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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
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9 changes: 6 additions & 3 deletions include/nlohmann/detail/output/serializer.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -128,7 +128,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

// first n-1 elements
Expand Down Expand Up @@ -201,7 +202,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

// first n-1 elements
Expand Down Expand Up @@ -262,7 +264,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

o->write_characters(indent_string.c_str(), new_indent);
Expand Down
9 changes: 6 additions & 3 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19302,7 +19302,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

// first n-1 elements
Expand Down Expand Up @@ -19375,7 +19376,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

// first n-1 elements
Expand Down Expand Up @@ -19436,7 +19438,8 @@ class serializer
const auto new_indent = current_indent + indent_step;
if (JSON_HEDLEY_UNLIKELY(indent_string.size() < new_indent))
{
indent_string.resize(indent_string.size() * 2, ' ');
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);
}

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