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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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tests/src/unit-conversions.cpp
+31
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@@ -430,6 +430,37 @@ TEST_CASE("value conversion")
CHECK(std::equal(std::begin(nbs[0][0][0]), std::end(nbs[1][1][1]), std::begin(nbs2[0][0][0])));
}
SECTION("built-in arrays: 5D")
{
// NOLINTBEGIN(misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
const int nbs[1][1][1][2][2] = {{{{{0, 1}, {2, 3}}}}};
int nbs2[1][1][1][2][2] = {{{{{0, 0}, {0, 0}}}}};
// NOLINTEND(misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
const json j2 = nbs;
j2.get_to(nbs2);
CHECK(std::equal(std::begin(nbs[0][0][0][0]), std::end(nbs[0][0][0][1]), std::begin(nbs2[0][0][0][0])));
}
SECTION("built-in arrays: mismatched shape")
{
// NOLINTBEGIN(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
int nbs2[2][3] = {{0, 0, 0}, {0, 0, 0}};
// NOLINTEND(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
SECTION("not an array")
{
const json j2 = 42;
CHECK_THROWS_WITH_AS(j2.get_to(nbs2), "[json.exception.type_error.304] cannot use at() with number", json::type_error&);
}
SECTION("too few elements")
{
const json j2 = {{0, 1, 2}};
CHECK_THROWS_WITH_AS(j2.get_to(nbs2), "[json.exception.out_of_range.401] array index 1 is out of range", json::out_of_range&);
}
}
SECTION("std::deque<json>")
{
std::deque<json> a{"previous", "value"};