2004 04:48:51 -0500, "Balog Pal" <pasa (AT) lib (DOT) hu> wrote: Quote: "Francis Glassborow" <francis (AT) robinton (DOT) demon.co.uk> wrote in message news:XIO0kdCJokFAFwQE (AT) robinton (DOT) demon.co.uk... Currently you can't declare two variables in a for() loop, unless they are the same type. Wouldn't it be great if the example above worked? Nonsense, we just declare them as a pair<type1, type2>. That reminds me another issue. Why is const discarded in multiple onject declaration: for(int i=0, const maxI = 10; i < maxI; ++i) ... It's not "discarded", it's an error. After the comma in a simple-declaration you can only have *declarators*, and they cannot start with a cv-qualifier (they may contain one, but not as the first token. Example: int i = 0, * const p = 0; // ok, the const in the second // declarator is after the * // (ptr-operator) ) Genny. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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