On Mon, Aug 11, 2008 at 9:33 AM, joel falcou <joel.falcou@u-psud.fr> wrote:
I was trying to use it as a simple traits to see if a given type fullfilled the result_of protocol whatever the arguments used. It works ok for the result_type variant but now I see it's still incomplete. The problem is that I can't see, for an arbitrary result structure, how I can see if it actually has at least one overload that has a proper nested type typedef.
I can't either :-)
Typical use is :
For F a given type, support_result_of_protocol<F>::type evaluates as a compile time boolean indicating if either F has a nested result structure or result_type typedef. It doesn't make any assumptions on wether or nor result is correcly implemented.
As for testing if a particuliar set of arguments produces a valid result_of, one can easily do :
template< class F , template<class> class T = boost::result_of<F>::type > struct valid_impl { typedef void type; };
template<class F, class EnableIf = void> struct produce_valid_result_of : boost::false_type {};
template<class F> struct produce_valid_result_of<F,typename valid_impl<F>::type> : boost::true_type {};
or did I miss something in your question ?
I think you got my question, but that won't work. If you fix it to: template< class F , typename T = typename boost::result_of<F>::type > struct valid_impl { typedef void type; }; template<class F, class EnableIf = void> struct produce_valid_result_of : boost::false_type {}; template<class F> struct produce_valid_result_of<F,typename valid_impl<F>::type> : boost::true_type {}; ...and then try: struct x {}; std::cout << produce_valid_result_of<x(x)>::type::value << std::endl; ... you will get an error. I get: /Development/boost/boost/utility/result_of.hpp: In instantiation of 'boost::detail::result_of_nested_result<x, x ()(x)>': /Development/boost/boost/utility/result_of.hpp:38: instantiated from 'boost::detail::result_of_impl<x, x ()(x), false>' /Development/boost/boost/utility/result_of.hpp:31: instantiated from 'boost::result_of<x ()(x)>' ../../libs/patterns/example/video_gesture_recognition.cpp:224: instantiated from here /Development/boost/boost/utility/result_of.hpp:73: error: no class template named 'result' in 'struct x' With the way boost::result_of is implemented, I couldn't find a way to make an approach like the above work. The best I could do was the result_of_defined code. Kind regards, Stjepan