C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  compiling difference b/w G++ and VC++          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Arun Kumar Guest Posted: Mon Sep 15, 2003 9:53 pm    Post subject: compiling difference b/w G++ and VC++ hello, The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h> int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? Arun Kumar. S [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Tue Sep 16, 2003 6:39 am    Post subject: Re: compiling difference b/w G++ and VC++ On 15 Sep 2003 17:53:12 -0400, [email]smsabu2002 (AT) yahoo (DOT) com[/email] (Arun Kumar) wrote: Quote: The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h Assuming corrected program of #include using namespace std; Quote: int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? Because dynamic arrays are not part of C++. They're part of C99, if I'm not terribly mistaken. With g++ try compiling with pedantic and language specification. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bo Persson Guest Posted: Tue Sep 16, 2003 6:40 am    Post subject: Re: compiling difference b/w G++ and VC++ "Arun Kumar" <smsabu2002 (AT) yahoo (DOT) com> skrev i meddelandet news:84afc8e.0309150639.799f0a3f (AT) posting (DOT) google.com... Quote: hello, The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? Because this is a C99 feature supported by G++ (as an extension). It is not part of standard C++. Bo Persson [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Tue Sep 16, 2003 6:40 am    Post subject: Re: compiling difference b/w G++ and VC++ "Arun Kumar" wrote: Quote: The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Presumably because you didn't turn on the warnings when you compiled. You don't mention what version of gcc you are using. But please keep in mind that gcc does not turn on warnings by default. A good habit is to include the following command line options when you compile with gcc: -Wall -W -pedantic When I compile your program with gcc 3.3 and the above command line options, I get the following three warnings: In file included from c:/djgpp/lang/cxx/3.3/backward/iostream.h:31, from test.cpp:1: c:/djgpp/lang/cxx/3.3/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please consider using one of the 32 headers found in section 17.4.1.2 of the C++ standard. Examples include substituting the <X> header for the <X.h> header for C++ includes, or <sstream> instead of the deprecated header <strstream.h>. To disable this warning u se -Wno-deprecated. (This warning results from your mistakenly using <iostream.h>, which is not a standard header, instead of <iostream>.) test.cpp: In function `int main()': test.cpp:6: warning: ISO C++ forbids variable-size array `a' (This warns you that you are using a gcc extension, and that standard C++ does not permit variable-sized arrays.) test.cpp:6: warning: unused variable `int a[((n - 1) + 1)]' (This warns you that you declare a variable, a, but never use it.) Quote: Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. Yes, as noted above, that's a gcc extension - it's not part of standard C++. Hence the warning. Please consult the gcc documentation for more details. Quote: But VC++ on windows doesn't allow the above feature. Why ?????? Because it's not standard C++. And neither is <iostream.h>. Best regards, Tom __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com [ 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: Tue Sep 16, 2003 6:40 am    Post subject: Re: compiling difference b/w G++ and VC++ "Arun Kumar" <smsabu2002 (AT) yahoo (DOT) com> wrote Quote: hello, The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. The program is non-standard (as are your compilers). Quote: #include This is not a standard header. Quote: int a[n]; This is not allowed in C++ (currently). The size of an array must be a constant expression. Use a vector if you want a variable array. Quote: Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? VC++ is doing the right thing. The dynamic array length is an G++ extension. Try using a CURRENT version of G++ or turn on the -pedantic-errors flag on older ones to identify these insidious extensions. