Construct the JSON Patch exceptions out of line

patch_inplace() built its error messages inline. In particular get_value()
concatenated the context-sensitive "operation 'x'" string on every call, even
when nothing was wrong. The patch exceptions now come from NEVER_INLINE
helpers: the index and key errors reuse basic_json's existing
throw_array_index_out_of_range/throw_key_not_found, and the seven
patch-specific messages get one helper each in detail/exceptions.hpp. Ids
and messages are unchanged, and unit-throw_messages pins them (with and
without JSON_DIAGNOSTICS).

citm_catalog patch: -6.6 % (Apple clang), -9.5 % (GCC 16).
The other remaining throw sites (parser::syntax_error, binary_writer,
serializer, input_adapters, enum_key_duplicate, format_error) were prototyped
and measured: no gain beyond noise, so they stay as they are.

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

include/nlohmann/detail/exceptions.hpp

@@ -428,7 +428,7 @@ void templated_json_throw(ExceptionType exception)
 @throw type_error.302 always
 */
 template<typename BasicJsonType>
-JSON_HEDLEY_NO_RETURN inline void throw_type_must_be(const char* expected, const BasicJsonType& j)
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_type_must_be(const char* expected, const BasicJsonType& j)
 {
     static_cast<void>(expected); // unused when JSON_NOEXCEPTION is defined
     static_cast<void>(j);
@@ -443,7 +443,7 @@ JSON_HEDLEY_NO_RETURN inline void throw_type_must_be(const char* expected, const
 @throw type_error always
 */
 template<typename BasicJsonType>
-JSON_HEDLEY_NO_RETURN inline void throw_cannot_use_with(const exception_id id_, const char* operation, const BasicJsonType& j)
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_cannot_use_with(const exception_id id_, const char* operation, const BasicJsonType& j)
 {
     static_cast<void>(id_); // unused when JSON_NOEXCEPTION is defined
     static_cast<void>(operation);
@@ -451,6 +451,77 @@ JSON_HEDLEY_NO_RETURN inline void throw_cannot_use_with(const exception_id id_,
     JSON_THROW(type_error::create(id_, concat("cannot use ", operation, " with ", j.type_name()), &j));
 }
 
+/*!
+@brief throws ExceptionType with a fixed message; kept out of line so that callers do not carry the exception construction
+*/
+template<typename ExceptionType, typename BasicJsonContext>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_literal(const exception_id id_, const char* what_arg, BasicJsonContext context)
+{
+    static_cast<void>(id_); // unused when JSON_NOEXCEPTION is defined
+    static_cast<void>(what_arg);
+    static_cast<void>(context);
+    JSON_THROW(ExceptionType::create(id_, what_arg, context));
+}
+
+// JSON Patch errors; constructed out of line so that patch_inplace() does not carry them
+
+template<typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_add_parent(const BasicJsonType& parent)
+{
+    static_cast<void>(parent);
+    JSON_THROW(out_of_range::create(exception_id::patch_add_parent_not_container, concat("cannot add value: the JSON Patch 'add' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
+}
+
+template<typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_remove_parent(const BasicJsonType& parent)
+{
+    static_cast<void>(parent);
+    JSON_THROW(out_of_range::create(exception_id::patch_remove_parent_not_container, concat("cannot remove value: the JSON Patch 'remove' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
+}
+
+template<typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_not_an_array(const BasicJsonType& j)
+{
+    static_cast<void>(j);
+    JSON_THROW(parse_error::create(exception_id::patch_not_an_array, 0, "JSON patch must be an array of objects", &j));
+}
+
+template<typename StringType, typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_missing_member(const StringType& op, const StringType& member, const bool string_type, const BasicJsonType& val)
+{
+    static_cast<void>(op);
+    static_cast<void>(member);
+    static_cast<void>(string_type);
+    static_cast<void>(val);
+    // context-sensitive error message
+    const std::string error_msg = (op == "op") ? std::string("operation") : concat("operation '", op, '\'');
+    JSON_THROW(parse_error::create(exception_id::patch_invalid_operation, 0, concat(error_msg, string_type ? " must have string member '" : " must have member '", member, "'"), &val));
+}
+
+template<typename StringType, typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_move_into_child(const StringType& from_path, const StringType& path, const BasicJsonType& result)
+{
+    static_cast<void>(from_path);
+    static_cast<void>(path);
+    static_cast<void>(result);
+    JSON_THROW(out_of_range::create(exception_id::patch_move_into_child, concat("cannot move value: 'from' path '", from_path, "' is a proper prefix of 'path' '", path, "'"), &result));
+}
+
+template<typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_test_failed(const BasicJsonType& val)
+{
+    static_cast<void>(val);
+    JSON_THROW(other_error::create(exception_id::patch_test_failed, concat("unsuccessful: ", val.dump()), &val));
+}
+
+template<typename StringType, typename BasicJsonType>
+JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_patch_invalid_operation(const StringType& op, const BasicJsonType& val)
+{
+    static_cast<void>(op);
+    static_cast<void>(val);
+    JSON_THROW(parse_error::create(exception_id::patch_invalid_operation, 0, concat("operation value '", op, "' is invalid"), &val));
+}
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
 

