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Merged
nlohmann merged 1 commit into from
Sep 30, 2026
Merged

Hide a discarded container's content from the parser callback #5706

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3 changes: 3 additions & 0 deletions docs/mkdocs/docs/api/basic_json/parser_callback_t.md
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Original file line number Diff line number Diff line change
Expand Up @@ -100,3 +100,6 @@ the latter case, it is skipped completely, or replaced by `null` if it is the to
- Added in version 1.0.0.
- Fixed in version 3.13.0 to also remove discarded values from a parent object; before, discarding an array or a value
stored under an object key left a discarded member behind, which made the parse result serialize to invalid JSON.
- Fixed in version 3.13.0 so that discarding an array or object at its start event also hides its content from the
callback, as documented above; before, the callback was still called for the content, and the key of every member of
a discarded object was kept in memory until the parse ended.
18 changes: 15 additions & 3 deletions include/nlohmann/detail/input/json_sax.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -582,8 +582,8 @@ class json_sax_dom_callback_parser

bool start_object(std::size_t len)
{
// check callback for object start
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
// check callback for object start; not called inside a discarded container
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
keep_stack.push_back(keep);

// the key this object will be stored under, read before handle_value()
Expand Down Expand Up @@ -619,6 +619,18 @@ class json_sax_dom_callback_parser

bool key(string_t& val)
{
if (!keep_stack.back() || !ref_stack.back())
{
// the object is not stored: the value of this key is dropped in
// handle_value() without touching the key stacks
if (keep_stack.back())
{
BasicJsonType k = BasicJsonType(val);
static_cast<void>(callback(static_cast<int>(ref_stack.size()), parse_event_t::key, k));
}
return true;
}

BasicJsonType k = BasicJsonType(val);

// check callback for the key
Expand Down Expand Up @@ -704,7 +716,7 @@ class json_sax_dom_callback_parser

bool start_array(std::size_t len)
{
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
keep_stack.push_back(keep);

// see start_object()
Expand Down
18 changes: 15 additions & 3 deletions single_include/nlohmann/json.hpp
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Expand Up @@ -11996,8 +11996,8 @@ class json_sax_dom_callback_parser

bool start_object(std::size_t len)
{
// check callback for object start
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
// check callback for object start; not called inside a discarded container
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
keep_stack.push_back(keep);

// the key this object will be stored under, read before handle_value()
Expand Down Expand Up @@ -12033,6 +12033,18 @@ class json_sax_dom_callback_parser

bool key(string_t& val)
{
if (!keep_stack.back() || !ref_stack.back())
{
// the object is not stored: the value of this key is dropped in
// handle_value() without touching the key stacks
if (keep_stack.back())
{
BasicJsonType k = BasicJsonType(val);
static_cast<void>(callback(static_cast<int>(ref_stack.size()), parse_event_t::key, k));
}
return true;
}

BasicJsonType k = BasicJsonType(val);

// check callback for the key
Expand Down Expand Up @@ -12118,7 +12130,7 @@ class json_sax_dom_callback_parser

bool start_array(std::size_t len)
{
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
keep_stack.push_back(keep);

// see start_object()
Expand Down
96 changes: 96 additions & 0 deletions tests/src/unit-class_parser.cpp
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Expand Up @@ -1966,6 +1966,102 @@ TEST_CASE("parser class")
}
}

SECTION("no callback for the content of a discarded container (#5643)")
{
// discarding a container at its start event must also hide
// everything inside it from the callback: none of the nested
// keys, values, or nested containers' own start/end events may
// be reported
std::vector<std::string> log;
bool first = true;
const json j = json::parse(R"({"skip": {"k1": 1, "k2": [2, {"k3": 3}]}, "keep": 1})",
[&](int depth, json::parse_event_t event, json & parsed)
{
static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"};
log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());

if (depth == 1 && event == json::parse_event_t::object_start && first)
{
// discard "skip" right at its object_start event
first = false;
return false;
}
return true;
});

CHECK(log == std::vector<std::string>
{
"0 object_start <discarded>",
"1 key \"skip\"",
"1 object_start <discarded>",
"1 key \"keep\"",
"1 value 1",
"0 object_end {\"keep\":1}"
});
CHECK(j == json({{"keep", 1}}));
}

SECTION("callback still called inside a container whose key was rejected (#5643)")
{
// rejecting a key does not discard its value's container at the
// container's own start event, so the callback is still called
// for that container's content; only storing the container
// under the rejected key is skipped
// (documented for parser_callback_t: "the callback is still
// called for the associated value, but its return value has no
// further effect")
const auto record = [](std::vector<std::string>& log, int depth, json::parse_event_t event, const json & parsed)
{
static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"};
log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());
};

std::vector<std::string> log_object;
const json j_object = json::parse(R"({"skip": {"k1": 1}, "keep": 2})",
[&](int depth, json::parse_event_t event, json & parsed)
{
record(log_object, depth, event, parsed);
return !(event == json::parse_event_t::key && parsed == json("skip"));
});

CHECK(log_object == std::vector<std::string>
{
"0 object_start <discarded>",
"1 key \"skip\"",
"1 object_start <discarded>",
"2 key \"k1\"",
"2 value 1",
"1 key \"keep\"",
"1 value 2",
"0 object_end {\"keep\":2}"
});
CHECK(j_object == json({{"keep", 2}}));

// same for a rejected key whose value is an array rather than an object
std::vector<std::string> log_array;
const json j_array = json::parse(R"({"skip": [1, {"k1": 2}], "keep": 2})",
[&](int depth, json::parse_event_t event, json & parsed)
{
record(log_array, depth, event, parsed);
return !(event == json::parse_event_t::key && parsed == json("skip"));
});

CHECK(log_array == std::vector<std::string>
{
"0 object_start <discarded>",
"1 key \"skip\"",
"1 array_start <discarded>",
"2 value 1",
"2 object_start <discarded>",
"3 key \"k1\"",
"3 value 2",
"1 key \"keep\"",
"1 value 2",
"0 object_end {\"keep\":2}"
});
CHECK(j_array == json({{"keep", 2}}));
}

SECTION("special cases")
{
// the following test cases cover the situation in which an empty
Expand Down
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