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Prevent stack overflow in json_pointer::flatten #5517

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841ee12
Fix stack overflow in json_pointer::flatten
dajiaohuang Sep 9, 2026
46d2b4d
Address iterative flatten review feedback
dajiaohuang Sep 10, 2026
fe2f71e
Fix amalgamation formatting
dajiaohuang Sep 10, 2026
0eabb21
Fix flatten task noexcept construction
dajiaohuang Sep 10, 2026
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108 changes: 68 additions & 40 deletions include/nlohmann/detail/json_pointer.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -876,58 +876,86 @@ class json_pointer
const BasicJsonType& value,
BasicJsonType& result)
{
switch (value.type())
struct flatten_task
{
case detail::value_t::array:
string_t reference_string;
const BasicJsonType* value;

flatten_task(string_t reference_string_, const BasicJsonType* value_) noexcept
: reference_string(std::move(reference_string_)), value(value_)
{}
};

std::vector<flatten_task> stack;
stack.emplace_back(reference_string, &value);

while (!stack.empty())
{
auto current = std::move(stack.back());
stack.pop_back();

switch (current.value->type())
{
if (value.m_data.m_value.array->empty())
{
// flatten empty array as null
result[reference_string] = nullptr;
}
else
case detail::value_t::array:
{
// iterate array and use index as a reference string
for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
if (current.value->m_data.m_value.array->empty())
{
flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
value.m_data.m_value.array->operator[](i), result);
// flatten empty array as null
result[current.reference_string] = nullptr;
}
else
{
// iterate array and use index as a reference string
for (std::size_t i = current.value->m_data.m_value.array->size(); i > 0; --i)

@gregmarr gregmarr Sep 9, 2026

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Comment about why this is iterating backwards?

@dajiaohuang dajiaohuang Sep 10, 2026

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Documented in 46d2b4d: children are pushed in reverse because the work stack is LIFO, preserving the original object iteration order. The issue #5393 regression passes in C++11 and C++20.

{
const auto index = i - 1;
stack.emplace_back(detail::concat<string_t>(current.reference_string, '/', std::to_string(index)),
&current.value->m_data.m_value.array->operator[](index));
}
}
break;
}
break;
}

case detail::value_t::object:
{
if (value.m_data.m_value.object->empty())
{
// flatten empty object as null
result[reference_string] = nullptr;
}
else
case detail::value_t::object:
{
// iterate object and use keys as reference string
for (const auto& element : *value.m_data.m_value.object)
if (current.value->m_data.m_value.object->empty())
{
flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
// flatten empty object as null
result[current.reference_string] = nullptr;
}
else
{
// iterate object and use keys as reference string
std::vector<flatten_task> children;
children.reserve(current.value->m_data.m_value.object->size());
for (const auto& element : *current.value->m_data.m_value.object)
{
children.emplace_back(detail::concat<string_t>(current.reference_string, '/', detail::escape(element.first)),
&element.second);
}
// Push children in reverse so the LIFO stack preserves object iteration order.
for (auto it = children.rbegin(); it != children.rend(); ++it)
{
stack.emplace_back(std::move(*it));
}
}
break;
}
break;
}

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[reference_string] = value;
break;
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[current.reference_string] = *current.value;
break;
}
}
}
}
Expand Down
108 changes: 68 additions & 40 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -16660,58 +16660,86 @@ class json_pointer
const BasicJsonType& value,
BasicJsonType& result)
{
switch (value.type())
struct flatten_task
{
case detail::value_t::array:
string_t reference_string;
const BasicJsonType* value;

flatten_task(string_t reference_string_, const BasicJsonType* value_) noexcept
: reference_string(std::move(reference_string_)), value(value_)
{}
};

std::vector<flatten_task> stack;
stack.emplace_back(reference_string, &value);

while (!stack.empty())
{
auto current = std::move(stack.back());
stack.pop_back();

switch (current.value->type())
{
if (value.m_data.m_value.array->empty())
{
// flatten empty array as null
result[reference_string] = nullptr;
}
else
case detail::value_t::array:
{
// iterate array and use index as a reference string
for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
if (current.value->m_data.m_value.array->empty())
{
// flatten empty array as null
result[current.reference_string] = nullptr;
}
else
{
flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
value.m_data.m_value.array->operator[](i), result);
// iterate array and use index as a reference string
for (std::size_t i = current.value->m_data.m_value.array->size(); i > 0; --i)
{
const auto index = i - 1;
stack.emplace_back(detail::concat<string_t>(current.reference_string, '/', std::to_string(index)),
&current.value->m_data.m_value.array->operator[](index));
}
}
break;
}
break;
}

case detail::value_t::object:
{
if (value.m_data.m_value.object->empty())
{
// flatten empty object as null
result[reference_string] = nullptr;
}
else
case detail::value_t::object:
{
// iterate object and use keys as reference string
for (const auto& element : *value.m_data.m_value.object)
if (current.value->m_data.m_value.object->empty())
{
// flatten empty object as null
result[current.reference_string] = nullptr;
}
else
{
flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
// iterate object and use keys as reference string
std::vector<flatten_task> children;
children.reserve(current.value->m_data.m_value.object->size());
for (const auto& element : *current.value->m_data.m_value.object)
{
children.emplace_back(detail::concat<string_t>(current.reference_string, '/', detail::escape(element.first)),
&element.second);
}
// Push children in reverse so the LIFO stack preserves object iteration order.
for (auto it = children.rbegin(); it != children.rend(); ++it)
{
stack.emplace_back(std::move(*it));
}
}
break;
}
break;
}

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[reference_string] = value;
break;
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[current.reference_string] = *current.value;
break;
}
}
}
}
Expand Down
10 changes: 10 additions & 0 deletions tests/src/unit-regression2.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1571,4 +1571,14 @@ TEST_CASE("issue #5402 - update(merge_objects=true) overwrites a primitive with
CHECK(mixed == json({{"keep", {{"a", 1}, {"b", 2}}}, {"replace", {{"x", 2}}}}));
}

TEST_CASE("issue #5393 - flatten deeply nested values without overflowing the stack")
{
constexpr std::size_t depth = 100000;
const auto deep = json::parse(std::string(depth, '[') + "0" + std::string(depth, ']'));
const auto flattened = deep.flatten();
CHECK(flattened.size() == 1);
CHECK(flattened.begin().value() == 0);
CHECK(flattened.begin().key().size() == depth * 2);
}

DOCTEST_CLANG_SUPPRESS_WARNING_POP
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