text: // Copyright (c) 2016-2021 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 PFR_CORE_HPP
  #define PFR_CORE_HPP
  #pragma once
  
  #include <pfr/detail/config.hpp>
  
  #include <pfr/detail/core.hpp>
  
  #include <pfr/detail/sequence_tuple.hpp>
  #include <pfr/detail/stdtuple.hpp>
  #include <pfr/detail/for_each_field_impl.hpp>
  #include <pfr/detail/make_integer_sequence.hpp>
  #include <pfr/detail/tie_from_structure_tuple.hpp>
  
  #include <type_traits>
  #include <utility>      // metaprogramming stuff
  
  #include <pfr/tuple_size.hpp>
  
  /// \file pfr/core.hpp
  /// Contains all the basic tuple-like interfaces \forcedlink{get}, \forcedlink{tuple_size}, \forcedlink{tuple_element_t}, and others.
  ///
  /// \b Synopsis:
  
  namespace pfr {
  
  /// \brief Returns reference or const reference to a field with index `I` in \aggregate `val`.
  ///
  /// \b Example:
  /// \code
  ///     struct my_struct { int i, short s; };
  ///     my_struct s {10, 11};
  ///     assert(pfr::get<0>(s) == 10);
  ///     pfr::get<1>(s) = 0;
  /// \endcode
  template <std::size_t I, class T>
  constexpr decltype(auto) get(const T& val) noexcept {
      return detail::sequence_tuple::get<I>( detail::tie_as_tuple(val) );
  }
  
  
  /// \overload get
  template <std::size_t I, class T>
  constexpr decltype(auto) get(T& val
  #if !PFR_USE_CPP17
      , std::enable_if_t<std::is_assignable<T, T>::value>* = nullptr
  #endif
  ) noexcept {
      return detail::sequence_tuple::get<I>( detail::tie_as_tuple(val) );
  }
  
  #if !PFR_USE_CPP17
  /// \overload get
  template <std::size_t I, class T>
  constexpr auto get(T&, std::enable_if_t<!std::is_assignable<T, T>::value>* = nullptr) noexcept {
      static_assert(sizeof(T) && false, "====================> Boost.PFR: Calling pfr::get on non const non assignable type is allowed only in C++17");
      return 0;
  }
  #endif
  
  
  /// \overload get
  template <std::size_t I, class T>
  constexpr auto get(T&& val, std::enable_if_t< std::is_rvalue_reference<T&&>::value>* = 0) noexcept {
      return std::move(detail::sequence_tuple::get<I>( detail::tie_as_tuple(val) ));
  }
  
  
  /// \brief `tuple_element` has a member typedef `type` that returns the type of a field with index I in \aggregate T.
  ///
  /// \b Example:
  /// \code
  ///     std::vector< pfr::tuple_element<0, my_structure>::type > v;
  /// \endcode
  template <std::size_t I, class T>
  using tuple_element = detail::sequence_tuple::tuple_element<I, decltype( ::pfr::detail::tie_as_tuple(std::declval<T&>()) ) >;
  
  
  /// \brief Type of a field with index `I` in \aggregate `T`.
  ///
  /// \b Example:
  /// \code
  ///     std::vector< pfr::tuple_element_t<0, my_structure> > v;
  /// \endcode
  template <std::size_t I, class T>
  using tuple_element_t = typename tuple_element<I, T>::type;
  
  
  /// \brief Creates a `std::tuple` from fields of an \aggregate `val`.
  ///
  /// \b Example:
  /// \code
  ///     struct my_struct { int i, short s; };
  ///     my_struct s {10, 11};
  ///     std::tuple<int, short> t = make_tuple(s);
  ///     assert(get<0>(t) == 10);
  /// \endcode
  template <class T>
  constexpr auto structure_to_tuple(const T& val) noexcept {
      return detail::make_stdtuple_from_tietuple(
          detail::tie_as_tuple(val),
          detail::make_index_sequence< tuple_size_v<T> >()
      );
  }
  
