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>
Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions
2026-09-07 08:47:57 +00:00 · 2026-09-06 12:18:38 +02:00 · 2026-09-05 20:51:58 +02:00 · 2026-09-05 20:40:48 +02:00 · 2026-09-05 20:30:22 +02:00
16 changed files with 161 additions and 21 deletions
@@ -125,6 +125,7 @@ The library uses the following mapping from JSON values types to BJData types ac

    - `"_ArrayType_"` is one of `uint8`, `int8`, `uint16`, `int16`, `uint32`, `int32`, `uint64`, `int64`, `single`,
      `double`, `char`, or `byte`,
+    - `"_ArraySize_"` is an array, since the dimensions are written as the ND-array header's length,
    - every entry of `"_ArraySize_"` is a non-negative integer, and their product is representable as a `std::size_t`,
    - `"_ArrayData_"` holds exactly that many elements, and
    - every element of `"_ArrayData_"` is a number of the kind named by `"_ArrayType_"` (a floating-point number for
@@ -1668,6 +1668,15 @@ class binary_writer
        CharType dtype = it->second;

        key = "_ArraySize_";
+        // the dimensions are written verbatim as the header length below, so a
+        // value that is not an array cannot produce a valid one: null emits 'Z'
+        // and an object emits '{', neither of which a reader accepts after '#'.
+        // Such an object is not a valid ndarray and falls back to a plain object.
+        if (!value.at(key).is_array())
+        {
+            return true;
+        }
+
        std::size_t len = (value.at(key).empty() ? 0 : 1);
        for (const auto& el : value.at(key))
        {
@@ -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)
@@ -4129,6 +4153,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    /// @brief check if the input is valid JSON
    /// @sa https://json.nlohmann.me/api/basic_json/accept/
    template<typename InputType>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
    static bool accept(InputType&& i,
                       const bool ignore_comments = false,
                       const bool ignore_trailing_commas = false)
@@ -4140,6 +4165,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    /// @sa https://json.nlohmann.me/api/basic_json/accept/
    template<typename IteratorType, typename SentinelType = IteratorType,
             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
    static bool accept(IteratorType first, SentinelType last,
                       const bool ignore_comments = false,
                       const bool ignore_trailing_commas = false)
@@ -4239,6 +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
    {
@@ -18676,6 +18676,15 @@ class binary_writer
        CharType dtype = it->second;

        key = "_ArraySize_";
+        // the dimensions are written verbatim as the header length below, so a
+        // value that is not an array cannot produce a valid one: null emits 'Z'
+        // and an object emits '{', neither of which a reader accepts after '#'.
+        // Such an object is not a valid ndarray and falls back to a plain object.
+        if (!value.at(key).is_array())
+        {
+            return true;
+        }
+
        std::size_t len = (value.at(key).empty() ? 0 : 1);
        for (const auto& el : value.at(key))
        {
@@ -22754,6 +22763,7 @@ 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,
@@ -22776,6 +22786,7 @@ 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;
@@ -22783,6 +22794,7 @@ 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();
@@ -22790,6 +22802,7 @@ 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();
@@ -22797,6 +22810,7 @@ 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;
@@ -22804,6 +22818,7 @@ 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;
@@ -22811,6 +22826,7 @@ 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();
@@ -22818,6 +22834,7 @@ 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;
@@ -22825,6 +22842,7 @@ 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;
@@ -22832,6 +22850,7 @@ 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;
@@ -22839,6 +22858,7 @@ 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;
@@ -22846,6 +22866,7 @@ 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;
@@ -22853,6 +22874,7 @@ 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;
@@ -22860,6 +22882,7 @@ 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;
@@ -22867,6 +22890,7 @@ 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;
@@ -24198,6 +24222,7 @@ 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
@@ -24208,6 +24233,7 @@ 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
@@ -24216,6 +24242,7 @@ 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();
@@ -24225,6 +24252,7 @@ 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();
@@ -24232,12 +24260,14 @@ 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
    {
@@ -24393,6 +24423,7 @@ 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)
@@ -24432,6 +24463,7 @@ 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)
@@ -24471,6 +24503,7 @@ 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)
@@ -25548,6 +25581,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    /// @brief check if the input is valid JSON
    /// @sa https://json.nlohmann.me/api/basic_json/accept/
    template<typename InputType>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
    static bool accept(InputType&& i,
                       const bool ignore_comments = false,
                       const bool ignore_trailing_commas = false)
@@ -25559,6 +25593,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
    /// @sa https://json.nlohmann.me/api/basic_json/accept/
    template<typename IteratorType, typename SentinelType = IteratorType,
             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    JSON_HEDLEY_WARN_UNUSED_RESULT
    static bool accept(IteratorType first, SentinelType last,
                       const bool ignore_comments = false,
                       const bool ignore_trailing_commas = false)
@@ -25658,6 +25693,7 @@ 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
    {
@@ -15,6 +15,15 @@
 namespace utils
 {

