pfr/detail/offset_based_getter.hpp

0001 // Copyright (c) 2017-2018 Chris Beck
0002 // Copyright (c) 2019-2023 Antony Polukhin
0003 //
0004 // Distributed under the Boost Software License, Version 1.0. (See accompanying
0005 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
0006
0007 #ifndef BOOST_PFR_DETAIL_OFFSET_BASED_GETTER_HPP
0008 #define BOOST_PFR_DETAIL_OFFSET_BASED_GETTER_HPP
0009 #pragma once
0010
0011 #include <boost/pfr/detail/config.hpp>
0012
0013 #include <type_traits>
0014 #include <utility>
0015 #include <memory>  // std::addressof
0016 #include <boost/pfr/detail/sequence_tuple.hpp>
0017 #include <boost/pfr/detail/rvalue_t.hpp>
0018 #include <boost/pfr/detail/size_t_.hpp>
0019
0020
0021 namespace boost { namespace pfr { namespace detail {
0022
0023 // Our own implementation of std::aligned_storage. On godbolt with MSVC, I have compilation errors
0024 // using the standard version, it seems the compiler cannot generate default ctor.
0025
0026 template<std::size_t s, std::size_t a>
0027 struct internal_aligned_storage {
0028    alignas(a) char storage_[s];
0029 };
0030
0031 // Metafunction that replaces tuple<T1, T2, T3, ...> with
0032 // tuple<std::aligned_storage_t<sizeof(T1), alignof(T1)>, std::aligned_storage<sizeof(T2), alignof(T2)>, ...>
0033 //
0034 // The point is, the new tuple is "layout compatible" in the sense that members have the same offsets,
0035 // but this tuple is constexpr constructible.
0036
0037 template <typename T>
0038 struct tuple_of_aligned_storage;
0039
0040 template <typename... Ts>
0041 struct tuple_of_aligned_storage<sequence_tuple::tuple<Ts...>> {
0042   using type = sequence_tuple::tuple<internal_aligned_storage<sizeof(Ts),
0043 #if defined(__GNUC__) && __GNUC__ < 8 && !defined(__x86_64__) && !defined(__CYGWIN__)
0044       // Before GCC-8 the `alignof` was returning the optimal alignment rather than the minimal one.
0045       // We have to adjust the alignment because otherwise we get the wrong offset.
0046       (alignof(Ts) > 4 ? 4 : alignof(Ts))
0047 #else
0048       alignof(Ts)
0049 #endif
0050   >...>;
0051 };
0052
0053 // Note: If pfr has a typelist also, could also have an overload for that here
0054
0055 template <typename T>
0056 using tuple_of_aligned_storage_t = typename tuple_of_aligned_storage<T>::type;
0057
0058 /***
0059  * Given a structure type and its sequence of members, we want to build a function
0060  * object "getter" that implements a version of `std::get` using offset arithmetic
0061  * and reinterpret_cast.
0062  *
0063  * typename U should be a user-defined struct
0064  * typename S should be a sequence_tuple which is layout compatible with U
0065  */
0066
0067 template <typename U, typename S>
0068 class offset_based_getter {
0069   using this_t = offset_based_getter<U, S>;
0070
0071   static_assert(sizeof(U) == sizeof(S), "====================> Boost.PFR: Member sequence does not indicate correct size for struct type! Maybe the user-provided type is not a SimpleAggregate?");
0072   static_assert(alignof(U) == alignof(S), "====================> Boost.PFR: Member sequence does not indicate correct alignment for struct type!");
0073
0074   static_assert(!std::is_const<U>::value, "====================> Boost.PFR: const should be stripped from user-defined type when using offset_based_getter or overload resolution will be ambiguous later, this indicates an error within pfr");
0075   static_assert(!std::is_reference<U>::value, "====================> Boost.PFR: reference should be stripped from user-defined type when using offset_based_getter or overload resolution will be ambiguous later, this indicates an error within pfr");
0076   static_assert(!std::is_volatile<U>::value, "====================> Boost.PFR: volatile should be stripped from user-defined type when using offset_based_getter or overload resolution will be ambiguous later. this indicates an error within pfr");
0077
0078   // Get type of idx'th member
0079   template <std::size_t idx>
0080   using index_t = typename sequence_tuple::tuple_element<idx, S>::type;
0081
0082   // Get offset of idx'th member
0083   // Idea: Layout object has the same offsets as instance of S, so if S and U are layout compatible, then these offset
0084   // calculations are correct.
0085   template <std::size_t idx>
0086   static constexpr std::ptrdiff_t offset() noexcept {
0087     constexpr tuple_of_aligned_storage_t<S> layout{};
0088     return &sequence_tuple::get<idx>(layout).storage_[0] - &sequence_tuple::get<0>(layout).storage_[0];
0089   }
0090
0091   // Encapsulates offset arithmetic and reinterpret_cast
0092   template <std::size_t idx>
0093   static index_t<idx> * get_pointer(U * u) noexcept {
0094     return reinterpret_cast<index_t<idx> *>(reinterpret_cast<char *>(u) + this_t::offset<idx>());
0095   }
0096
0097   template <std::size_t idx>
0098   static const index_t<idx> * get_pointer(const U * u) noexcept {
0099     return reinterpret_cast<const index_t<idx> *>(reinterpret_cast<const char *>(u) + this_t::offset<idx>());
0100   }
0101
0102   template <std::size_t idx>
0103   static volatile index_t<idx> * get_pointer(volatile U * u) noexcept {
0104     return reinterpret_cast<volatile index_t<idx> *>(reinterpret_cast<volatile char *>(u) + this_t::offset<idx>());
0105   }
0106
0107   template <std::size_t idx>
0108   static const volatile index_t<idx> * get_pointer(const volatile U * u) noexcept {
0109     return reinterpret_cast<const volatile index_t<idx> *>(reinterpret_cast<const volatile char *>(u) + this_t::offset<idx>());
0110   }
0111
0112 public:
0113   template <std::size_t idx>
0114   index_t<idx> & get(U & u, size_t_<idx>) const noexcept {
0115     return *this_t::get_pointer<idx>(std::addressof(u));
0116   }
0117
0118   template <std::size_t idx>
0119   index_t<idx> const & get(U const & u, size_t_<idx>) const noexcept {
0120     return *this_t::get_pointer<idx>(std::addressof(u));
0121   }
0122
0123   template <std::size_t idx>
0124   index_t<idx> volatile & get(U volatile & u, size_t_<idx>) const noexcept {
0125     return *this_t::get_pointer<idx>(std::addressof(u));
0126   }
0127
0128   template <std::size_t idx>
0129   index_t<idx> const volatile & get(U const volatile & u, size_t_<idx>) const noexcept {
0130     return *this_t::get_pointer<idx>(std::addressof(u));
0131   }
0132
0133   // rvalues must not be used here, to avoid template instantiation bloats.
0134   template <std::size_t idx>
0135   index_t<idx> && get(rvalue_t<U> u, size_t_<idx>) const = delete;
0136 };
0137
0138
0139 }}} // namespace boost::pfr::detail
0140
0141 #endif // BOOST_PFR_DETAIL_OFFSET_LIST_HPP