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top llewelly Guest Posted: Tue Sep 16, 2003 6:46 am    Post subject: Re: compiling difference b/w G++ and VC++ [email]smsabu2002 (AT) yahoo (DOT) com[/email] (Arun Kumar) writes: Quote: hello, The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. [snip] Try using -pedantic: $ g++ -pedantic arun.cc arun.cc: In function `int main()': arun.cc:6: warning: ISO C++ forbids variable-size array `a' [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jack Klein Guest Posted: Tue Sep 16, 2003 6:47 am    Post subject: Re: compiling difference b/w G++ and VC++ On 15 Sep 2003 17:53:12 -0400, [email]smsabu2002 (AT) yahoo (DOT) com[/email] (Arun Kumar) wrote in comp.lang.c++.moderated: Quote: hello, The following C Prog is reporting different messages while compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? Arun Kumar. S Because in this particular case, Visual C++ is being more standard conforming than the version of g++ you are using. This is a g++ extension that is not part of the C++ language at all. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq [ 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 Sep 16, 2003 7:06 am    Post subject: Re: compiling difference b/w G++ and VC++ Arun Kumar wrote: Quote: #include<iostream.h You might consider writing a not-so-ancient tongue af C++ ... get ideas about which are good books at accu.org. Quote: int main() { int n; cin >> n; int a[n]; GCC accept arrays with a not-compile-time size as an extension. Use alloca or new/malloc in combination with a smart pointer. Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Spangenberg Guest Posted: Tue Sep 16, 2003 7:08 am    Post subject: Re: compiling difference b/w G++ and VC++ Hello, Arun Kumar! Arun Kumar schrieb: Quote: hello, The following C Prog is reporting different messages while This is neither a Standard C program (C programs don't know of a <iostream.h> header)... Quote: compiling with G++ and Microsoft Visual C++ compiler. #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Thus above C++ program (compiled by G++, running on Linux) is supporting runtime array memory allocation. But VC++ on windows doesn't allow the above feature. Why ?????? ... nor a valid C++ program. Reasons are: 1) <iostream.h> is not a valid C++ header, normally you should use <iostream>. <iostream.h> was part of the "old", pre-standard C++ library. The only reason for possibly using this header is in cases, where the compiler is not aware of or does not provide compliant standard headers. 2) Everything from a C++ library which is **pure** C++ is contained in the namespace std. Thus you have to qualify cin by std::cin. 3) Current C++98 as well as C89 do not provide variable-length C arrays. All C arrays need a integral constant expression which describes the dimensions of this array. Dynamic or variable-length C arrays are part of C99 (But even C99 does not help for the points (1) and (2)). Greetings from Bremen, Daniel Spangenberg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marco Manfredini Guest Posted: Wed Sep 17, 2003 9:36 am    Post subject: Re: compiling difference b/w G++ and VC++ Arun Kumar wrote: Quote: #include<iostream.h int main() { int n; cin >> n; int a[n]; } The following errors were reported after compiling in VC++ error C2057: expected constant expression error C2466: cannot allocate an array of constant size 0 error C2133: 'a' : unknown size But G++ didn't report any error or warnings. Current versions g++ of implement a substantial subset of the new C9x standard in both the C and the C++ compiler - it actually compiles a kind of C9x++. Declaring an array with runtime size is allowed according to this standard. A nice roundup is here: http://home.tiscalinet.ch/t_wolf/tw/c/c9x_changes.html You should be able to select language standard with the -std= switch and -pedantic-errors. Marco -- The text above is a result of a bug in my newsreader and I take no responsibility for this text appearing in my post. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top André Pönitz Guest Posted: Fri Sep 19, 2003 10:12 am    Post subject: Re: compiling difference b/w G++ and VC++ In comp.lang.c++.moderated Jack Klein <jackklein (AT) spamcop (DOT) net> wrote: Quote: Because in this particular case, Visual C++ is being more standard conforming than the version of g++ you are using. This is a g++ extension that is not part of the C++ language at all. g++ -pedantic-errors says: ISO C++ forbids variable-size array `a' So g++ is well aware that this is an extension and informs you if you wish to be informed... Andre' [ 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 Sep 20, 2003 9:58 am    Post subject: Re: compiling difference b/w G++ and VC++ "André Pönitz" <poenitz (AT) gmx (DOT) net> wrote Quote: So g++ is well aware that this is an extension and informs you if you wish to be informed... And if you use a CURRENT version of G++, you don't even have to add options. The pedantic warnings are on by default. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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