C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  detecting existence of a member function (SFINAE ?)          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Samuel Krempp Guest Posted: Mon Mar 14, 2005 12:04 am    Post subject: detecting existence of a member function (SFINAE ?) I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? -- Samuel.Krempp cout << "@" << "crans." << "ens-cachan.fr" << endl; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Mon Mar 14, 2005 8:58 am    Post subject: Re: detecting existence of a member function (SFINAE ?) Samuel Krempp <krempp (AT) crans (DOT) ens-cachan.fr> writes: Quote: I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? #include <boost/mpl/has_xxx.hpp> #include <boost/static_assert.hpp> #include <utility> BOOST_MPL_HAS_XXX_TRAIT_DEF(first_type) BOOST_STATIC_ASSERT((has_first_type<std::pair::value)); -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Turkanis Guest Posted: Mon Mar 14, 2005 9:00 am    Post subject: Re: detecting existence of a member function (SFINAE ?) Samuel Krempp wrote: Quote: I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? See these threads from the Boost developers list: http://lists.boost.org/MailArchives/boost/msg76679.php http://lists.boost.org/MailArchives/boost/msg55484.php And this thread from comp.lang.c++: http://groups-beta.google.com/group/comp.lang.c++/msg/ec08415ef5b4b0d6 Jonathan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Mon Mar 14, 2005 9:05 am    Post subject: Re: detecting existence of a member function (SFINAE ?) Samuel Krempp <krempp (AT) crans (DOT) ens-cachan.fr> wrote: Quote: I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? Something along these lines: #include <iostream> template <class T, T val> struct member_wrapper{}; template <class T> char test_for_swap(member_wrapper<void (T::*)(T&), &T::swap>* p); template <class T> double test_for_swap(...); template <class T> struct has_member_swap { static const bool value = (1 == sizeof(test_for_swap<T>(0))); }; struct A {}; struct B { void swap(B&); }; int main() { using namespace std; cout << has_member_swap cout << has_member_swap } -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Vladimir Marko Guest Posted: Mon Mar 14, 2005 1:31 pm    Post subject: Re: detecting existence of a member function (SFINAE ?) Samuel Krempp wrote: Quote: I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? If you know the name and the _exact signature_ of the function it is possible. If you are just trying to find out if there is an unambigous member function with a given name that can be called with a given set of arguments you will soon run into DR339: http://www2.open-std.org/jtc1/sc22/WG21/docs/cwg_active.html#339 I didn't find any workaroud, but it may still exist. So, for the easy case: struct no { }; struct yes { no no_[2]; }; template <typename T,void (T::*)()> // exact signature struct yes_holder { typedef yes type; }; no tester(...); template <typename T> typename yes_holder<T,&T::foo>::type tester(const T&); // test: struct s1 { }; struct s2 { void foo() { } }; static const bool s1_has_foo= sizeof(yes)==sizeof(tester(s1())); static const bool s2_has_foo= sizeof(yes)==sizeof(tester(s2())); #include <iostream> #include <ostream> int main(){ std::cout << s1_has_foo << " " << s2_has_foo << std::endl; } Note, however, that this is not very useful. A template like template struct has_foo{ static const bool value= sizeof(yes)==sizeof(tester(*(T*)0)); }; would be much better, but according to April 2003 Note from DR339 even this should be invalid. On the other hand, N1705 (quoted in March 2005 Note in DR339) suggests a different resolution. For now, we have to work with existing compilers, so let's also talk about what the compilers say if we add the template has_foo to the program and std::cout << has_foo std::endl; to the main(). gcc 3.3 fails to apply SFINAE in instantiation of has_foo accept the code and give correct results. Comeau C/C++ Online issues an "NULL reference is not allowed" in instantiation of has_foo<s2> which is really funny since the NULL reference is in a sizeof expression. Regards, Vladimir Marko [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Samuel Krempp Guest Posted: Mon Mar 14, 2005 9:26 pm    Post subject: Re: detecting existence of a member function (SFINAE ?) Vladimir Marko wrote: Quote: If you know the name and the _exact signature_ of the function it is possible. If you are just trying to find out if there is an unambigous member function with a given name that can be called with a given set of arguments you will soon run into DR339: http://www2.open-std.org/jtc1/sc22/WG21/docs/cwg_active.html#339 I didn't find any workaroud, but it may still exist. So, for the easy case: struct no { }; struct yes { no no_[2]; }; template <typename T,void (T::*)()> // exact signature struct yes_holder { typedef yes type; }; no tester(...); template <typename T typename yes_holder thanks for the code, and the precisions on the situation with overload resolution. Even if it requires the exact prototype, it can make things a little more transparent in my situation. this kind of techniques really takes some getting used to, tricking the compiler to work for us this way isn't very intuitive IMO :) -- Sam [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Mon Mar 14, 2005 9:33 pm    Post subject: Re: detecting existence of a member function (SFINAE ?) "Jonathan Turkanis" <technews (AT) kangaroologic (DOT) com> writes: Quote: Samuel Krempp wrote: I thought I remembered seeing some application of SFINAE that could detect whether a class had a given member function. but I can't find any evidence of it now.. and my short experiments didnt bring much of an answer. Isn't it feasible ? anyone has pointers about that ? See these threads from the Boost developers list: http://lists.boost.org/MailArchives/boost/msg76679.php http://lists.boost.org/MailArchives/boost/msg55484.php And this thread from comp.lang.c++: http://groups-beta.google.com/group/comp.lang.c++/msg/ec08415ef5b4b0d6 Whoops; I didn't read your question carefully enough. What I posted only finds member types. Jonathan gave the right references. -- Dave Abrahams Boost Consulting www.boost-consulting.com [ 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.