2009/12/22 Edward Diener <eldiener@tropicsoft.com>
Is there any way one can determine whether a type has been registered or not with Boost.Typeof at compile time so that the information can be used in TMP ? I am thinking, of course, of the use of typeof in a template, where a template parameter may or may not be aregistered type when the template has been instantiated. I would like to provide my own TMP fallback code if the type has not been registered rather than see an immediate compiler error generated. Something like:
Here is a possible implementation of boost::typeof::is_registered:
namespace boost { namespace type_of { class AllTypesRegisteredVector {}; class NotAllTypesRegisteredVector {}; template<typename V> struct vector_handler { template<typename Type_Not_Registered_With_Typeof_System> struct handle_unregistered_type; }; template<> struct vector_handler<AllTypesRegisteredVector> { template<typename T> struct handle_unregistered_type { typedef NotAllTypesRegisteredVector type; }; }; template<> struct vector_handler<NotAllTypesRegisteredVector> { template<typename T> struct handle_unregistered_type { typedef NotAllTypesRegisteredVector type; }; }; template<class T> struct push_back<NotAllTypesRegisteredVector,T> { typedef NotAllTypesRegisteredVector type; }; template<class T> struct push_back<AllTypesRegisteredVector,T> { typedef AllTypesRegisteredVector type; }; }} BOOST_TYPEOF_BEGIN_ENCODE_NS template<class V,class T> struct encode_type_impl { typedef typename boost::type_of::vector_handler<V>::handle_unregistered_type<T>::type type; }; BOOST_TYPEOF_END_ENCODE_NS namespace boost {namespace type_of { template<typename T> struct is_registered : boost::is_same<AllTypesRegisteredVector,typename encode_type<AllTypesRegisteredVector,T>::type> {}; }}
boost::mpl::eval_if < boost::typeof::registered<T>, // this would be nice to have boost::mpl::identity<BOOST_TYPEOF_TPL(some_expression)>, boost::mpl::identity</* use another facility to get the type I want into a metafunction*/>
The problem is that BOOST_TYPEOF_TPL is evaluated even if is_registered is false. One way to workaroun this is to allow the user to add a default type: typedef BOOST_TYPEOF_WITH_DEFAULT(some_expression,some_backup_type_if_some_expression_not_registered) type; Is this acceptable? _______________________________________________
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