C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Compile time testing for "type equality"?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Guest Posted: Thu May 11, 2006 10:21 pm    Post subject: Compile time testing for "type equality"? Hi, I would like to know if there is a way to test if two types are indeed the same. The simplest example is typedef int A; typedef float B; [magic statement that results in error iff A is not same type as B] The ability to convert from A to B and back is not sufficient. The code above is contrived, but the situation where it happens is that I have something as follows: template <class T, class I = unsigned int> class Alph { typedef I index_type; typedef T alphabet_type; } // Only makes sense if A1 and A2 are of type Alph<> template <class A1, class A2> Alph< std::pair<A1,A2> > cartesian_product(A1& a1, A2& a2) { [magic code that verifies that A1::index_type == A2::index_type] // Construct cartesian product ... } Thanks, Patrick [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top johnchx2@yahoo.com Guest Posted: Fri May 12, 2006 12:21 am    Post subject: Re: Compile time testing for "type equality"? pmitran (AT) gmail (DOT) com wrote: Quote: I would like to know if there is a way to test if two types are indeed the same. You might have a look at is_same, from the Boost Type Traits library: http://www.boost.org/doc/html/boost_typetraits/reference.html#boost_typetraits.is_same [ 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: Fri May 12, 2006 3:21 am    Post subject: Re: Compile time testing for "type equality"? pmitran (AT) gmail (DOT) com writes: Quote: Hi, I would like to know if there is a way to test if two types are indeed the same. The simplest example is typedef int A; typedef float B; [magic statement that results in error iff A is not same type as B] #include <boost/mpl/assert.hpp> #include <boost/type_traits/is_same.hpp> BOOST_MPL_ASSERT((boost::is_same<A,B>)); http://www.boost.org http://www.boost-consulting.com/mplbook HTH, -- 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 Jeffrey Schwab Guest Posted: Fri May 12, 2006 4:21 am    Post subject: Re: Compile time testing for "type equality"? pmitran (AT) gmail (DOT) com wrote: Quote: I would like to know if there is a way to test if two types are indeed the same. template<typename T> bool same_type(T const&, T const&) { return true; } template<typename T, typename U> bool same_type(T const&, U const&) { return false; } #include <iostream> int main() { std::cout << "same_type(int, int)\t" << same_type(1, 2) << '\n'; std::cout << "same_type(int, float)\t" << same_type(1, 2.0) << '\n'; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carlos Moreno Guest Posted: Mon May 15, 2006 12:22 am    Post subject: Re: Compile time testing for "type equality"? Jeffrey Schwab wrote: Quote: template<typename T bool same_type(T const&, T const&) { return true; } template<typename T, typename U bool same_type(T const&, U const&) { return false; } #include <iostream int main() { std::cout << "same_type(int, int)\t" << same_type(1, 2) << '\n'; std::cout << "same_type(int, float)\t" << same_type(1, 2.0) '\n'; Tiny nit pick: that should be same_type(int, double), which is really what you're testing (2.0 is a *double* literal, not a float literal) Carlos -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jon Colverson Guest Posted: Wed May 17, 2006 10:21 am    Post subject: Re: Compile time testing for "type equality"? pmitran (AT) gmail (DOT) com wrote: Quote: but the situation where it happens is that I have something as follows: template <class T, class I = unsigned int class Alph { typedef I index_type; typedef T alphabet_type; } // Only makes sense if A1 and A2 are of type Alph template <class A1, class A2 Alph< std::pair<A1,A2> > cartesian_product(A1& a1, A2& a2) { [magic code that verifies that A1::index_type == A2::index_type] // Construct cartesian product ... } You can state your requirements more precisely in the template and function parameters to avoid the need for an explicit check: template <class T1, class T2, class I> Alph< std::pair<Alph<T1, I>,Alph<T2, I> > > cartesian_product(Alph<T1, I>& a1, Alph<T2, I>& a2) { // Construct cartesian product ... } -- Jon Colverson -- looking for a game programming job in London or the south-east of England: http://vcxz.co.uk/cv.pdf [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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