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4daca40d7b03d05d1af730003f826c1ff91a90e8
json/tests/src/unit-diagnostics.cpp
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Niels Lohmann 4daca40d7b Merge deeply nested objects without recursing per nesting level (#5547) 
* Merge deeply nested objects without recursing per nesting level

merge_patch() and update(j, true) merged a nested object by calling
themselves on it, once per nesting level. A value nested deeply enough -
50,000 levels of objects on an 8 MiB stack - exhausted the call stack
and terminated the process, although parse() accepts such values without
complaint.

Bound the descent the same way dump() does. The recursion now carries
the nesting level, and once merge_depth_limit() (128) levels have been
entered, update_members_iteratively() and merge_patch_iteratively()
finish the merge on an explicit stack. They still merge a nested object
completely before the next member, and in the same order, so the results,
including the parents JSON_DIAGNOSTICS reports paths from, are unchanged.
Values nested less deeply than the bound run the same code as before, so
the common case does not pay for the stack: merging only on it cost
10-14% in a first version.

The public signatures are unchanged. The recursive worker behind
merge_patch() has its own name rather than being a private overload, so
that &basic_json::merge_patch stays unambiguous.

Tests check every depth up to 300 against recursive reference
implementations of both operations, check the diagnostic paths past the
bound, and merge objects nested 100,000 levels deep.

Fixes #5545 for update(j, true), and #5393 for merge_patch().

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

* Use the shared recursion limit in update() and merge_patch()

merge_depth_limit() is gone in favor of detail::recursion_depth_limit().
The two identical function-local frame structs become one member struct,
merge_frame, with a constructor, so both loops emplace_back() their
frames. merge_patch_iteratively() copies the frame it works on out of the
stack and changes it only through stack.back().

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

* Build the update()/merge_patch() diagnostics test values instead of parsing them

Parsed values carry byte positions under JSON_DIAGNOSTIC_POSITIONS, which
the expected messages do not include.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-09-24 17:12:03 +02:00

