1 /*! 2 @file 3 Defines `boost::hana::tag_of` and `boost::hana::tag_of_t`. 4 5 @copyright Louis Dionne 2013-2016 6 Distributed under the Boost Software License, Version 1.0. 7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 8 */ 9 10 #ifndef BOOST_HANA_CORE_TAG_OF_HPP 11 #define BOOST_HANA_CORE_TAG_OF_HPP 12 13 #include <boost/hana/fwd/core/tag_of.hpp> 14 15 #include <boost/hana/config.hpp> 16 #include <boost/hana/core/when.hpp> 17 18 19 BOOST_HANA_NAMESPACE_BEGIN 20 //! @cond 21 template <typename T, typename> 22 struct tag_of : tag_of<T, when<true>> { }; 23 //! @endcond 24 25 namespace core_detail { 26 template <typename ...> 27 struct is_valid { static constexpr bool value = true; }; 28 } 29 30 template <typename T, bool condition> 31 struct tag_of<T, when<condition>> { 32 using type = T; 33 }; 34 35 template <typename T> 36 struct tag_of<T, when< 37 core_detail::is_valid<typename T::hana_tag>::value 38 >> { 39 using type = typename T::hana_tag; 40 }; 41 42 template <typename T> struct tag_of<T const> : tag_of<T> { }; 43 template <typename T> struct tag_of<T volatile> : tag_of<T> { }; 44 template <typename T> struct tag_of<T const volatile> : tag_of<T> { }; 45 template <typename T> struct tag_of<T&> : tag_of<T> { }; 46 template <typename T> struct tag_of<T&&> : tag_of<T> { }; 47 BOOST_HANA_NAMESPACE_END 48 49 #endif // !BOOST_HANA_CORE_TAG_OF_HPP 50