Boost C++ Libraries

boost/pfr/detail/core14_loophole.hpp

// Copyright (c) 2017-2018 Alexandr Poltavsky, Antony Polukhin.
// Copyright (c) 2019-2022 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)


// The Great Type Loophole (C++14)
// Initial implementation by Alexandr Poltavsky, http://alexpolt.github.io
//
// Description: 
//  The Great Type Loophole is a technique that allows to exchange type information with template
//  instantiations. Basically you can assign and read type information during compile time.
//  Here it is used to detect data members of a data type. I described it for the first time in
//  this blog post http://alexpolt.github.io/type-loophole.html .
//
// This technique exploits the http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2118
// CWG 2118. Stateful metaprogramming via friend injection
// Note: CWG agreed that such techniques should be ill-formed, although the mechanism for prohibiting them is as yet undetermined.

#ifndef BOOST_PFR_DETAIL_CORE14_LOOPHOLE_HPP
#define BOOST_PFR_DETAIL_CORE14_LOOPHOLE_HPP

#include <boost/pfr/detail/config.hpp>

#include <type_traits>
#include <utility>

#include <boost/pfr/detail/cast_to_layout_compatible.hpp> // still needed for enums
#include <boost/pfr/detail/offset_based_getter.hpp>
#include <boost/pfr/detail/fields_count.hpp>
#include <boost/pfr/detail/make_flat_tuple_of_references.hpp>
#include <boost/pfr/detail/make_integer_sequence.hpp>
#include <boost/pfr/detail/sequence_tuple.hpp>
#include <boost/pfr/detail/rvalue_t.hpp>
#include <boost/pfr/detail/unsafe_declval.hpp>


#ifdef __clang__
#   pragma clang diagnostic push
#   pragma clang diagnostic ignored "-Wmissing-braces"
#   pragma clang diagnostic ignored "-Wundefined-inline"
#   pragma clang diagnostic ignored "-Wundefined-internal"
#   pragma clang diagnostic ignored "-Wmissing-field-initializers"
#elif defined(__GNUC__)
#   pragma GCC diagnostic push
#   pragma GCC diagnostic ignored "-Wnon-template-friend"
#endif


namespace boost { namespace pfr { namespace detail {

// tag<T,N> generates friend declarations and helps with overload resolution.
// There are two types: one with the auto return type, which is the way we read types later.
// The second one is used in the detection of instantiations without which we'd get multiple
// definitions.

template <class T, std::size_t N>
struct tag {
    friend auto loophole(tag<T,N>);
};

// The definitions of friend functions.
template <class T, class U, std::size_t N, bool B>
struct fn_def_lref {
    friend auto loophole(tag<T,N>) {
        // Standard Library containers do not SFINAE on invalid copy constructor. Because of that std::vector<std::unique_ptr<int>> reports that it is copyable,
        // which leads to an instantiation error at this place.
        //
        // To workaround the issue, we check that the type U is movable, and move it in that case.
        using no_extents_t = std::remove_all_extents_t<U>;
        return static_cast< std::conditional_t<std::is_move_constructible<no_extents_t>::value, no_extents_t&&, no_extents_t&> >(
            boost::pfr::detail::unsafe_declval<no_extents_t&>()
        );
    }
};
template <class T, class U, std::size_t N, bool B>
struct fn_def_rref {
    friend auto loophole(tag<T,N>) { return std::move(boost::pfr::detail::unsafe_declval< std::remove_all_extents_t<U>& >()); }
};


// Those specializations are to avoid multiple definition errors.
template <class T, class U, std::size_t N>
struct fn_def_lref<T, U, N, true> {};

template <class T, class U, std::size_t N>
struct fn_def_rref<T, U, N, true> {};