+// Some tests intentionally discard the [[nodiscard]]/JSON_HEDLEY_WARN_UNUSED_RESULT
+// return value of a call they only make to exercise its side effects (e.g. checking
+// that it does not throw). A plain (void) cast on the call expression does not
+// suppress GCC's warning for functions using the GNU __attribute__((warn_unused_result))
+// form (as opposed to the C++17 [[nodiscard]] attribute) -- passing the value into an
+// ordinary function call does.
+template<typename T>
+inline void ignore_return_value(T&& /*unused*/) noexcept {}
+
 inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
 {
    std::ifstream file(filename, std::ios::binary);
@@ -2751,6 +2751,31 @@ TEST_CASE("BJData")
                CHECK(json::to_bjdata(j_ok) == std::vector<uint8_t>({'[', '$', 'U', '#', '[', 'i', 2, 'i', 3, ']', 1, 2, 3, 4, 5, 6}));
                CHECK(json::from_bjdata(json::to_bjdata(j_ok), true, true) == j_ok);
            }
+
+            SECTION("ndarray whose _ArraySize_ is not an array stays as object")
+            {
+                // the shape is written verbatim as the header length, so a
+                // value that is not an array cannot produce a valid one: null
+                // would emit 'Z' and an object '{', neither of which a reader
+                // accepts after '#'. Both have to stay plain objects.
+                json const j_null = json({{"_ArrayType_", "uint8"}, {"_ArraySize_", nullptr}, {"_ArrayData_", json::array()}});
+                const auto out_null = json::to_bjdata(j_null);
+                CHECK(out_null.at(0) == '{');
+                CHECK(json::from_bjdata(out_null) == j_null);
+
+                // an object shape passes the per-entry check by iterating its
+                // values rather than dimensions, so it needs rejecting too
+                json const j_obj = json({{"_ArrayType_", "uint8"}, {"_ArraySize_", {{"a", 1}}}, {"_ArrayData_", {1}}});
+                const auto out_obj = json::to_bjdata(j_obj);
+                CHECK(out_obj.at(0) == '{');
+                CHECK(json::from_bjdata(out_obj) == j_obj);
+
+                // a scalar shape is not a dimension list either
+                json const j_num = json({{"_ArrayType_", "uint8"}, {"_ArraySize_", 1}, {"_ArrayData_", {1}}});
+                const auto out_num = json::to_bjdata(j_num);
+                CHECK(out_num.at(0) == '{');
+                CHECK(json::from_bjdata(out_num) == j_num);
+            }
        }
    }

@@ -23,6 +23,8 @@ using nlohmann::json;
 #include <utility>
 #include <vector>

+#include "test_utils.hpp"
+
 namespace
 {
 class SaxEventLogger
@@ -624,7 +626,8 @@ TEST_CASE("parser class")
            SECTION("overflow")
            {
                // overflows during parsing yield an exception
-                CHECK_THROWS_WITH_AS(parser_helper("1.18973e+4932").empty(), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
+                // empty() is nodiscard; the exception is thrown by parser_helper() itself, before empty() would run
+                CHECK_THROWS_WITH_AS(utils::ignore_return_value(parser_helper("1.18973e+4932").empty()), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
            }

            SECTION("invalid numbers")
@@ -22,6 +22,8 @@ using nlohmann::json;

