C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Testing existence of template type          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message korusef@gmail.com Guest Posted: Sat Dec 02, 2006 10:10 am    Post subject: Testing existence of template type Class TypeTraits tries to verify, whether its templated type contains required types. It works for non-template types, but I don't know, how to check for the template one. So is there any way to check for the existence of TestType::B? #include <boost/utility/enable_if.hpp> #include <boost/concept_check.hpp> #include <iostream> using namespace std; template <typename T> struct TypeTautology { enum { value = true};}; template <class GP, typename Enable = void> struct TypeTraitsImpl { enum { value = false }; }; template <class GP> struct TypeTraitsImpl< GP ,typename boost::enable_if< TypeTautology< typename GP::A//works ok, checks existence of A //typename GP::B//compiles, but returns "test: 0" //typename GP::template B//doesn't compile //typename GP::template <class> B//doesn't compile Quote: ::type { enum { value = true }; }; template <class GP> struct TypeTraits: public TypeTraitsImpl<GP, void> {}; struct TestType { struct A {}; template <class PG> struct B {}; }; int main(int const, char const* []) { try { clog << "test: " << TypeTraits<TestType>::value << endl; //Expected output: //test: 1 return EXIT_SUCCESS; } catch (...){ cerr << "error" << endl; } exit(EXIT_FAILURE); } -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg Herlihy Guest Posted: Sun Dec 03, 2006 5:24 pm    Post subject: Re: Testing existence of template type korusef (AT) gmail (DOT) com wrote: Quote: Class TypeTraits tries to verify, whether its templated type contains required types. It works for non-template types, but I don't know, how to check for the template one. So is there any way to check for the existence of TestType::B? #include <boost/utility/enable_if.hpp #include <boost/concept_check.hpp #include <iostream using namespace std; template <typename T> struct TypeTautology { enum { value = true};}; template <class GP, typename Enable = void struct TypeTraitsImpl { enum { value = false }; }; template <class GP struct TypeTraitsImpl GP ,typename boost::enable_if TypeTautology typename GP::A//works ok, checks existence of A //typename GP::B//compiles, but returns "test: 0" //typename GP::template B//doesn't compile //typename GP::template <class> B//doesn't compile ::type { enum { value = true }; }; A template is not a type, so TypeTraitsImpl (since it detects the existence of types only) is not able to test for the existence of a template - directly. TypeTraitsImpl is able to test for a template indirectly, by testing for one of its instantiations (since an instantiation which would be a type). If any one of its instantiated types is found (it does not matter which one), then the template itself must also exist: template <class GP> struct TypeTraitsImpl< GP, typename boost::enable_if< TypeTautology< typename GP::template B<int> // returns "test: 1" Quote: ::type Greg -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.