C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What is wrong with this class?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Jason Carucci Guest Posted: Wed Dec 10, 2003 8:35 pm    Post subject: What is wrong with this class? Here's a little test program where I define a class that derives from an iterator. The first commented out version fails to compile. The error I get from Visual C++ 7.1 is the following: c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : warning C4346: 'T::value_type' : dependent name is not a type prefix with 'typename' to indicate a type c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(10) : see reference to class template instantiation 'Test1<T>' being compiled c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : error C2923: 'std::iterator' : 'T::value_type' is invalid as template argument '#2', type expected The second version does compile, I can't figure out the difference between them. #include <iterator> #include <string> /* Version 1, This fails to compile template<class T> class Test1 : public std::iterator<std::output_iterator_tag, T::value_type > { public: }; */ // Version 2, this does compile template<class T> class Test1 : public std::iterator<std::output_iterator_tag, wchar_t > { public: }; int main(int argc, char* argv[]) { Test1<std::wstring> testing; return 0; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Hansen Guest Posted: Thu Dec 11, 2003 10:48 am    Post subject: Re: What is wrong with this class? On 10 Dec 2003 15:35:52 -0500, there came a drop of sanity from [email]jcarucci (AT) yahoo (DOT) com[/email] (Jason Carucci) containing: Quote: Here's a little test program where I define a class that derives from an iterator. The first commented out version fails to compile. The error I get from Visual C++ 7.1 is the following: c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : warning C4346: 'T::value_type' : dependent name is not a type prefix with 'typename' to indicate a type c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(10) : see reference to class template instantiation 'Test1<T>' being compiled c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : error C2923: 'std::iterator' : 'T::value_type' is invalid as template argument '#2', type expected The second version does compile, I can't figure out the difference between them. #include <iterator #include /* Version 1, This fails to compile template class Test1 : public std::iterator { public: }; */ // Version 2, this does compile template class Test1 : public std::iterator { public: }; int main(int argc, char* argv[]) { Test1 return 0; } Well, the error says it all...! Change inheritance to: public std::iterator<std::output_iterator_tag, typename T::value_type> Reason is that the compiler can't possible know at compile time if this is a type or a value! You clearify things by adding the "typename" keyword... I have just gone through a similar problem in my library SmartWin when porting to GCC! Reason to problem is that MSVC++7.1 doesn't require the "typename" keyword unless it HAVE to have it, so us Microsoft C++ coders aren't really that used to the keyword! -- http://smartwin.sourceforge.net THE template based Windows API GUI Library My email: "john.johnson (AT) theJohnsons (DOT) com" * Replace "john" with "thomas", replace "johnson" with "hansen", replace "theJohnsons" with "adramatch" [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gianni Mariani Guest Posted: Thu Dec 11, 2003 11:44 am    Post subject: Re: What is wrong with this class? Jason Carucci wrote: Quote: Here's a little test program where I define a class that derives from an iterator. The first commented out version fails to compile. The error I get from Visual C++ 7.1 is the following: c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : warning C4346: 'T::value_type' : dependent name is not a type prefix with 'typename' to indicate a type c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(10) : see reference to class template instantiation 'Test1<T>' being compiled c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : error C2923: 'std::iterator' : 'T::value_type' is invalid as template argument '#2', type expected GCC 3.3.1 gives this error message... typename.cpp:8: error: to refer to a type member of a template parameter, use `typename T::value_type' Quote: The second version does compile, I can't figure out the difference between them. #include <iterator #include /* Version 1, This fails to compile template class Test1 : public std::iterator try this ------ std::iterator T::value_type > Quote: { public: }; */ There exists lots of history about why you need to use typename. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (nee Spang Guest Posted: Thu Dec 11, 2003 12:06 pm    Post subject: Re: What is wrong with this class? Good Morning, Jason Carucci! Jason Carucci schrieb: Quote: Here's a little test program where I define a class that derives from an iterator. The first commented out version fails to compile. The error I get from Visual C++ 7.1 is the following: c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : warning C4346: 'T::value_type' : dependent name is not a type prefix with 'typename' to indicate a type c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(10) : see reference to class template instantiation 'Test1<T>' being compiled c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : error C2923: 'std::iterator' : 'T::value_type' is invalid as template argument '#2', type expected The second version does compile, I can't figure out the difference between them. #include <iterator #include /* Version 1, This fails to compile template class Test1 : public std::iterator Missing typename in front of T::value_type: template class Test1 : public std::iterator<std::output_iterator_tag, typename T::value_type > { }; Hope that helps, Daniel Krügler [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mike Capp Guest Posted: Thu Dec 11, 2003 8:43 pm    Post subject: Re: What is wrong with this class? [email]jcarucci (AT) yahoo (DOT) com[/email] (Jason Carucci) wrote in message news:<58c726b1.0312100933.721b0cde (AT) posting (DOT) google.com>... Quote: /* Version 1, This fails to compile template<class T class Test1 : public std::iterator The compiler has no way of knowing that T::value_type is a type as opposed to, say, a static data member. Just tell it explicitly using the "typename" keyword, i.e. template class Test1 : public std::iterator<std::output_iterator_tag, typename T::value_type> and you should be fine. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gareth Stockwell Guest Posted: Thu Dec 11, 2003 9:09 pm    Post subject: Re: What is wrong with this class? Jason, The difference is that, in the first version, the type of 'T::value_type' is dependent on the template parameter T. So the base class should be written using the 'typename' keyword, like this: public std::iterator<std::output_iterator_tag, typename T::value_type> Gareth [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Mon Dec 15, 2003 10:20 am    Post subject: Re: What is wrong with this class? "Jason Carucci" <jcarucci (AT) yahoo (DOT) com> wrote Quote: Here's a little test program where I define a class that derives from an iterator. The first commented out version fails to compile. The error I get from Visual C++ 7.1 is the following: c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : warning C4346: 'T::value_type' : dependent name is not a type prefix with 'typename' to indicate a type c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(10) : see reference to class template instantiation 'Test1<T>' being compiled c:VSS_RDCConversionExamplesIteratorExampleIteratorExample.cpp(7) : error C2923: 'std::iterator' : 'T::value_type' is invalid as template argument '#2', type expected The second version does compile, I can't figure out the difference between them. #include <iterator #include /* Version 1, This fails to compile template class Test1 : public std::iterator { public: }; */ As the error message said, what you need to do is: class Test1 : public std::iterator // etc [second example using wchar_t instead of T::value_type snipped]. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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