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// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++ (supporting code)
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
#include "doctest_compatibility.h"
#ifdef JSON_DIAGNOSTICS
#undef JSON_DIAGNOSTICS
#endif
#define JSON_DIAGNOSTICS 1
#include <nlohmann/json.hpp>
using nlohmann::json;
TEST_CASE("Better diagnostics")
{
SECTION("empty JSON Pointer")
{
json const j = 1;
std::string s;
CHECK_THROWS_WITH_AS(s = j.get<std::string>(), "[json.exception.type_error.302] type must be string, but is number", json::type_error);
}
SECTION("invalid type")
{
json j;
j["a"]["b"]["c"] = 1;
std::string s;
CHECK_THROWS_WITH_AS(s = j["a"]["b"]["c"].get<std::string>(), "[json.exception.type_error.302] (/a/b/c) type must be string, but is number", json::type_error);
}
SECTION("missing key")
{
json j;
j["object"]["object"] = true;
CHECK_THROWS_WITH_AS(j["object"].at("not_found"), "[json.exception.out_of_range.403] (/object) key 'not_found' not found", json::out_of_range);
}
SECTION("array index out of range")
{
json j;
j["array"][4] = true;
CHECK_THROWS_WITH_AS(j["array"].at(5), "[json.exception.out_of_range.401] (/array) array index 5 is out of range", json::out_of_range);
}
SECTION("array index at wrong type")
{
json j;
j["array"][4] = true;
CHECK_THROWS_WITH_AS(j["array"][4][5], "[json.exception.type_error.305] (/array/4) cannot use operator[] with a numeric argument with boolean", json::type_error);
}
SECTION("wrong iterator")
{
json j;
j["array"] = json::array();
CHECK_THROWS_WITH_AS(j["array"].erase(j.begin()), "[json.exception.invalid_iterator.202] (/array) iterator does not fit current value", json::invalid_iterator);
}
SECTION("JSON Pointer escaping")
{
json j;
j["a/b"]["m~n"] = 1;
std::string s;
CHECK_THROWS_WITH_AS(s = j["a/b"]["m~n"].get<std::string>(), "[json.exception.type_error.302] (/a~1b/m~0n) type must be string, but is number", json::type_error);
}
SECTION("Parse error")
{
json _;
CHECK_THROWS_WITH_AS(_ = json::parse(""), "[json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON", json::parse_error);
}
SECTION("Wrong type in update()")
{
json j = {{"foo", "bar"}};
json k = {{"bla", 1}};
CHECK_THROWS_WITH_AS(j.update(k["bla"].begin(), k["bla"].end()), "[json.exception.type_error.312] (/bla) cannot use update() with number", json::type_error);
CHECK_THROWS_WITH_AS(j.update(k["bla"]), "[json.exception.type_error.312] (/bla) cannot use update() with number", json::type_error);
}
}
TEST_CASE("Regression tests for extended diagnostics")
{
SECTION("Regression test for https://github.com/nlohmann/json/pull/2562#pullrequestreview-574858448")
{
CHECK_THROWS_WITH_AS(json({"0", "0"})[1].get<int>(), "[json.exception.type_error.302] (/1) type must be number, but is string", json::type_error);
CHECK_THROWS_WITH_AS(json({"0", "1"})[1].get<int>(), "[json.exception.type_error.302] (/1) type must be number, but is string", json::type_error);
}
SECTION("Regression test for https://github.com/nlohmann/json/pull/2562/files/380a613f2b5d32425021129cd1f371ddcfd54ddf#r563259793")
{
json j;
j["/foo"] = {1, 2, 3};
CHECK_THROWS_WITH_AS(j.unflatten(), "[json.exception.type_error.315] (/~1foo) values in object must be primitive", json::type_error);
}
SECTION("Regression test for issue #2838 - Assertion failure when inserting into arrays with JSON_DIAGNOSTICS set")
{
// void push_back(basic_json&& val)
{
json j_arr = json::array();
j_arr.push_back(json::object());
j_arr.push_back(json::object());
j_arr.push_back(json::object());
j_arr.push_back(json::object());
json j_obj = json::object();
j_obj["key"] = j_arr;
}
// void push_back(const basic_json& val)
{
json j_arr = json::array();
auto object = json::object();
j_arr.push_back(object);
j_arr.push_back(object);
j_arr.push_back(object);
j_arr.push_back(object);
json j_obj = json::object();
j_obj["key"] = j_arr;
}
// reference emplace_back(Args&& ... args)
{
json j_arr = json::array();
j_arr.emplace_back(json::object());
j_arr.emplace_back(json::object());
j_arr.emplace_back(json::object());
j_arr.emplace_back(json::object());
json j_obj = json::object();
j_obj["key"] = j_arr;
}
// iterator insert(const_iterator pos, const basic_json& val)
{
json j_arr = json::array();
j_arr.insert(j_arr.begin(), json::object());
j_arr.insert(j_arr.begin(), json::object());
j_arr.insert(j_arr.begin(), json::object());
j_arr.insert(j_arr.begin(), json::object());
json j_obj = json::object();
j_obj["key"] = j_arr;
}
// iterator insert(const_iterator pos, size_type cnt, const basic_json& val)
{
json j_arr = json::array();
j_arr.insert(j_arr.begin(), 2, json::object());
json j_obj = json::object();
j_obj["key"] = j_arr;
}
// iterator insert(const_iterator pos, const_iterator first, const_iterator last)
{
json j_arr = json::array();
json j_objects = {json::object(), json::object()};
j_arr.insert(j_arr.begin(), j_objects.begin(), j_objects.end());
json j_obj = json::object();
j_obj["key"] = j_arr;
}
}
SECTION("Regression test for issue #2962 - JSON_DIAGNOSTICS assertion for ordered_json")
{
nlohmann::ordered_json j;
nlohmann::ordered_json j2;
const std::string value;
j["first"] = value;
j["second"] = value;
j2["something"] = j;
}
SECTION("Regression test for issue #3007 - Parent pointers properly set when using update()")
{
// void update(const_reference j)
{
json j = json::object();
{
json j2 = json::object();
j2["one"] = 1;
j.update(j2);
}
// Must call operator[] on const element, otherwise m_parent gets updated.
auto const& constJ = j;
CHECK_THROWS_WITH_AS(constJ["one"].at(0), "[json.exception.type_error.304] (/one) cannot use at() with number", json::type_error);
}
// void update(const_iterator first, const_iterator last)
{
json j = json::object();
{
json j2 = json::object();
j2["one"] = 1;
