C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  variable as an array size          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message HappyHippy Guest Posted: Wed Oct 12, 2005 4:31 pm    Post subject: variable as an array size Hi, I'm wondering what you think about this piece of code: #include<iostream> int main() { int size; std::cin >> size; int array[size]; std::cout< return 0; } Is it legal or not (according to C/C++ standard)? A couple of days ago I would say it is not, only constant expression can be used to specify the size, of the array, but now I'm not sure The reason is that gcc 3.4.4 compiles at with no errors and warnings. And the result printed by the program is correct: sizeof(arr) = size * sizeof(int) P.S. MS Visual C++ 7.1 does not compile this code. It gives the following errors: main.cpp(7) : error C2057: expected constant expression main.cpp(7) : error C2466: cannot allocate an array of constant size 0 main.cpp(7) : error C2133: 'array' : unknown size main.cpp(9) : error C2070: 'int []': illegal sizeof operand [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Wed Oct 12, 2005 10:47 pm    Post subject: Re: variable as an array size HappyHippy <karenman (AT) mail (DOT) ru> writes: Quote: #include<iostream This declares std::cin and std::cout, but not all the required operator<< overloads. #include To err on the safe side, I also #include <istream> if I call an operator>> (the one your code uses is a member of basic_istream, so that wouldn't strictly be necessary here). Quote: int main() { int size; std::cin >> size; int array[size]; I don't think much of this, but not for the reason you expect: The input operation can fail. If that happens, size is left unitialized. Reading an uninitialized variable has undefined behavior. As for your real question, variably sized arrays were added to C in 1999. The ISO C++ Standard is based on C 90, though. Quote: std::cout< return 0; } Is it legal or not (according to C/C++ standard)? There is no such thing as a "C/C++ standard". The program isn't legal according to the C Standard, but only because it uses components of the C++ Standard Library not present in the C Standard Library. The problem isn't legal according to the ISO C++ Standard because it doesn't (yet) support variably sized arrays. You can use std::vector instead. Quote: A couple of days ago I would say it is not, only constant expression can be used to specify the size, of the array, but now I'm not sure The reason is that gcc 3.4.4 compiles at with no errors and warnings. That's an implementation-specific extension then. One that makes a lot of sense given the fact that gcc compiles both C and C++. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Valentin Samko Guest Posted: Wed Oct 12, 2005 10:50 pm    Post subject: Re: variable as an array size HappyHippy wrote: Quote: Hi, I'm wondering what you think about this piece of code: #include<iostream int main() { int size; std::cin >> size; int array[size]; std::cout< return 0; } Is it legal or not (according to C/C++ standard)? It is illegal according to the current C++ standard. g++ compiles this because it is allowed by C99, although it is not a part of the C++ standard. g++ does not compile the same code with -pedantic. -- Valentin Samko - http://val.samko.info [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Thu Oct 13, 2005 1:14 pm    Post subject: Re: variable as an array size HappyHippy wrote: Quote: int main() { int size; std::cin >> size; int array[size]; std::cout< return 0; } Is it legal or not (according to C/C++ standard)? There is no C/C++ standard! C and C++ are two different languages covered by two different standards. In fact, the "variable length array" feature is allowed in C (to be precise in C99), but not in C++. Quote: A couple of days ago I would say it is not, only constant expression can be used to specify the size, of the array, but now I'm not sure The reason is that gcc 3.4.4 compiles at with no errors and warnings. AFAIK gcc is implementing C99 quite well, so it's no surprise that it supports variable length arrays in C programs. Apparently, gcc is also is allowing this C-only feature even in C++ programs. This has to be regarded as a non-standard extension, however. Other compilers are not required to do the same. In case you are wondering, I didn't get signals about a possible inclusion of variable length arrays in C++, mainly because the object model (read: construction and destruction of UDTs) is rather different from C. Of course I might have missed some discussion. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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