整理

include/boost/pfr/detail/offset_based_getter.hpp

@@ -0,0 +1,143 @@
+// Copyright (c) 2017-2018 Chris Beck
+// Copyright (c) 2019-2025 Antony Polukhin
+//
+// 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_PFR_DETAIL_OFFSET_BASED_GETTER_HPP
+#define BOOST_PFR_DETAIL_OFFSET_BASED_GETTER_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#include <boost/pfr/detail/sequence_tuple.hpp>
+#include <boost/pfr/detail/rvalue_t.hpp>
+#include <boost/pfr/detail/size_t_.hpp>
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#include <utility>
+#include <memory>  // std::addressof
+#endif
+
+namespace boost { namespace pfr { namespace detail {
+
+// Our own implementation of std::aligned_storage. On godbolt with MSVC, I have compilation errors
+// using the standard version, it seems the compiler cannot generate default ctor.
+
+template<std::size_t s, std::size_t a>
+struct internal_aligned_storage {
+   alignas(a) char storage_[s];
+};
+
+// Metafunction that replaces tuple<T1, T2, T3, ...> with
+// tuple<std::aligned_storage_t<sizeof(T1), alignof(T1)>, std::aligned_storage<sizeof(T2), alignof(T2)>, ...>
+//
+// The point is, the new tuple is "layout compatible" in the sense that members have the same offsets,
+// but this tuple is constexpr constructible.
+
+template <typename T>
+struct tuple_of_aligned_storage;
+
+template <typename... Ts>
+struct tuple_of_aligned_storage<sequence_tuple::tuple<Ts...>> {
+  using type = sequence_tuple::tuple<internal_aligned_storage<sizeof(Ts),
+#if defined(__GNUC__) && __GNUC__ < 8 && !defined(__x86_64__) && !defined(__CYGWIN__)
+      // Before GCC-8 the `alignof` was returning the optimal alignment rather than the minimal one.
+      // We have to adjust the alignment because otherwise we get the wrong offset.
+      (alignof(Ts) > 4 ? 4 : alignof(Ts))
+#else
+      alignof(Ts)
+#endif
+  >...>;
+};
+
+// Note: If pfr has a typelist also, could also have an overload for that here
+
+template <typename T>
+using tuple_of_aligned_storage_t = typename tuple_of_aligned_storage<T>::type;
+
+/***
+ * Given a structure type and its sequence of members, we want to build a function
+ * object "getter" that implements a version of `std::get` using offset arithmetic
+ * and reinterpret_cast.
+ *
+ * typename U should be a user-defined struct
+ * typename S should be a sequence_tuple which is layout compatible with U
+ */
+
+template <typename U, typename S>
+class offset_based_getter {
+  using this_t = offset_based_getter<U, S>;
+
+  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?");
+  static_assert(alignof(U) == alignof(S), "====================> Boost.PFR: Member sequence does not indicate correct alignment for struct type!");
+
+  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");
+  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");
+  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");
+
+  // Get type of idx'th member
+  template <std::size_t idx>
+  using index_t = typename sequence_tuple::tuple_element<idx, S>::type;
+
+  // Get offset of idx'th member
+  // Idea: Layout object has the same offsets as instance of S, so if S and U are layout compatible, then these offset
+  // calculations are correct.
+  template <std::size_t idx>
+  static constexpr std::ptrdiff_t offset() noexcept {
+    constexpr tuple_of_aligned_storage_t<S> layout{};
+    return &sequence_tuple::get<idx>(layout).storage_[0] - &sequence_tuple::get<0>(layout).storage_[0];
+  }
+
+  // Encapsulates offset arithmetic and reinterpret_cast
+  template <std::size_t idx>
+  static index_t<idx> * get_pointer(U * u) noexcept {
+    return reinterpret_cast<index_t<idx> *>(reinterpret_cast<char *>(u) + this_t::offset<idx>());
+  }
+
+  template <std::size_t idx>
+  static const index_t<idx> * get_pointer(const U * u) noexcept {
+    return reinterpret_cast<const index_t<idx> *>(reinterpret_cast<const char *>(u) + this_t::offset<idx>());
+  }
+
+  template <std::size_t idx>
+  static volatile index_t<idx> * get_pointer(volatile U * u) noexcept {
+    return reinterpret_cast<volatile index_t<idx> *>(reinterpret_cast<volatile char *>(u) + this_t::offset<idx>());
+  }
+
+  template <std::size_t idx>
+  static const volatile index_t<idx> * get_pointer(const volatile U * u) noexcept {
+    return reinterpret_cast<const volatile index_t<idx> *>(reinterpret_cast<const volatile char *>(u) + this_t::offset<idx>());
+  }
+
+public:
+  template <std::size_t idx>
+  index_t<idx> & get(U & u, size_t_<idx>) const noexcept {
+    return *this_t::get_pointer<idx>(std::addressof(u));
+  }
+
+  template <std::size_t idx>
+  index_t<idx> const & get(U const & u, size_t_<idx>) const noexcept {
+    return *this_t::get_pointer<idx>(std::addressof(u));
+  }
+
+  template <std::size_t idx>
+  index_t<idx> volatile & get(U volatile & u, size_t_<idx>) const noexcept {
+    return *this_t::get_pointer<idx>(std::addressof(u));
+  }
+
+  template <std::size_t idx>
+  index_t<idx> const volatile & get(U const volatile & u, size_t_<idx>) const noexcept {
+    return *this_t::get_pointer<idx>(std::addressof(u));
+  }
+
+  // rvalues must not be used here, to avoid template instantiation bloats.
+  template <std::size_t idx>
+  index_t<idx> && get(rvalue_t<U> u, size_t_<idx>) const = delete;
+};
+
+
+}}} // namespace boost::pfr::detail
+
+#endif // BOOST_PFR_DETAIL_OFFSET_LIST_HPP