Hi, is it possible to conclude on that? Do we need more reviewers? Is the issue with operator< returning void a stopper for including the actual code in the library? Regards, Frédéric
Please find at https://svn.boost.org/trac/boost/browser/sandbox/type_traits and at http://dl.free.fr/tK0BwIzYy my new proposal according to review comments.
* now using 2 template parameters so that it is possible to check if 2 different types are comparable * now using factory functions instead of static references * now using type_traits yes_type and no_type * the doc. is alphabetically sorted * the doc. has been updated for clarity * the tests have been modified to include new possibilities
The code has the following properties: * returns true if t<u is meaningful and returns a value convertible to bool * returns false if t<u is meaningless. * fails with compile time error if t<u is meaningful and returns void; on this point, please have a look at the comment below.
Thank you for your review of this new version (please find below the main piece of code).
Frédéric
I'd like to see something done to detect the void return, with a meaningful error message though. It is possible to extract the return type from a function pointer and check for that being the same as void, so it might be possible to form a pointer to the operator. You'd have to know whether the argument types are built-in types, in which case you can short-circuit the answer. If not, you'd have to determine whether the operator is a free function or a member in order to create the proper pointer type. Once you have that, you can extract the return type and compare it. Finally, you can use BOOST_STATIC_ASSERT to complain if the return type is void.
This looks too difficult for me! Can we keep it for a future update, unless you can show me how to do it?
type_traits/detail/is_comparable.hpp
#include <boost/config.hpp> #include <boost/type_traits/remove_cv.hpp> #include <boost/type_traits/integral_constant.hpp> #include <boost/type_traits/detail/yes_no_type.hpp>
// should be the last #include #include <boost/type_traits/detail/bool_trait_def.hpp>
namespace boost { namespace detail {
// This namespace ensures that ADL does not mess things up. namespace BOOST_JOIN(BOOST_TT_TRAIT_NAME,_impl) {
// a type returned from comparison operator when no such operator is found in the // type's own namespace struct tag { } ;
// any soaks up implicit conversions and makes the following // comparison operators less-preferred than any other such operators that // might be found via ADL. struct any { template <class T> any(T const&) ; } ;
// when operator< is not available, this one is used tag operator BOOST_TT_TRAIT_OP (const any&, const any&) ;
::boost::type_traits::yes_type is_bool(bool) ; // this version is preferred for types convertible to bool ::boost::type_traits::no_type is_bool(...) ; // this version is used otherwise
template < typename T, typename U > struct BOOST_JOIN(BOOST_TT_TRAIT_NAME,_impl) { static T make_T() ; static U make_U() ; static const bool value = sizeof(is_bool(make_T() BOOST_TT_TRAIT_OP make_U())) == sizeof(::boost::type_traits::yes_type) ; } ;
} // namespace impl } // namespace detail
BOOST_TT_AUX_BOOL_TRAIT_DEF2(BOOST_TT_TRAIT_NAME,T,U=T,(::boost::detail::BOOST_JOIN(BOOST_TT_TRAIT_NAME,_impl)::BOOST_JOIN(BOOST_TT_TRAIT_NAME,_impl)<T,U>::value))
} // namespace boost
#include <boost/type_traits/detail/bool_trait_undef.hpp>
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