j.update(j2.begin(), j2.end());
}
// Must call operator[] on const element, otherwise m_parent gets updated.
auto const& constJ = j;
CHECK_THROWS_WITH_AS(constJ["one"].at(0), "[json.exception.type_error.304] (/one) cannot use at() with number", json::type_error);
}
// Code from #3007 triggering unwanted assertion without fix to update().
{
json root = json::array();
json lower = json::object();
{
json lowest = json::object();
lowest["one"] = 1;
lower.update(lowest);
}
root.push_back(lower);
}
}
SECTION("Regression test for issue #3032 - Yet another assertion failure when inserting into arrays with JSON_DIAGNOSTICS set")
{
// reference operator[](size_type idx)
{
json j_arr = json::array();
j_arr[0] = 0;
j_arr[1] = 1;
j_arr[2] = 2;
j_arr[3] = 3;
j_arr[4] = 4;
j_arr[5] = 5;
j_arr[6] = 6;
j_arr[7] = 7;
json const j_arr_copy = j_arr;
}
}
SECTION("Regression test for issue #3915 - JSON_DIAGNOSTICS trigger assertion")
{
json j = json::object();
j["root"] = "root_str";
json jj = json::object();
jj["child"] = json::object();
// If do not push anything in object, then no assert will be produced
jj["child"]["prop1"] = "prop1_value";
// Push all properties of child in parent
j.insert(jj.at("child").begin(), jj.at("child").end());
// Here assert is generated when construct new json
const json k(j);
CHECK(k.dump() == "{\"prop1\":\"prop1_value\",\"root\":\"root_str\"}");
}
SECTION("Regression test for issue #4813 - update() with merge_objects=true triggers JSON_ASSERT with JSON_DIAGNOSTICS")
{
// https://github.com/nlohmann/json/issues/4813
nlohmann::ordered_json j1 = {{"numbers", {{"one", 1}}}};
nlohmann::ordered_json const j2 = {{"numbers", {{"two", 2}}}, {"string", "t"}};
CHECK_NOTHROW(j1.update(j2, true));
CHECK(j1["numbers"]["one"] == 1);
CHECK(j1["numbers"]["two"] == 2);
CHECK(j1["string"] == "t");
}
SECTION("Regression test for issue #5387 - copying keeps the parents of nested values")
{
// A value nested deeper than the copy constructor's descent bound is
// copied without the call stack. Every container that path creates has
// to have the parents of its children set, or the JSON Pointer in the
// diagnostic is cut short.
const std::size_t depth = 300;
SECTION("objects")
{
json j = "not a number";
std::string pointer;
for (std::size_t i = 0; i < depth; ++i)
{
j = json{{"a", j}};
pointer += "/a";
}
json const copy(j); // NOLINT(performance-unnecessary-copy-initialization)
const json* inner = &copy;
for (std::size_t i = 0; i < depth; ++i)
{
inner = &inner->at("a");
}
std::string const expected = "[json.exception.type_error.302] (" + pointer + ") type must be number, but is string";
int i = 0;
CHECK_THROWS_WITH_AS(i = inner->get<int>(), expected.c_str(), json::type_error);
CHECK(i == 0);
}
SECTION("arrays")
{
json j = "not a number";
std::string pointer;
for (std::size_t i = 0; i < depth; ++i)
{
j = json::array({j});
pointer += "/0";
}
json const copy(j); // NOLINT(performance-unnecessary-copy-initialization)
const json* inner = &copy;
for (std::size_t i = 0; i < depth; ++i)
{
inner = &inner->at(0);
}
std::string const expected = "[json.exception.type_error.302] (" + pointer + ") type must be number, but is string";
int i = 0;
CHECK_THROWS_WITH_AS(i = inner->get<int>(), expected.c_str(), json::type_error);
CHECK(i == 0);
}
}
SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
{
// swap(array_t&)
{
json j = json::array();
json::array_t arr = {json::array({1})};
j.swap(arr);
// parent pointers of the moved-in elements must point into j, not
// into the now-defunct free-standing array_t
CHECK_THROWS_WITH_AS(j[0][0].get<std::string>(), "[json.exception.type_error.302] (/0/0) type must be string, but is number", json::type_error);
// must not trigger assert_invariant() in a debug/assert-enabled build
json const k = j;
CHECK(k == j);
}
// swap(object_t&)
{
json o = json::object();
json::object_t obj = {{"a", json::array({1})}};
o.swap(obj);
CHECK_THROWS_WITH_AS(o["a"][0].get<std::string>(), "[json.exception.type_error.302] (/a/0) type must be string, but is number", json::type_error);
// must not trigger assert_invariant() in a debug/assert-enabled build
json const p = o;
CHECK(p == o);
}
}
}
TEST_CASE("Better diagnostics past the descent bound of update() and merge_patch()")
{
// Both merge objects nested more than detail::recursion_depth_limit()
// (128) levels deep without recursing; the values they add or replace
// there must still know their parents.
// The values are built rather than parsed, so that the expected messages
// carry no byte positions under JSON_DIAGNOSTIC_POSITIONS.
const std::size_t depth = 200;
json target = {{"x", 1}};
json patch = {{"y", 2}};
std::string path;
for (std::size_t i = 0; i < depth; ++i)
{
target = json{{"a", std::move(target)}};
patch = json{{"a", std::move(patch)}};
path += "/a";
}
const std::string expected_x = "[json.exception.type_error.304] (" + path + "/x) cannot use at() with number";
const std::string expected_y = "[json.exception.type_error.304] (" + path + "/y) cannot use at() with number";
SECTION("update()")
{
json j = target;
j.update(patch, true);
// walk down through const references, which leave m_parent alone
const json* p = &j;
for (std::size_t i = 0; i < depth; ++i)
{
p = &p->at("a");
}
CHECK_THROWS_WITH_AS(p->at("x").at(0), expected_x.c_str(), json::type_error);
CHECK_THROWS_WITH_AS(p->at("y").at(0), expected_y.c_str(), json::type_error);
}
SECTION("merge_patch()")
{
json j = target;
j.merge_patch(patch);
const json* p = &j;
for (std::size_t i = 0; i < depth; ++i)
{
p = &p->at("a");
}
CHECK_THROWS_WITH_AS(p->at("x").at(0), expected_x.c_str(), json::type_error);
CHECK_THROWS_WITH_AS(p->at("y").at(0), expected_y.c_str(), json::type_error);
}
}
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