boost/hana/core/to.hpp
/*! @file Defines `boost::hana::to` and related utilities. Copyright Louis Dionne 2013-2022 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) */ #ifndef BOOST_HANA_CORE_TO_HPP #define BOOST_HANA_CORE_TO_HPP #include <boost/hana/fwd/core/to.hpp> #include <boost/hana/concept/constant.hpp> #include <boost/hana/concept/foldable.hpp> #include <boost/hana/concept/sequence.hpp> #include <boost/hana/config.hpp> #include <boost/hana/core/common.hpp> #include <boost/hana/core/dispatch.hpp> #include <boost/hana/core/make.hpp> #include <boost/hana/detail/wrong.hpp> #include <boost/hana/unpack.hpp> #include <boost/hana/value.hpp> #include <type_traits> #include <utility> namespace boost { namespace hana { ////////////////////////////////////////////////////////////////////////// // to ////////////////////////////////////////////////////////////////////////// //! @cond template <typename To, typename From, typename> struct to_impl : to_impl<To, From, when<true>> { }; //! @endcond namespace convert_detail { struct no_conversion { }; template <typename To, typename From, typename = void> struct maybe_static_cast : no_conversion { template <typename X> static constexpr auto apply(X const&) { static_assert(detail::wrong<to_impl<To, From>, X>{}, "no conversion is available between the provided types"); } }; template <typename To, typename From> struct maybe_static_cast<To, From, decltype((void) static_cast<To>(std::declval<From>()) )> { template <typename X> static constexpr To apply(X&& x) { return static_cast<To>(static_cast<X&&>(x)); } }; } // end namespace convert_detail template <typename To, typename From, bool condition> struct to_impl<To, From, when<condition>> : convert_detail::maybe_static_cast<To, From> { }; template <typename To> struct to_impl<To, To> : embedding<> { template <typename X> static constexpr X apply(X&& x) { return static_cast<X&&>(x); } }; //! @cond template <typename To> template <typename X> constexpr decltype(auto) to_t<To>::operator()(X&& x) const { using From = typename hana::tag_of<X>::type; return to_impl<To, From>::apply(static_cast<X&&>(x)); } //! @endcond #define BOOST_HANA_DEFINE_EMBEDDING_IMPL(TO, FROM) \ template <> \ struct to_impl<TO, FROM> : embedding<> \ { static constexpr TO apply(FROM x) { return x; } } \ /**/ BOOST_HANA_DEFINE_EMBEDDING_IMPL(long double, double); BOOST_HANA_DEFINE_EMBEDDING_IMPL(long double, float); BOOST_HANA_DEFINE_EMBEDDING_IMPL(double , float); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long long, signed long); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long long, signed int); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long long, signed short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long long, signed char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long , signed int); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long , signed short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed long , signed char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed int , signed short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed int , signed char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(signed short , signed char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long long, unsigned long); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long long, unsigned int); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long long, unsigned short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long long, unsigned char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long , unsigned int); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long , unsigned short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned long , unsigned char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned int , unsigned short); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned int , unsigned char); BOOST_HANA_DEFINE_EMBEDDING_IMPL(unsigned short , unsigned char); #undef BOOST_HANA_DEFINE_EMBEDDING_IMPL namespace detail { template <typename T> struct copy_char_signedness { using type = typename std::conditional<std::is_signed<char>::value, std::make_signed<T>, std::make_unsigned<T> >::type::type; }; } // If `char` is signed, we define an embedding from `char` to any signed // integral type. Otherwise, we define one from `char` to any unsigned // integral type. #define BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL(TO) \ template <> \ struct to_impl<detail::copy_char_signedness<TO>::type, char> \ : embedding<> \ { \ static constexpr detail::copy_char_signedness<TO>::type \ apply(char x) \ { return x; } \ } \ /**/ BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL(long long); BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL(long); BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL(int); BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL(short); #undef BOOST_HANA_DEFINE_CHAR_EMBEDDING_IMPL template <typename T> struct to_impl<T*, decltype(nullptr)> : embedding<> { static constexpr T* apply(decltype(nullptr)) { return nullptr; } }; ////////////////////////////////////////////////////////////////////////// // is_convertible ////////////////////////////////////////////////////////////////////////// template <typename From, typename To, typename> struct is_convertible : std::true_type { }; template <typename From, typename To> struct is_convertible<From, To, decltype((void) static_cast<convert_detail::no_conversion>(*(to_impl<To, From>*)0) )> : std::false_type { }; ////////////////////////////////////////////////////////////////////////// // is_embedded ////////////////////////////////////////////////////////////////////////// template <typename From, typename To, typename> struct is_embedded : std::false_type { }; template <typename From, typename To> struct is_embedded<From, To, decltype((void) static_cast<embedding<true>>(*(to_impl<To, From>*)0) )> : std::true_type { }; ////////////////////////////////////////////////////////////////////////// // Conversion for Constants ////////////////////////////////////////////////////////////////////////// template <typename To, typename From> struct to_impl<To, From, when< hana::Constant<From>::value && is_convertible<typename From::value_type, To>::value >> : embedding<is_embedded<typename From::value_type, To>::value> { template <typename X> static constexpr decltype(auto) apply(X const&) { return hana::to<To>(hana::value<X>()); } }; ////////////////////////////////////////////////////////////////////////// // Foldable -> Sequence ////////////////////////////////////////////////////////////////////////// template <typename S, typename F> struct to_impl<S, F, when< hana::Sequence<S>::value && hana::Foldable<F>::value >> : embedding<Sequence<F>::value> { template <typename Xs> static constexpr decltype(auto) apply(Xs&& xs) { return hana::unpack(static_cast<Xs&&>(xs), hana::make<S>); } }; }} // end namespace boost::hana #endif // !BOOST_HANA_CORE_TO_HPP