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Fix indentation overflow and correctness bug #5186

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Fix indentation overflow and correctness bug #5186

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b3effb6
:bug: use correct indentation character on resize
nlohmann May 20, 2026
61f0d68
:bug: avoid overflow
nlohmann May 20, 2026
3249b18
:rotating_light: fix compilation
nlohmann May 21, 2026
250787c
:recycle: add reserve_indent helper
nlohmann Jun 30, 2026
526c8c5
:recycle: check for truncation
nlohmann Jun 30, 2026
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45 changes: 30 additions & 15 deletions include/nlohmann/detail/output/serializer.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -125,11 +125,7 @@ class serializer
o->write_characters("{\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

// first n-1 elements
auto i = val.m_data.m_value.object->cbegin();
Expand Down Expand Up @@ -198,11 +194,7 @@ class serializer
o->write_characters("[\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

// first n-1 elements
for (auto i = val.m_data.m_value.array->cbegin();
Expand Down Expand Up @@ -259,11 +251,7 @@ class serializer
o->write_characters("{\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

o->write_characters(indent_string.c_str(), new_indent);

Expand Down Expand Up @@ -373,6 +361,33 @@ class serializer
}
}

/*!
@brief compute the next indentation level and grow the indentation string

Computes the new indentation level @a current_indent + @a indent_step and
grows the indentation string (by doubling its size, but at least to the new
level) so that it can be used as a source for writing that many indentation
characters. The newly added characters use the configured @ref indent_char.

@param[in] current_indent the current indentation level
@param[in] indent_step the number of characters to indent per level

@return the new indentation level @a current_indent + @a indent_step
*/
unsigned int reserve_indent(const unsigned int current_indent, const unsigned int indent_step)
{
const unsigned int new_indent = current_indent + indent_step;
// a very large indent_step can wrap the unsigned accumulation on deep
// nesting, which would silently truncate the indentation
JSON_ASSERT(new_indent >= current_indent);
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize((std::max)(indent_string.size() * 2, static_cast<std::size_t>(new_indent)), indent_char);
}
JSON_ASSERT(indent_string.size() >= new_indent);
return new_indent;
}

JSON_PRIVATE_UNLESS_TESTED:
/*!
@brief dump escaped string
Expand Down
45 changes: 30 additions & 15 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19299,11 +19299,7 @@ class serializer
o->write_characters("{\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

// first n-1 elements
auto i = val.m_data.m_value.object->cbegin();
Expand Down Expand Up @@ -19372,11 +19368,7 @@ class serializer
o->write_characters("[\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

// first n-1 elements
for (auto i = val.m_data.m_value.array->cbegin();
Expand Down Expand Up @@ -19433,11 +19425,7 @@ class serializer
o->write_characters("{\n", 2);

// variable to hold indentation for recursive calls
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
}
const auto new_indent = reserve_indent(current_indent, indent_step);

o->write_characters(indent_string.c_str(), new_indent);

Expand Down Expand Up @@ -19547,6 +19535,33 @@ class serializer
}
}

/*!
@brief compute the next indentation level and grow the indentation string

Computes the new indentation level @a current_indent + @a indent_step and
grows the indentation string (by doubling its size, but at least to the new
level) so that it can be used as a source for writing that many indentation
characters. The newly added characters use the configured @ref indent_char.

@param[in] current_indent the current indentation level
@param[in] indent_step the number of characters to indent per level

@return the new indentation level @a current_indent + @a indent_step
*/
unsigned int reserve_indent(const unsigned int current_indent, const unsigned int indent_step)
{
const unsigned int new_indent = current_indent + indent_step;
// a very large indent_step can wrap the unsigned accumulation on deep
// nesting, which would silently truncate the indentation
JSON_ASSERT(new_indent >= current_indent);
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize((std::max)(indent_string.size() * 2, static_cast<std::size_t>(new_indent)), indent_char);
}
JSON_ASSERT(indent_string.size() >= new_indent);
return new_indent;
}

JSON_PRIVATE_UNLESS_TESTED:
/*!
@brief dump escaped string
Expand Down
35 changes: 35 additions & 0 deletions tests/src/unit-inspection.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -245,6 +245,41 @@ TEST_CASE("object inspection")
CHECK(binary.dump(1024).size() == 2086);
}

SECTION("indentation and resize")
{
SECTION("array")
{
const auto j_array = json::parse("[[[[[[]]]]]]");
// check right size after indentation triggering a resize
CHECK(j_array.dump(1024).size() == 25622);
// check if right indentation symbol is used
CHECK(j_array.dump(1024, '\t')[4096] == '\t');
// check resize is large enough
CHECK(j_array.dump(10000).size() == 250022);
}

SECTION("object")
{
const auto j_object = json::parse(R"({"":{"":{"":{"":{"":{}}}}}})");
// check right size after indentation triggering a resize
CHECK(j_object.dump(1024).size() == 25642);
// check if right indentation symbol is used
CHECK(j_object.dump(1024, '\t')[4096] == '\t');
// check resize is large enough
CHECK(j_object.dump(10000).size() == 250042);
}

SECTION("binary")
{
const auto j_binary = json::binary({1, 2, 3}, 128);
// check right size after indentation triggering a resize
CHECK(j_binary.dump(1024).size() == 2086);
CHECK(j_binary.dump(1024, '\t')[1024] == '\t');
// check resize is large enough
CHECK(j_binary.dump(10000).size() == 20038);
}
}

SECTION("dump and floating-point numbers")
{
auto s = json(42.23).dump();
Expand Down
18 changes: 10 additions & 8 deletions tests/src/unit-serialization.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -311,14 +311,16 @@ TEST_CASE("dump for basic_json with long double number_float_t")
SECTION("round-trip dump/parse")
{
constexpr std::array<long double, 13> values =
{{
0.0L, -0.0L, 1.0L, -1.0L,
0.5L, -0.5L, 1.5L, -2.25L,
1.23e45L, 1.23e-45L,
(std::numeric_limits<long double>::min)(),
std::numeric_limits<long double>::lowest(),
(std::numeric_limits<long double>::max)()
}};
{
{
0.0L, -0.0L, 1.0L, -1.0L,
0.5L, -0.5L, 1.5L, -2.25L,
1.23e45L, 1.23e-45L,
(std::numeric_limits<long double>::min)(),
std::numeric_limits<long double>::lowest(),
(std::numeric_limits<long double>::max)()
}
};

for (long double v : values)
{
Expand Down
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