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Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions (#5477)

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* Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions

Add JSON_HEDLEY_WARN_UNUSED_RESULT to the unambiguous, const,
side-effect-free observer functions whose return value is the entire
purpose of the call:

- dump()
- type(), type_name()
- all is_* predicates (is_primitive, is_structured, is_null,
  is_boolean, is_number, is_number_integer, is_number_unsigned,
  is_number_float, is_object, is_array, is_string, is_binary,
  is_discarded)
- empty(), size(), max_size()
- count(...) (both overloads) and contains(...) (all overloads,
  including the deprecated json_pointer<BasicJsonType> overload)

This mirrors the direction the standard library has taken with
[[nodiscard]] on the analogous std::vector/std::map members, and
catches real bugs such as `j.empty();` (meant `j.clear();`) or
`j.contains(k);` with the result thrown away.

Deliberately out of scope (left for a separate, later policy
decision, per the issue): at(), value(), get*(), flatten(),
unflatten(), patch(), merge_patch(), begin()/end(), comparison
operators, erase(), and emplace().

Compiling the full test suite (tests/src/unit-*.cpp) with
-Wunused-result -Werror uncovered one real hit: a regression test in
unit-regression2.cpp called dump() purely to check it does not throw,
discarding the result. Fixed by explicitly casting to void, since the
call is intentionally result-less there.

Fixes #5410

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

* Fix discarded nodiscard results across the test suite for GCC's warn_unused_result

A plain (void) cast on a call expression suppresses the C++17 [[nodiscard]]
warning but not GCC's warning for functions annotated via the GNU
__attribute__((warn_unused_result)) form -- which is what
JSON_HEDLEY_WARN_UNUSED_RESULT expands to on GCC. Several existing tests
that call a newly-annotated function (dump(), empty()) purely to check
that it throws/does not throw, discarding the result via (void), newly
warned (and failed -Werror builds) once the annotation was broadened.
Route those discards through a small ignore_return_value() helper
instead, which actually consumes the value and suppresses the warning
on both attribute forms.