 #include <valarray>

+#include "test_utils.hpp"
+
 namespace
 {
 class SaxEventLogger
@@ -629,7 +631,8 @@ TEST_CASE("parser class")
            SECTION("overflow")
            {
                // overflows during parsing yield an exception
-                CHECK_THROWS_WITH_AS(parser_helper("1.18973e+4932").empty(), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
+                // empty() is nodiscard; the exception is thrown by parser_helper() itself, before empty() would run
+                CHECK_THROWS_WITH_AS(utils::ignore_return_value(parser_helper("1.18973e+4932").empty()), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
            }

            SECTION("invalid numbers")
@@ -29,6 +29,7 @@ using nlohmann::json;
 #include <limits>
 #include <cstdio>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 #ifdef JSON_HAS_CPP_17
    #include <variant>
@@ -1373,7 +1374,8 @@ TEST_CASE("regression tests 1")
        std::array<uint8_t, 28> key1 = {{ 103, 92, 117, 48, 48, 48, 55, 92, 114, 215, 126, 214, 95, 92, 34, 174, 40, 71, 38, 174, 40, 71, 38, 223, 134, 247, 127, 0 }};
        std::string const key1_str(reinterpret_cast<char*>(key1.data()));
        json const j = key1_str;
-        CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 10: 0x7E", json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 10: 0x7E", json::type_error&);
    }

 #if JSON_USE_IMPLICIT_CONVERSIONS
@@ -31,6 +31,8 @@ using ordered_json = nlohmann::ordered_json;
 #include <type_traits>
 #include <utility>

+#include "test_utils.hpp"
+
 #ifdef JSON_HAS_CPP_17
    #include <any>
    #include <variant>
@@ -639,7 +641,12 @@ TEST_CASE("regression tests 2")
                s += static_cast<char>(i);
            }
            dump_test["1"] = s;
-            dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
+            // dump() is nodiscard; this only checks that dumping does not throw/crash.
+            // A (void) cast on the call itself does not suppress GCC's warning for the
+            // GNU warn_unused_result attribute (unlike a real C++17 [[nodiscard]]), so
+            // capture the result in a variable and discard that instead.
+            auto dump_result = dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
+            (void)dump_result;
        }
    }

@@ -731,12 +738,14 @@ TEST_CASE("regression tests 2")
    {
        const std::array<unsigned char, 23> data = {{0x81, 0xA4, 0x64, 0x61, 0x74, 0x61, 0xC4, 0x0F, 0x33, 0x30, 0x30, 0x32, 0x33, 0x34, 0x30, 0x31, 0x30, 0x37, 0x30, 0x35, 0x30, 0x31, 0x30}};
        const json j = json::from_msgpack(data.data(), data.size());
+        // dump() is nodiscard; this only checks that dumping does not throw
        CHECK_NOTHROW(
-            j.dump(4,                             // Indent
-                   ' ',                           // Indent char
-                   false,                         // Ensure ascii
-                   json::error_handler_t::strict  // Error
-                  ));
+            utils::ignore_return_value(
+                j.dump(4,                             // Indent
+                       ' ',                           // Indent char
+                       false,                         // Ensure ascii
+                       json::error_handler_t::strict  // Error
+                      )));
    }

    SECTION("PR #2181 - regression bug with lvalue")
@@ -15,6 +15,8 @@ using nlohmann::json;
 #include <sstream>
 #include <iomanip>

+#include "test_utils.hpp"
+
 TEST_CASE("serialization")
 {
    SECTION("operator<<")
@@ -84,8 +86,9 @@ TEST_CASE("serialization")
        {
            const json j = "ä\xA9ü";

-            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
-            CHECK_THROWS_WITH_AS(j.dump(1, ' ', false, json::error_handler_t::strict), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
+            // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+            CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
+            CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"äü\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"ä\xEF\xBF\xBDü\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"\\u00e4\\ufffd\\u00fc\"");
@@ -95,8 +98,9 @@ TEST_CASE("serialization")
        {
            const json j = "123\xC2";

-            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
-            CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
+            // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+            CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
+            CHECK_THROWS_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd\"");
@@ -106,8 +110,9 @@ TEST_CASE("serialization")
        {
            const json j = "123\xF1\xB0\x34\x35\x36";