// This has a templated conversion operator which in turn triggers instantiations.
// Important point, using sizeof seems to be more reliable. Also default template
// arguments are "cached" (I think). To fix that I provide a U template parameter to
// the ins functions which do the detection using constexpr friend functions and SFINAE.
template <class T, std::size_t N>
struct loophole_ubiq_lref {
    template<class U, std::size_t M> static std::size_t ins(...);
    template<class U, std::size_t M, std::size_t = sizeof(loophole(tag<T,M>{})) > static char ins(int);

    template<class U, std::size_t = sizeof(fn_def_lref<T, U, N, sizeof(ins<U, N>(0)) == sizeof(char)>)>
    constexpr operator U&() const&& noexcept; // `const&&` here helps to avoid ambiguity in loophole instantiations. optional_like test validate that behavior.
};

template <class T, std::size_t N>
struct loophole_ubiq_rref {
    template<class U, std::size_t M> static std::size_t ins(...);
    template<class U, std::size_t M, std::size_t = sizeof(loophole(tag<T,M>{})) > static char ins(int);

    template<class U, std::size_t = sizeof(fn_def_rref<T, U, N, sizeof(ins<U, N>(0)) == sizeof(char)>)>
    constexpr operator U&&() const&& noexcept; // `const&&` here helps to avoid ambiguity in loophole instantiations. optional_like test validate that behavior.
};


// This is a helper to turn a data structure into a tuple.
template <class T, class U>
struct loophole_type_list_lref;

template <typename T, std::size_t... I>
struct loophole_type_list_lref< T, std::index_sequence<I...> >
     // Instantiating loopholes:
    : sequence_tuple::tuple< decltype(T{ loophole_ubiq_lref<T, I>{}... }, 0) >
{
    using type = sequence_tuple::tuple< decltype(loophole(tag<T, I>{}))... >;
};


template <class T, class U>
struct loophole_type_list_rref;

template <typename T, std::size_t... I>
struct loophole_type_list_rref< T, std::index_sequence<I...> >
     // Instantiating loopholes:
    : sequence_tuple::tuple< decltype(T{ loophole_ubiq_rref<T, I>{}... }, 0) >
{
    using type = sequence_tuple::tuple< decltype(loophole(tag<T, I>{}))... >;
};


// Lazily returns loophole_type_list_{lr}ref.
template <bool IsCopyConstructible /*= true*/, class T, class U>
struct loophole_type_list_selector {
    using type = loophole_type_list_lref<T, U>;
};

template <class T, class U>
struct loophole_type_list_selector<false /*IsCopyConstructible*/, T, U> {
    using type = loophole_type_list_rref<T, U>;
};

template <class T>
auto tie_as_tuple_loophole_impl(T& lvalue) noexcept {
    using type = std::remove_cv_t<std::remove_reference_t<T>>;
    using indexes = detail::make_index_sequence<fields_count<type>()>;
    using loophole_type_list = typename detail::loophole_type_list_selector<
        std::is_copy_constructible<std::remove_all_extents_t<type>>::value, type, indexes
    >::type;
    using tuple_type = typename loophole_type_list::type;

    return boost::pfr::detail::make_flat_tuple_of_references(
        lvalue,
        offset_based_getter<type, tuple_type>{},
        size_t_<0>{},
        size_t_<tuple_type::size_v>{}
    );
}

template <class T>
auto tie_as_tuple(T& val) noexcept {
    static_assert(
        !std::is_union<T>::value,
        "====================> Boost.PFR: For safety reasons it is forbidden to reflect unions. See `Reflection of unions` section in the docs for more info."
    );
    return boost::pfr::detail::tie_as_tuple_loophole_impl(
        val
    );
}

template <class T, class F, std::size_t... I>
void for_each_field_dispatcher(T& t, F&& f, std::index_sequence<I...>) {
    static_assert(
        !std::is_union<T>::value,
        "====================> Boost.PFR: For safety reasons it is forbidden to reflect unions. See `Reflection of unions` section in the docs for more info."
    );
    std::forward<F>(f)(
        boost::pfr::detail::tie_as_tuple_loophole_impl(t)
    );
}

}}} // namespace boost::pfr::detail


#ifdef __clang__
#   pragma clang diagnostic pop
#elif defined(__GNUC__)
#   pragma GCC diagnostic pop
#endif


#endif // BOOST_PFR_DETAIL_CORE14_LOOPHOLE_HPP