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Do not instantiate a hash map with an incomplete basic_json in the tests

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object_t is probed for key_compare inside the definition of basic_json, so
it is instantiated while basic_json is still incomplete. Whether a hash map
survives that depends on the standard library: libstdc++ 9 needs the size of
the mapped type to instantiate std::unordered_map's node type and rejects
the adapter, which broke the GCC 9 builds.

The test now derives its no-key_compare object type from std::map -- which
does cope -- and shadows the inherited key_compare member type with an
entity that is not a type, so the library's probe finds none, exactly as for
a hash map. The unflatten() order-independence checks in unit-json_pointer
already cover the behaviour that the unordered object type was there for.
The limitation is documented for std::unordered_map.

Also address two Clang-Tidy findings the earlier commits introduced:
erase_from_object() declares its iterator with auto, and at(size_type) checks
the type first and then falls through to the return instead of throwing from
an else branch.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 18:17:15 +00:00
parent 110cd31e8f
commit 43afb5bebc
4 changed files with 71 additions and 64 deletions
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include/nlohmann/json.hpp
+17 -19
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@@ -798,7 +798,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
typename object_t::iterator erase_from_object(It pos)
{
typename object_t::iterator next = std::next(pos);
auto next = std::next(pos);
m_data.m_value.object->erase(pos);
return next;
}
@@ -2063,18 +2063,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
reference at(size_type idx)
{
// at only works for arrays
if (JSON_HEDLEY_LIKELY(is_array()))
{
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
}
return set_parent((*m_data.m_value.array)[idx]);
}
else
if (JSON_HEDLEY_UNLIKELY(!is_array()))
{
JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
}
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
}
return set_parent((*m_data.m_value.array)[idx]);
}
/// @brief access specified array element with bounds checking
@@ -2082,18 +2081,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const_reference at(size_type idx) const
{
// at only works for arrays
if (JSON_HEDLEY_LIKELY(is_array()))
{
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
}
return (*m_data.m_value.array)[idx];
}
else
if (JSON_HEDLEY_UNLIKELY(!is_array()))
{
JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
}
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
}
return (*m_data.m_value.array)[idx];
}
/// @brief access specified object element with bounds checking