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Speed up container destruction, iterator access and diff()

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- destroy: free a container whose children are all leaves in one go
  (object of 100k ints: 41.8 -> 17.4 ns/member); the walk stays
  non-recursive and allocation-free.
- iter_impl: check array, then object, before the type switch in
  ++, *, -> and ==; cold throws moved out of line (range-for over an
  array 2.5 -> 1.3 ns/element).
- diff(): one find per source key, lazily built "add" operations,
  children compared with == before building their path (diff of citm
  with 1% changes 1.09 -> 0.70 ms). Output is byte-identical.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-14 22:19:07 +02:00
1 parent 4e294f4b18
commit a8e4e38284
3 files changed
+416 -238
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include/nlohmann/detail/iterators/iter_impl.hpp
+46 -1
View File
@@ -280,6 +280,11 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
}
/// @throw invalid_iterator.214 always
JSON_HEDLEY_NEVER_INLINE JSON_HEDLEY_NO_RETURN void throw_compare_different_values() const
{
JSON_THROW(invalid_iterator::create(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object));
}
JSON_HEDLEY_NO_RETURN void throw_cannot_get_value() const
{
JSON_THROW(invalid_iterator::create(exception_id::iterator_value_unavailable, "cannot get value", m_object));
@@ -294,6 +299,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return *m_it.array_iterator;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
return m_it.object_iterator->second;
}
switch (m_object->m_data.m_type)
{
case value_t::object:
@@ -338,6 +355,12 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return &*m_it.array_iterator;
}
switch (m_object->m_data.m_type)
{
case value_t::object:
@@ -391,6 +414,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
++m_it.array_iterator;
return *this;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
std::advance(m_it.object_iterator, 1);
return *this;
}
switch (m_object->m_data.m_type)
{
case value_t::object:
@@ -484,7 +519,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
// if objects are not the same, the comparison is undefined
if (JSON_HEDLEY_UNLIKELY(m_object != other.m_object))
{
JSON_THROW(invalid_iterator::create(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object));
throw_compare_different_values();
}
// value-initialized forward iterators can be compared, and must compare equal to other value-initialized iterators of the same type #4493
@@ -493,6 +528,16 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
return true;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
return (m_it.array_iterator == other.m_it.array_iterator);
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
return (m_it.object_iterator == other.m_it.object_iterator);
}
switch (m_object->m_data.m_type)
{
case value_t::object: