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Do not treat std::array as implicitly reflectable (#169) #234

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18 changes: 15 additions & 3 deletions include/boost/pfr/detail/possible_reflectable.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,11 +11,23 @@
#include <boost/pfr/traits_fwd.hpp>

#if !defined(BOOST_PFR_INTERFACE_UNIT)
#include <array> // for std::array
#include <cstddef> // for std::size_t
#include <type_traits> // for std::is_aggregate
#endif

namespace boost { namespace pfr { namespace detail {

// std::array<T, N> is an aggregate, but it stores its elements in a C array

@apolukhin apolukhin Jun 11, 2026

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The workaround is too specific... How about treating all the types with name array (std::array, boost::array, ...) as non reflectable?

This could be done in the following way:

template <class T>
constexpr std::false_type is_array(int);

template <class T>
constexpr auto is_array(long) -> decltype(std::declval<T>().~array(), std::true_type{});

Call it as decltype(detail::is_array<T>(1L))::value

That way there's no need to include <array> and it would work with boost::array too

// data member. Boost.PFR can not reflect types with C array members yet (see
// https://github.com/boostorg/pfr/issues/20), so std::array must not be
// treated as implicitly reflectable.
template <class T>
struct is_stdarray : std::false_type {};

template <class T, std::size_t N>
struct is_stdarray<std::array<T, N>> : std::true_type {};

///////////////////// Returns false when the type exactly wasn't be reflectable
template <class T, class WhatFor>
constexpr decltype(is_reflectable<T, WhatFor>::value) possible_reflectable(long) noexcept {
Expand All @@ -26,11 +38,11 @@ constexpr decltype(is_reflectable<T, WhatFor>::value) possible_reflectable(long)

template <class T, class WhatFor>
constexpr bool possible_reflectable(int) noexcept {
# if defined(__cpp_lib_is_aggregate)
using type = std::remove_cv_t<T>;
return std::is_aggregate<type>();
# if defined(__cpp_lib_is_aggregate)
return std::is_aggregate<type>() && !detail::is_stdarray<type>::value;
# else
return true;
return !detail::is_stdarray<type>::value;
# endif
}

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11 changes: 11 additions & 0 deletions test/core/run/is_implicitly_reflectable.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
// 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)

#include <array>
#include <iostream>
#include <boost/pfr/traits.hpp>
#include <type_traits> // for std::true_type, std::false_type and std::is_aggregate
Expand Down Expand Up @@ -71,6 +72,12 @@ int main() {
assert_non_reflectable<Nonrefrectable, tag>();
assert_reflectable<ReflectableBoostJson, tag>();
assert_non_reflectable<NonrefrectableBoostJson, tag>();

// std::array is an aggregate, but it holds a C array member that
// Boost.PFR can not reflect correctly, so it must not be considered
// implicitly reflectable (https://github.com/boostorg/pfr/issues/169).
assert_non_reflectable<std::array<int, 3>, tag>();
assert_non_reflectable<std::array<char, 1>, tag>();
}

{
Expand All @@ -81,6 +88,10 @@ int main() {
assert_non_reflectable<Nonrefrectable, tag>();
assert_reflectable<ReflectableBoostFusion, tag>();
assert_non_reflectable<NonrefrectableBoostFusion, tag>();

// See the comment in the boost_json_tag block above.
assert_non_reflectable<std::array<int, 3>, tag>();
assert_non_reflectable<std::array<char, 1>, tag>();
}
#endif // #if BOOST_PFR_ENABLE_IMPLICIT_REFLECTION
}
Expand Down
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