/*============================================================================= Copyright (c) 2015 Paul Fultz II alias.h Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ==============================================================================*/ #ifndef BOOST_HOF_GUARD_ALIAS_H #define BOOST_HOF_GUARD_ALIAS_H #include <boost/hof/returns.hpp> #include <boost/hof/detail/delegate.hpp> #include <boost/hof/detail/move.hpp> #include <boost/hof/detail/holder.hpp> #include <boost/hof/config.hpp> /// alias /// ===== /// /// Description /// ----------- /// /// The `alias` class wraps a type with a new type that can be tagged by the /// user. This allows defining extra attributes about the type outside of the /// type itself. There are three different ways the value can be stored: as a /// member variable, by inheritance, or as a static member variable. The value /// can be retrieved uniformily using the `alias_value` function. /// /// Synopsis /// -------- /// /// // Alias the type using a member variable /// template<class T, class Tag=void> /// class alias; /// /// // Alias the type by inheriting /// template<class T, class Tag=void> /// class alias_inherit; /// /// // Alias the type using a static variable /// template<class T, class Tag=void> /// class alias_static; /// /// // Retrieve tag from alias /// template<class Alias> /// class alias_tag; /// /// // Check if type has a certian tag /// template<class T, class Tag> /// class has_tag; /// /// // Retrieve value from alias /// template<class Alias> /// constexpr auto alias_value(Alias&&); /// #ifdef _MSC_VER #pragma warning(push) #pragma warning(disable: 4579) #endif namespace boost { namespace hof { template<class T> struct alias_tag; template<class T, class Tag, class=void> struct has_tag : std::false_type {}; template<class T, class Tag> struct has_tag<T, Tag, typename detail::holder< typename alias_tag<T>::type >::type> : std::is_same<typename alias_tag<T>::type, Tag> {}; namespace detail { template<class T> constexpr T& lvalue(T& x) noexcept { return x; } template<class T> constexpr const T& lvalue(const T& x) noexcept { return x; } } #define BOOST_HOF_UNARY_PERFECT_FOREACH(m) \ m(const&, boost::hof::detail::lvalue) \ m(&, boost::hof::detail::lvalue) \ m(&&, boost::hof::move) \ template<class T, class Tag=void> struct alias { T value; BOOST_HOF_DELEGATE_CONSTRUCTOR(alias, T, value) }; #define BOOST_HOF_DETAIL_ALIAS_GET_VALUE(ref, move) \ template<class Tag, class T, class... Ts> \ constexpr auto alias_value(alias<T, Tag> ref a, Ts&&...) BOOST_HOF_RETURNS(move(a.value)) BOOST_HOF_UNARY_PERFECT_FOREACH(BOOST_HOF_DETAIL_ALIAS_GET_VALUE) template<class T, class Tag> struct alias_tag<alias<T, Tag>> { typedef Tag type; }; template<class T, class Tag=void> struct alias_inherit #if (defined(__GNUC__) && !defined (__clang__)) : std::conditional<(std::is_class<T>::value), T, alias<T>>::type #else : T #endif { BOOST_HOF_INHERIT_CONSTRUCTOR(alias_inherit, T) }; #define BOOST_HOF_DETAIL_ALIAS_INHERIT_GET_VALUE(ref, move) \ template<class Tag, class T, class... Ts, class=typename std::enable_if<(BOOST_HOF_IS_CLASS(T))>::type> \ constexpr T ref alias_value(alias_inherit<T, Tag> ref a, Ts&&...) BOOST_HOF_RETURNS_DEDUCE_NOEXCEPT(move(a)) \ { \ return move(a); \ } BOOST_HOF_UNARY_PERFECT_FOREACH(BOOST_HOF_DETAIL_ALIAS_INHERIT_GET_VALUE) template<class T, class Tag> struct alias_tag<alias_inherit<T, Tag>> { typedef Tag type; }; namespace detail { template<class T, class Tag> struct alias_static_storage { #ifdef _MSC_VER // Since we disable the error for 4579 on MSVC, which leaves the static // member unitialized at runtime, it is, therefore, only safe to use this // class on types that are empty with constructors that have no possible // side effects. static_assert(BOOST_HOF_IS_EMPTY(T) && BOOST_HOF_IS_LITERAL(T) && BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(T), "In-class initialization is not yet implemented on MSVC"); #endif static constexpr T value = T(); }; template<class T, class Tag> constexpr T alias_static_storage<T, Tag>::value; } template<class T, class Tag=void> struct alias_static { template<class... Ts, BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(T, Ts...)> constexpr alias_static(Ts&&...) noexcept {} }; template<class Tag, class T, class... Ts> constexpr const T& alias_value(const alias_static<T, Tag>&, Ts&&...) noexcept { return detail::alias_static_storage<T, Tag>::value; } template<class T, class Tag> struct alias_tag<alias_static<T, Tag>> { typedef Tag type; }; namespace detail { template<class T, class Tag> struct alias_try_inherit : std::conditional<(BOOST_HOF_IS_CLASS(T) && !BOOST_HOF_IS_FINAL(T) && !BOOST_HOF_IS_POLYMORPHIC(T)), alias_inherit<T, Tag>, alias<T, Tag> > {}; #if BOOST_HOF_HAS_EBO template<class T, class Tag> struct alias_empty : std::conditional<(BOOST_HOF_IS_EMPTY(T)), typename alias_try_inherit<T, Tag>::type, alias<T, Tag> > {}; #else template<class T, class Tag> struct alias_empty : std::conditional< BOOST_HOF_IS_EMPTY(T) && BOOST_HOF_IS_LITERAL(T) && BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(T), alias_static<T, Tag>, alias<T, Tag> > {}; #endif } }} // namespace boost::hof #ifdef _MSC_VER #pragma warning(pop) #endif #endif