  
  /// \brief std::tie` like function that ties fields of a structure.
  ///
  /// \returns a `std::tuple` with lvalue and const lvalue references to fields of an \aggregate `val`.
  ///
  /// \b Example:
  /// \code
  ///     void foo(const int&, const short&);
  ///     struct my_struct { int i, short s; };
  ///
  ///     const my_struct const_s{1, 2};
  ///     std::apply(foo, structure_tie(const_s));
  ///
  ///     my_struct s;
  ///     structure_tie(s) = std::tuple<int, short>{10, 11};
  ///     assert(s.s == 11);
  /// \endcode
  template <class T>
  constexpr auto structure_tie(const T& val) noexcept {
      return detail::make_conststdtiedtuple_from_tietuple(
          detail::tie_as_tuple(const_cast<T&>(val)),
          detail::make_index_sequence< tuple_size_v<T> >()
      );
  }
  
  
  /// \overload structure_tie
  template <class T>
  constexpr auto structure_tie(T& val
  #if !PFR_USE_CPP17
      , std::enable_if_t<std::is_assignable<T, T>::value>* = nullptr
  #endif
  ) noexcept {
      return detail::make_stdtiedtuple_from_tietuple(
          detail::tie_as_tuple(val),
          detail::make_index_sequence< tuple_size_v<T> >()
      );
  }
  
  #if !PFR_USE_CPP17
  /// \overload structure_tie
  template <class T>
  constexpr auto structure_tie(T&, std::enable_if_t<!std::is_assignable<T, T>::value>* = nullptr) noexcept {
      static_assert(sizeof(T) && false, "====================> Boost.PFR: Calling pfr::structure_tie on non const non assignable type is allowed only in C++17");
      return 0;
  }
  #endif
  
  
  /// \overload structure_tie
  template <class T>
  constexpr auto structure_tie(T&&, std::enable_if_t< std::is_rvalue_reference<T&&>::value>* = 0) noexcept {
      static_assert(sizeof(T) && false, "====================> Boost.PFR: Calling pfr::structure_tie on rvalue references is forbidden");
      return 0;
  }
  
  /// Calls `func` for each field of a `value`.
  ///
  /// \param func must have one of the following signatures:
  ///     * any_return_type func(U&& field)                // field of value is perfect forwarded to function
  ///     * any_return_type func(U&& field, std::size_t i)
  ///     * any_return_type func(U&& value, I i)           // Here I is an `std::integral_constant<size_t, field_index>`
  ///
  /// \param value To each field of this variable will be the `func` applied.
  ///
  /// \b Example:
  /// \code
  ///     struct my_struct { int i, short s; };
  ///     int sum = 0;
  ///     for_each_field(my_struct{20, 22}, [&sum](const auto& field) { sum += field; });
  ///     assert(sum == 42);
  /// \endcode
  template <class T, class F>
  void for_each_field(T&& value, F&& func) {
      constexpr std::size_t fields_count_val = pfr::detail::fields_count<std::remove_reference_t<T>>();
  
      ::pfr::detail::for_each_field_dispatcher(
          value,
          [f = std::forward<F>(func)](auto&& t) mutable {
              // MSVC related workaround. Its lambdas do not capture constexprs.
              constexpr std::size_t fields_count_val_in_lambda
                  = pfr::detail::fields_count<std::remove_reference_t<T>>();
  
              ::pfr::detail::for_each_field_impl(
                  t,
                  std::forward<F>(f),
                  detail::make_index_sequence<fields_count_val_in_lambda>{},
                  std::is_rvalue_reference<T&&>{}
              );
          },
          detail::make_index_sequence<fields_count_val>{}
      );
  }
  
  /// \brief std::tie-like function that allows assigning to tied values from aggregates.
  ///
  /// \returns an object with lvalue references to `args...`; on assignment of an \aggregate value to that
  /// object each field of an aggregate is assigned to the corresponding `args...` reference.
  ///
  /// \b Example:
  /// \code
  ///     auto f() {
  ///       struct { struct { int x, y } p; short s; } res { { 4, 5 }, 6 };
  ///       return res;
  ///     }
  ///     auto [p, s] = f();
  ///     tie_from_structure(p, s) = f();
  /// \endcode
  template <typename... Elements>
  constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Elements&... args) noexcept {
      return detail::tie_from_structure_tuple<Elements...>(args...);
  }
  
  } // namespace pfr
  
  #endif // PFR_CORE_HPP
  