include/nlohmann/detail/input/binary_reader.hpp

@@ -195,7 +195,7 @@ class binary_reader
 
             if (JSON_HEDLEY_UNLIKELY(current != char_traits<char_type>::eof()))
             {
-                return last_byte_error(exception_id::unexpected_end_of_input, concat("expected end of input; last byte: 0x", get_token_string()), "value");
+                return last_byte_error(exception_id::unexpected_end_of_input, "expected end of input", "value", "");
             }
         }
 
@@ -317,7 +317,7 @@ class binary_reader
     {
         if (JSON_HEDLEY_UNLIKELY(document_size < 0 || static_cast<std::size_t>(document_size) != chars_read - document_start))
         {
-            return last_byte_error(exception_id::unexpected_byte, concat("document size ", std::to_string(document_size), " does not match the number of bytes read (", std::to_string(chars_read - document_start), ")"), "document");
+            return bson_document_size_error(document_size, chars_read - document_start);
         }
         return true;
     }
@@ -509,7 +509,7 @@ class binary_reader
     {
         if (JSON_HEDLEY_UNLIKELY(len < 1))
         {
-            return last_byte_error(exception_id::unexpected_byte, concat("string length must be at least 1, is ", std::to_string(len)), "string");
+            return bson_length_error("string length must be at least 1, is ", len, "string");
         }
 
         if (JSON_HEDLEY_UNLIKELY(!get_string(len - static_cast<NumberType>(1), result)))
@@ -539,7 +539,7 @@ class binary_reader
     {
         if (JSON_HEDLEY_UNLIKELY(len < 0))
         {
-            return last_byte_error(exception_id::unexpected_byte, concat("byte array length cannot be negative, is ", std::to_string(len)), "binary");
+            return bson_length_error("byte array length cannot be negative, is ", len, "binary");
         }
 
         // All BSON binary values have a subtype
@@ -629,9 +629,7 @@ class binary_reader
 
             default: // anything else is not supported (yet)
             {
-                const std::string cr_str = hex_byte(static_cast<std::uint8_t>(element_type));
+                return bson_unsupported_type_error(element_type_parse_position, element_type);
-                return sax->parse_error(element_type_parse_position, cr_str,
-                                        parse_error::create(exception_id::bson_unsupported_type, element_type_parse_position, concat("Unsupported BSON record type 0x", cr_str), nullptr));
             }
         }
     }
@@ -1060,6 +1058,7 @@ class binary_reader
     @param[in] context  parsing context for the error message
     @return whether the SAX consumer accepts the parse error
     */
+    JSON_HEDLEY_NEVER_INLINE
     bool cbor_indefinite_string_error(const char* type_name, const char* context)
     {
         return last_byte_error(exception_id::invalid_string_or_size, concat("indefinite-length ", type_name, " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", get_token_string()), context);
@@ -1139,7 +1138,7 @@ class binary_reader
 
             default:
             {
-                return last_byte_error(exception_id::invalid_string_or_size, concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", get_token_string()), "string");
+                return last_byte_error(exception_id::invalid_string_or_size, "expected length specification (0x60-0x7B)", "string", inside_indefinite ? "" : " or indefinite string type (0x7F)");
             }
         }
     }
@@ -1269,7 +1268,7 @@ class binary_reader
                 break;
         }
 
-        return last_byte_error(exception_id::invalid_string_or_size, concat("only string keys are supported, but found ", found, "; last byte: 0x", get_token_string()), "object key");
+        return last_byte_error(exception_id::invalid_string_or_size, "only string keys are supported, but found ", "object key", found);
     }
 