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

* Use utils::ignore_return_value() for the issue #1445 dump() discard too

Addresses review feedback from @gregmarr on PR #5477: this call site was
still using the older "capture in a variable, then (void) it" pattern
from before this PR introduced utils::ignore_return_value(), instead of
the helper now used at every other discarded-nodiscard-result call site
this PR touches.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-09 09:50:39 +02:00
committed by GitHub
parent a316738cfa
commit c9f1d2d646
13 changed files with 109 additions and 21 deletions
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include/nlohmann/json.hpp
+25
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@@ -1336,6 +1336,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief serialization
/// @sa https://json.nlohmann.me/api/basic_json/dump/
JSON_HEDLEY_WARN_UNUSED_RESULT
string_t dump(const int indent = -1,
const char indent_char = ' ',
const bool ensure_ascii = false,
@@ -1358,6 +1359,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return the type of the JSON value (explicit)
/// @sa https://json.nlohmann.me/api/basic_json/type/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr value_t type() const noexcept
{
return m_data.m_type;
@@ -1365,6 +1367,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether type is primitive
/// @sa https://json.nlohmann.me/api/basic_json/is_primitive/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_primitive() const noexcept
{
return is_null() || is_string() || is_boolean() || is_number() || is_binary();
@@ -1372,6 +1375,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether type is structured
/// @sa https://json.nlohmann.me/api/basic_json/is_structured/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_structured() const noexcept
{
return is_array() || is_object();
@@ -1379,6 +1383,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is null
/// @sa https://json.nlohmann.me/api/basic_json/is_null/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_null() const noexcept
{
return m_data.m_type == value_t::null;
@@ -1386,6 +1391,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is a boolean
/// @sa https://json.nlohmann.me/api/basic_json/is_boolean/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_boolean() const noexcept
{
return m_data.m_type == value_t::boolean;
@@ -1393,6 +1399,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is a number
/// @sa https://json.nlohmann.me/api/basic_json/is_number/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_number() const noexcept
{
return is_number_integer() || is_number_float();
@@ -1400,6 +1407,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is an integer number
/// @sa https://json.nlohmann.me/api/basic_json/is_number_integer/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_number_integer() const noexcept
{
return m_data.m_type == value_t::number_integer || m_data.m_type == value_t::number_unsigned;
@@ -1407,6 +1415,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is an unsigned integer number
/// @sa https://json.nlohmann.me/api/basic_json/is_number_unsigned/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_number_unsigned() const noexcept
{
return m_data.m_type == value_t::number_unsigned;
@@ -1414,6 +1423,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is a floating-point number
/// @sa https://json.nlohmann.me/api/basic_json/is_number_float/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_number_float() const noexcept
{
return m_data.m_type == value_t::number_float;
@@ -1421,6 +1431,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is an object
/// @sa https://json.nlohmann.me/api/basic_json/is_object/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_object() const noexcept
{
return m_data.m_type == value_t::object;
@@ -1428,6 +1439,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is an array
/// @sa https://json.nlohmann.me/api/basic_json/is_array/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_array() const noexcept
{
return m_data.m_type == value_t::array;
@@ -1435,6 +1447,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is a string
/// @sa https://json.nlohmann.me/api/basic_json/is_string/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_string() const noexcept
{
return m_data.m_type == value_t::string;
@@ -1442,6 +1455,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is a binary array
/// @sa https://json.nlohmann.me/api/basic_json/is_binary/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_binary() const noexcept
{
return m_data.m_type == value_t::binary;
@@ -1449,6 +1463,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return whether value is discarded
/// @sa https://json.nlohmann.me/api/basic_json/is_discarded/
JSON_HEDLEY_WARN_UNUSED_RESULT
constexpr bool is_discarded() const noexcept
{
return m_data.m_type == value_t::discarded;
@@ -2780,6 +2795,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief returns the number of occurrences of a key in a JSON object
/// @sa https://json.nlohmann.me/api/basic_json/count/
JSON_HEDLEY_WARN_UNUSED_RESULT
size_type count(const typename object_t::key_type& key) const
{
// return 0 for all nonobject types
@@ -2790,6 +2806,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/count/
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
JSON_HEDLEY_WARN_UNUSED_RESULT
size_type count(KeyType && key) const
{
// return 0 for all nonobject types
@@ -2798,6 +2815,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief check the existence of an element in a JSON object
/// @sa https://json.nlohmann.me/api/basic_json/contains/
JSON_HEDLEY_WARN_UNUSED_RESULT
bool contains(const typename object_t::key_type& key) const
{
return is_object() && m_data.m_value.object->find(key) != m_data.m_value.object->end();
@@ -2807,6 +2825,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @sa https://json.nlohmann.me/api/basic_json/contains/
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
JSON_HEDLEY_WARN_UNUSED_RESULT
bool contains(KeyType && key) const
{
return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
@@ -2814,12 +2833,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief check the existence of an element in a JSON object given a JSON pointer
/// @sa https://json.nlohmann.me/api/basic_json/contains/
JSON_HEDLEY_WARN_UNUSED_RESULT
bool contains(const json_pointer& ptr) const
{
return ptr.contains(this);
}
template<typename BasicJsonType, detail::enable_if_t<detail::is_basic_json<BasicJsonType>::value, int> = 0>
JSON_HEDLEY_WARN_UNUSED_RESULT
JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
bool contains(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr) const
{
@@ -2975,6 +2996,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief checks whether the container is empty.
/// @sa https://json.nlohmann.me/api/basic_json/empty/
JSON_HEDLEY_WARN_UNUSED_RESULT
bool empty() const noexcept
{
switch (m_data.m_type)
@@ -3014,6 +3036,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief returns the number of elements
/// @sa https://json.nlohmann.me/api/basic_json/size/
JSON_HEDLEY_WARN_UNUSED_RESULT
size_type size() const noexcept
{
switch (m_data.m_type)
@@ -3053,6 +3076,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief returns the maximum possible number of elements
/// @sa https://json.nlohmann.me/api/basic_json/max_size/
JSON_HEDLEY_WARN_UNUSED_RESULT
size_type max_size() const noexcept
{
switch (m_data.m_type)
@@ -4242,6 +4266,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// @brief return the type as string
/// @sa https://json.nlohmann.me/api/basic_json/type_name/
JSON_HEDLEY_WARN_UNUSED_RESULT
JSON_HEDLEY_RETURNS_NON_NULL
const char* type_name() const noexcept
{