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Support any-rank C arrays in from_json, not just rank 1-4

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from_json() for C arrays had four hand-unrolled overloads (rank 1-4,
added incrementally in #4262), each with its own nested loops. to_json()
already handles any rank recursively, so a rank-5+ C array could be
serialized but not read back with get_to()/get<>().

Replace the four overloads with one from_json() SFINAE-constrained on
get<remove_all_extents<T>::type>() existing, forwarding to a pair of
mutually recursive from_json_c_array_element() helpers: one assigns a
non-array element via get<T>(), the other loops over a array element and
recurses one dimension at a time. Each dimension still goes through at(),
so type_error.304/out_of_range.401 stay unchanged; ranks 1-4 keep their
existing behavior and semantics.

Adds rank-5 round-trip and mismatched-shape tests to unit-conversions.cpp.

Public API: additive only (rank 5+ C arrays become readable).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

#5708 item 1
This commit is contained in:
Niels Lohmann
2026-09-30 18:00:16 +02:00
parent b5fad30499
commit 23aa069e63
3 changed files with 61 additions and 96 deletions
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include/nlohmann/detail/conversions/from_json.hpp
+15 -48
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@@ -206,62 +206,29 @@ inline void from_json(const BasicJsonType& j, std::valarray<T>& l)
});
}
// element is not itself a C array: read it directly
template<typename BasicJsonType, typename T>
auto from_json_c_array_element(const BasicJsonType& j, T& e)
-> decltype(e = j.template get<T>(), void())
{
e = j.template get<T>();
}
// element is itself a C array: recurse one dimension at a time, so any rank is supported
template<typename BasicJsonType, typename T, std::size_t N>
auto from_json(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-> decltype(j.template get<T>(), void())
void from_json_c_array_element(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
{
for (std::size_t i = 0; i < N; ++i)
{
arr[i] = j.at(i).template get<T>();
from_json_c_array_element(j.at(i), arr[i]);
}
}
template<typename BasicJsonType, typename T, std::size_t N1, std::size_t N2>
auto from_json(const BasicJsonType& j, T (&arr)[N1][N2]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-> decltype(j.template get<T>(), void())
template<typename BasicJsonType, typename T, std::size_t N>
auto from_json(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-> decltype(j.template get<typename std::remove_all_extents<T>::type>(), void())
{
for (std::size_t i1 = 0; i1 < N1; ++i1)
{
for (std::size_t i2 = 0; i2 < N2; ++i2)
{
arr[i1][i2] = j.at(i1).at(i2).template get<T>();
}
}
}
template<typename BasicJsonType, typename T, std::size_t N1, std::size_t N2, std::size_t N3>
auto from_json(const BasicJsonType& j, T (&arr)[N1][N2][N3]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-> decltype(j.template get<T>(), void())
{
for (std::size_t i1 = 0; i1 < N1; ++i1)
{
for (std::size_t i2 = 0; i2 < N2; ++i2)
{
for (std::size_t i3 = 0; i3 < N3; ++i3)
{
arr[i1][i2][i3] = j.at(i1).at(i2).at(i3).template get<T>();
}
}
}
}
template<typename BasicJsonType, typename T, std::size_t N1, std::size_t N2, std::size_t N3, std::size_t N4>
auto from_json(const BasicJsonType& j, T (&arr)[N1][N2][N3][N4]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-> decltype(j.template get<T>(), void())
{
for (std::size_t i1 = 0; i1 < N1; ++i1)
{
for (std::size_t i2 = 0; i2 < N2; ++i2)
{
for (std::size_t i3 = 0; i3 < N3; ++i3)
{
for (std::size_t i4 = 0; i4 < N4; ++i4)
{
arr[i1][i2][i3][i4] = j.at(i1).at(i2).at(i3).at(i4).template get<T>();
}
}
}
}
from_json_c_array_element(j, arr);
}
template<typename BasicJsonType>