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Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions

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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>
This commit is contained in:
Niels Lohmann
2026-09-05 20:51:58 +02:00
parent ae6f737976
commit 354d5fa723
3 changed files with 52 additions and 1 deletions
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include/nlohmann/json.hpp
+25
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@@ -1335,6 +1335,7 @@ 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,
@@ -1357,6 +1358,7 @@ 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;
@@ -1364,6 +1366,7 @@ 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();
@@ -1371,6 +1374,7 @@ 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();
@@ -1378,6 +1382,7 @@ 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;
@@ -1385,6 +1390,7 @@ 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;
@@ -1392,6 +1398,7 @@ 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();
@@ -1399,6 +1406,7 @@ 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;
@@ -1406,6 +1414,7 @@ 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;
@@ -1413,6 +1422,7 @@ 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;
@@ -1420,6 +1430,7 @@ 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;
@@ -1427,6 +1438,7 @@ 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;
@@ -1434,6 +1446,7 @@ 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;
@@ -1441,6 +1454,7 @@ 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;
@@ -1448,6 +1462,7 @@ 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;
@@ -2779,6 +2794,7 @@ 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
@@ -2789,6 +2805,7 @@ 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
@@ -2797,6 +2814,7 @@ 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();
@@ -2806,6 +2824,7 @@ 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();
@@ -2813,12 +2832,14 @@ 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
{
@@ -2974,6 +2995,7 @@ 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)
@@ -3013,6 +3035,7 @@ 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)
@@ -3052,6 +3075,7 @@ 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)
@@ -4241,6 +4265,7 @@ 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
{