     /*!
@@ -1350,7 +1349,7 @@ class binary_reader
 
             default:
             {
-                return last_byte_error(exception_id::invalid_string_or_size, concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", get_token_string()), "binary");
+                return last_byte_error(exception_id::invalid_string_or_size, "expected length specification (0x40-0x5B)", "binary", inside_indefinite ? "" : " or indefinite binary array type (0x5F)");
             }
         }
     }
@@ -1496,8 +1495,7 @@ class binary_reader
     {
         if (JSON_HEDLEY_UNLIKELY(!value_in_range_of<std::size_t>(len) || len == detail::unknown_size()))
         {
-            return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large,
+            return container_size_error(context);
-                                    exception_message(concat("excessive ", context, " size"), "size"), nullptr));
         }
         result = conditional_static_cast<std::size_t>(len);
         return true;
@@ -2065,7 +2063,7 @@ class binary_reader
 
             default:
             {
-                return last_byte_error(exception_id::invalid_string_or_size, concat("expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0x", get_token_string()), "string");
+                return last_byte_error(exception_id::invalid_string_or_size, "expected length specification (0xA0-0xBF, 0xD9-0xDB)", "string", "");
             }
         }
     }
@@ -2157,7 +2155,7 @@ class binary_reader
                 break;
         }
 
-        return last_byte_error(exception_id::invalid_string_or_size, concat("only string keys are supported, but found ", found, "; last byte: 0x", get_token_string()), "object key");
+        return last_byte_error(exception_id::invalid_string_or_size, "only string keys are supported, but found ", "object key", found);
     }
 
     /*!
@@ -2655,8 +2653,7 @@ class binary_reader
         }
         if (JSON_HEDLEY_UNLIKELY(!value_in_range_of<std::size_t>(number)))
         {
-            return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large,
+            return out_of_range_error(exception_id::container_too_large, "integer value overflow", "size");
-                                    exception_message("integer value overflow", "size"), nullptr));
         }
         result = static_cast<std::size_t>(number); // NOLINT(bugprone-signed-char-misuse,cert-str34-c): number is not a char
         return true;
@@ -2753,8 +2750,7 @@ class binary_reader
                 }
                 if (!value_in_range_of<std::size_t>(number))
                 {
-                    return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large,
+                    return out_of_range_error(exception_id::container_too_large, "integer value overflow", "size");
-                                            exception_message("integer value overflow", "size"), nullptr));
                 }
                 result = detail::conditional_static_cast<std::size_t>(number);
                 return true;
@@ -2832,7 +2828,7 @@ class binary_reader
                         // or SIZE_MAX.
                         if (JSON_HEDLEY_UNLIKELY(result > (std::numeric_limits<std::size_t>::max)() / i))
                         {
-                            return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large, exception_message("excessive ndarray size caused overflow", "size"), nullptr));
+                            return out_of_range_error(exception_id::container_too_large, "excessive ndarray size caused overflow", "size");
                         }
                         result *= i;
                         // the pre-check above already rules out result becoming 0
@@ -2841,7 +2837,7 @@ class binary_reader
                         // unknown-size container (see get_ubjson_size_type())
                         if (result == npos)
                         {
-                            return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large, exception_message("excessive ndarray size caused overflow", "size"), nullptr));
+                            return out_of_range_error(exception_id::container_too_large, "excessive ndarray size caused overflow", "size");
                         }
                         if (JSON_HEDLEY_UNLIKELY(!emit_unsigned(i)))
                         {
@@ -2891,7 +2887,7 @@ class binary_reader
             if (input_format == input_format_t::bjdata
                     && JSON_HEDLEY_UNLIKELY(is_bjd_excluded_optimized_type(result.second)))
             {
-                return last_byte_error(exception_id::unexpected_byte, concat("marker 0x", get_token_string(), " is not a permitted optimized array type"), "type");
+                return optimized_type_error();
             }
 
             if (JSON_HEDLEY_UNLIKELY(!unexpect_eof("type")))
@@ -2906,7 +2902,7 @@ class binary_reader
                 {
                     return false;
                 }
-                return last_byte_error(exception_id::unexpected_byte, concat("expected '#' after type information; last byte: 0x", get_token_string()), "size");
+                return last_byte_error(exception_id::unexpected_byte, "expected '#' after type information", "size", "");
             }
 
