John M. Dlugosz schreef op 04-Feb-14 7:03 PM:

I want to mark certain types as being of a category of my invention (e.g. is_pretty), for purposes of using enable_if in the same manner is standard type traits such as is_arithmetic etc.

The trait will be false by default, and I'd declare something to nominate types that should be seen to have that trait. I want it to follow inheritance; if B has been declared to be in my category, and C is derived from B, then C will also be in that category without needing to do anything more (though some way to turn it _off_ would be available).

I use typedef void has_some_interface; inside the class to identify that a class implements some interface. I could write out a global test template and specialization to do facilitate the checking, but I prefer the macro #define ASSERT_PROVIDES_INTERFACE( T, V ) \ template< class t, class x = void > struct V \ { static constexpr bool value = false; }; \ template< class t > struct V< t, typename t::V > \ { static constexpr bool value = true; }; \ static_assert( \ V< T >::value, \ "the " #T " template argument doesn't provide " #V \ ); (the \'s line up in a non-proporitionally font) which writes them 'locally'. Use it like this: struct implements_some_interface { typedef void has_some_interface; . . . }; template< class t > class uses_some_interface { ASSERT_PROVIDES_INTERFACE( t, some_interface ); . . . }; The error message issued by GCC when the feature id type is not provided is reasonably close to the top of the error list. In practice I write the identification type in an 'archetype' class that serves to identify the interface elements that an real implementation must provide (sort of an abstract base class, but for compile-time inheritance): struct char_out_channel_archetype { typedef void has_char_out_channel; static void put( char c ); }; struct char_in_channel_archetype { typedef void has_char_in_channel; static char get( ); }; struct uart : public char_out_channel_archetype, public char_in_channel_archetype { static void put( char c ){ . . . } static char get(){ . . . } }; A pity that I can't use 'override' to make sure that the implementations realy override a declaration in a base class, that works only for non-static methods. (WHY??) Wouter van Ooijen

-----Original Message----- From: Boost-users [mailto:boost-users-bounces@lists.boost.org] On Behalf Of John M. Dlugosz I want to mark certain types as being of a category of my invention (e.g. is_pretty), for purposes of using enable_if in the same manner is standard type traits such as is_arithmetic etc. The trait will be false by default, and I'd declare something to nominate types that should be seen to have that trait. I want it to follow inheritance; if B has been declared to be in my category, and C is derived from B, then C will also be in that category without needing to do anything more (though some way to turn it _off_ would be available). -----End Original Message----- The following works for me, but I am not sure if it is because of a working or a failing compiler: Best wishes, Alex #include <iostream> #include <string> #include <type_traits> #include <utility> class A{}; //is_pretty; class B{}; // don't know class C : public A{}; // is_pretty because it derives from A class D : public A{}; // not pretty even though it derives from A // default because conversion pointer-to-bool has lowest overload precedence std::false_type is_pretty_function(bool); std::true_type is_pretty_function(A*); std::false_type is_pretty_function(D*); struct is_pretty_object { template <typename T> auto operator()(T&& t) -> decltype(is_pretty_function(std::forward<T>(t))) { return is_pretty_function(std::forward<T>(t)); } }; template<class T> struct trait { typedef typename std::result_of<is_pretty_object(T*)>::type is_pretty; }; int main() { std::cout << "A looks " << (trait<A>::is_pretty::value ? "pretty": "nice" )<<std::endl; std::cout << "B looks " << (trait<B>::is_pretty::value ? "pretty": "nice" )<<std::endl; std::cout << "C looks " << (trait<C>::is_pretty::value ? "pretty": "nice" )<<std::endl; std::cout << "D looks " << (trait<D>::is_pretty::value ? "pretty": "nice" )<<std::endl; return 0; } http://www.compileonline.com/compile_cpp11_online.php A looks pretty B looks nice C looks pretty D looks nice