boost/pfr/detail/offset_based_getter.hpp
// Copyright (c) 2017-2018 Chris Beck
// Copyright (c) 2019-2024 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 <type_traits>
#include <utility>
#include <memory> // std::addressof
#include <boost/pfr/detail/sequence_tuple.hpp>
#include <boost/pfr/detail/rvalue_t.hpp>
#include <boost/pfr/detail/size_t_.hpp>
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