-            CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0x34", json::type_error&);
-            CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
+            // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+            CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0x34", json::type_error&);
+            CHECK_THROWS_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), json::type_error&);
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123456\"");
            CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\x34\x35\x36\"");
            CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd456\"");
@@ -17,6 +17,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iomanip>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 TEST_CASE("Unicode (1/5)" * doctest::skip())
 {
@@ -240,7 +241,8 @@ void roundtrip(bool success_expected, const std::string& s)
    if (success_expected)
    {
        // serialization succeeds
-        CHECK_NOTHROW(j.dump());
+        // dump() is nodiscard; this only checks that dumping does not throw
+        CHECK_NOTHROW(utils::ignore_return_value(j.dump()));

        // exclude parse test for U+0000
        if (s[0] != '\0')
@@ -259,7 +261,8 @@ void roundtrip(bool success_expected, const std::string& s)
    else
    {
        // serialization fails
-        CHECK_THROWS_AS(j.dump(), json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);

        // parsing JSON text fails
        CHECK_THROWS_AS(_ = json::parse(ps), json::parse_error&);
@@ -19,6 +19,7 @@ using nlohmann::json;
 #include <iostream>
 #include <iomanip>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
    else
    {
        // strict mode must throw if success is not expected
-        CHECK_THROWS_AS(j.dump(), json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
        // ignore and replace must create different dumps
        CHECK(s_ignored != s_replaced);

@@ -19,6 +19,7 @@ using nlohmann::json;
 #include <iostream>
 #include <iomanip>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
    else
    {
        // strict mode must throw if success is not expected
-        CHECK_THROWS_AS(j.dump(), json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
        // ignore and replace must create different dumps
        CHECK(s_ignored != s_replaced);

@@ -19,6 +19,7 @@ using nlohmann::json;
 #include <iostream>
 #include <iomanip>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
    else
    {
        // strict mode must throw if success is not expected
-        CHECK_THROWS_AS(j.dump(), json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
        // ignore and replace must create different dumps
        CHECK(s_ignored != s_replaced);

@@ -19,6 +19,7 @@ using nlohmann::json;
 #include <iostream>
 #include <iomanip>
 #include "make_test_data_available.hpp"
+#include "test_utils.hpp"

 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
    else
    {
        // strict mode must throw if success is not expected
-        CHECK_THROWS_AS(j.dump(), json::type_error&);
+        // dump() is nodiscard; the exception is thrown by dump() itself before it would return
+        CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
        // ignore and replace must create different dumps
        CHECK(s_ignored != s_replaced);
Author	SHA1	Message	Date
Niels Lohmann	7cb4d3284e	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>	2026-09-06 12:18:38 +02:00
Niels Lohmann	354d5fa723	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>	2026-09-05 20:51:58 +02:00
Niels Lohmann	ae6f737976	Add JSON_HEDLEY_WARN_UNUSED_RESULT to the current accept() overloads accept() is a pure query whose only effect is the returned bool; both parse() overloads and the deprecated accept(span_input_adapter&&, ...) overload already carry JSON_HEDLEY_WARN_UNUSED_RESULT, but the two current, recommended accept() overloads were missing it. Add the annotation to match, so discarding accept()'s result now warns under -Wunused-result / [[nodiscard]], as it already does for parse(). Fixes #5407 Signed-off-by: Niels Lohmann <mail@nlohmann.me>	2026-09-05 20:40:48 +02:00
Qatadaha Bin Matloob	09b6b6b5ba	Fix to_bjdata() emitting unparsable output when _ArraySize_ is not an array (#5455 ) * Fix to_bjdata() emitting unparsable output when _ArraySize_ is not an array write_bjdata_ndarray() never checked that _ArraySize_ is an array. The shape is written verbatim as the header length, so a null shape emitted 'Z' and an object shape emitted '{' after the '#', neither of which from_bjdata() accepts, and the round-trip guarantee in the BJData docs was broken. Both slipped through the existing validation: for null, empty() is true so the element count starts at 0 and the per-dimension loop never runs, and for an object the loop walks its values, which can satisfy the non-negative integer check. When _ArrayData_ then matched that count, the writer took the ndarray path. Require the shape to be an array, so anything else falls back to a plain object encoding that round-trips, as the fallback rule in the docs already specifies. Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com> * Document that _ArraySize_ must be an array in the ndarray requirements The list at bjdata.md is the exhaustive set of conditions for the ndarray encoding, but it only implied this one through 'every entry of'. Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com> --------- Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>	2026-09-05 20:30:22 +02:00