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Flatten deeply nested values without recursing per nesting level (#5792)

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* Flatten deeply nested values without recursing per nesting level

json_pointer::flatten() called itself once per nesting level, so
flatten() on a value nested deeply enough exhausted the call stack.
#5547 and #5548 fixed merge_patch() and diff() from #5393, but flatten()
was left out.

flatten() now walks the value with an explicit stack and keeps the path
in one buffer that grows and shrinks with it. It has a single code path
and no depth limit: the old version built a new path string per child,
so the iterative one is no slower on shallow values and much faster on
deep ones. The output, including the order of an ordered_json result,
is unchanged.

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

* Construct flatten frames in place

Give the frame a constructor so both call sites can use emplace_back, as
suggested in the review; index starts at 0 for every frame.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann authored and GitHub committed 2026-10-09 17:06:41 +02:00
1 parent 374dfe4f0f
commit 44e8597701
3 files changed
+308 -96
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single_include/nlohmann/json.hpp
+115 -48
View File
@@ -20796,64 +20796,131 @@ class json_pointer
@param[in,out] result the result object to insert values to
@note Empty objects or arrays are flattened to `null`.
The value is walked with an explicit stack rather than the call stack, so
arbitrarily deeply nested values can be flattened.
@sa https://github.com/nlohmann/json/issues/5393
*/
template<typename BasicJsonType>
static void flatten(const string_t& reference_string,
const BasicJsonType& value,
BasicJsonType& result)
{
switch (value.type())
using object_const_iterator = typename BasicJsonType::object_t::const_iterator;
// an array or object being walked: the container, the array index or
// object iterator of the next child, and the length of the path of the
// container itself
struct frame
{
case detail::value_t::array:
{
if (value.m_data.m_value.array->empty())
{
// flatten empty array as null
result[reference_string] = nullptr;
}
else
{
// iterate array and use index as a reference string
for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
{
flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
value.m_data.m_value.array->operator[](i), result);
}
}
break;
}
frame(const BasicJsonType* container_, object_const_iterator member_, const std::size_t path_length_) noexcept
: container(container_), member(std::move(member_)), path_length(path_length_)
{}
case detail::value_t::object:
{
if (value.m_data.m_value.object->empty())
{
// flatten empty object as null
result[reference_string] = nullptr;
}
else
{
// iterate object and use keys as reference string
for (const auto& element : *value.m_data.m_value.object)
{
flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
}
}
break;
}
const BasicJsonType* container;
std::size_t index = 0;
object_const_iterator member;
std::size_t path_length;
};
case detail::value_t::null:
case detail::value_t::string:
case detail::value_t::boolean:
case detail::value_t::number_integer:
case detail::value_t::number_unsigned:
case detail::value_t::number_float:
case detail::value_t::binary:
case detail::value_t::discarded:
default:
// The containers being flattened are kept on an explicit stack, and
// every child is flattened completely before the next one, so the
// entries come out in the same order as with a recursive walk. The
// path of the value being flattened is kept in one buffer that grows
// and shrinks with the stack, rather than in a new string per level.
std::vector<frame> stack;
string_t path = reference_string;
// flatten `v`, whose path is `path`: primitives and empty containers
// are added to the result right away; other containers get a frame
const auto enter = [&stack, &path, &result](const BasicJsonType & v)
{
switch (v.type())
{
// add a primitive value with its reference string
result[reference_string] = value;
break;
case detail::value_t::array:
{
if (v.m_data.m_value.array->empty())
{
// flatten empty array as null
result[path] = nullptr;
}
else
{
stack.emplace_back(&v, object_const_iterator(), path.size());
}
return;
}
case detail::value_t::object:
{
if (v.m_data.m_value.object->empty())
{
// flatten empty object as null
result[path] = nullptr;
}
else
{
stack.emplace_back(&v, v.m_data.m_value.object->begin(), path.size());
}
return;
}
case detail::value_t::null:
case detail::value_t::string:
case detail::value_t::boolean:
case detail::value_t::number_integer:
case detail::value_t::number_unsigned:
case detail::value_t::number_float:
case detail::value_t::binary:
case detail::value_t::discarded:
default:
{
// add a primitive value with its reference string
result[path] = v;
return;
}
}
};
enter(value);
while (!stack.empty())
{
// the frame is changed through stack.back(): enter() may push a
// frame, which would invalidate a reference to it
const BasicJsonType* const container = stack.back().container;
// drop the path of the previous child
path.resize(stack.back().path_length);
if (container->is_array())
{
const auto& array = *container->m_data.m_value.array;
const std::size_t i = stack.back().index;
if (i == array.size())
{
stack.pop_back();
continue;
}
// iterate array and use index as a reference string
++stack.back().index;
detail::concat_into(path, '/', detail::to_string<string_t>(i));
enter(array[i]);
}
else
{
const object_const_iterator it = stack.back().member;
if (it == container->m_data.m_value.object->end())
{
stack.pop_back();
continue;
}
// iterate object and use keys as reference string
++stack.back().member;
detail::concat_into(path, '/', detail::escape(it->first));
enter(it->second);
}
}
}