             const bool is_error = get_ubjson_size_value(result.first, is_ndarray, 0, result.second);
@@ -3057,7 +3053,7 @@ class binary_reader
                 }
                 if (JSON_HEDLEY_UNLIKELY(current > 127))
                 {
-                    return last_byte_error(exception_id::invalid_string_or_size, concat("byte after 'C' must be in range 0x00..0x7F; last byte: 0x", get_token_string()), "char");
+                    return last_byte_error(exception_id::invalid_string_or_size, "byte after 'C' must be in range 0x00..0x7F", "char", "");
                 }
                 string_t s(1, static_cast<typename string_t::value_type>(current));
                 return sax->string(s);
@@ -3141,8 +3137,7 @@ class binary_reader
             if (JSON_HEDLEY_UNLIKELY((size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F')
                                      && size_and_type.first > max_valueless_container_size))
             {
-                return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large,
+                return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
-                                        exception_message("excessive array size", "size"), nullptr));
             }
 
             if (JSON_HEDLEY_UNLIKELY(!enter_array(size_and_type.first, size_and_type.second)))
@@ -3854,7 +3849,7 @@ class binary_reader
         {
             // in case of failure, advance position by 1 to report the failing location
             ++chars_read;
-            sax->parse_error(chars_read, "<end of file>", parse_error::create(exception_id::unexpected_end_of_input, chars_read, exception_message("unexpected end of input", context), nullptr));
+            static_cast<void>(report_eof(context));
             return false;
         }
         return true;
@@ -4007,8 +4002,7 @@ class binary_reader
         const auto result = static_cast<number_float_t>(number);
         if (JSON_HEDLEY_UNLIKELY(std::isfinite(number) && !std::isfinite(result)))
         {
-            return sax->parse_error(chars_read, get_token_string(),
+            return out_of_range_error(exception_id::number_overflow, "number overflow", "value");
-                                    out_of_range::create(exception_id::number_overflow, exception_message("number overflow", "value"), nullptr));
         }
         return sax->number_float(result, "");
     }
@@ -4223,22 +4217,82 @@ class binary_reader
     {
         if (JSON_HEDLEY_UNLIKELY(current == char_traits<char_type>::eof()))
         {
-            return sax->parse_error(chars_read, "<end of file>",
+            return report_eof(context);
-                                    parse_error::create(exception_id::unexpected_end_of_input, chars_read, exception_message("unexpected end of input", context), nullptr));
         }
         return true;
     }
 
+    /*!
+    @brief reports an unexpected end of input (out of line to keep the exception
+           construction out of the hot reading functions)
+    @param[in] context  further context information (for diagnostics)
+    @return the result of the SAX parser's parse_error()
+    */
+    JSON_HEDLEY_NON_NULL(2)
+    JSON_HEDLEY_NEVER_INLINE
+    bool report_eof(const char* context) const
+    {
+        return sax->parse_error(chars_read, "<end of file>",
+                                parse_error::create(exception_id::unexpected_end_of_input, chars_read, exception_message("unexpected end of input", context), nullptr));
+    }
+
+    /*!
+    @brief reports an out_of_range error at the last read byte (out of line)
+    */
+    JSON_HEDLEY_NEVER_INLINE
+    bool out_of_range_error(const exception_id id_, const char* detail, const char* context) const
+    {
+        return sax->parse_error(chars_read, get_token_string(), out_of_range::create(id_, exception_message(detail, context), nullptr));
+    }
+
+    JSON_HEDLEY_NEVER_INLINE
+    bool container_size_error(const char* what) const
+    {
+        return sax->parse_error(chars_read, get_token_string(), out_of_range::create(exception_id::container_too_large,
+                                exception_message(concat("excessive ", what, " size"), "size"), nullptr));
+    }
+
+    JSON_HEDLEY_NEVER_INLINE
+    bool optimized_type_error() const
+    {
+        return last_byte_error(exception_id::unexpected_byte, concat("marker 0x", get_token_string(), " is not a permitted optimized array type"), "type");
+    }
+
+    template<typename NumberType>
+    JSON_HEDLEY_NEVER_INLINE
+    bool bson_length_error(const char* prefix, const NumberType len, const char* context) const
+    {
+        return last_byte_error(exception_id::unexpected_byte, concat(prefix, std::to_string(len)), context);
+    }
+
+    JSON_HEDLEY_NEVER_INLINE
+    bool bson_document_size_error(const std::int32_t document_size, const std::size_t bytes_read) const
+    {
+        return last_byte_error(exception_id::unexpected_byte, concat("document size ", std::to_string(document_size), " does not match the number of bytes read (", std::to_string(bytes_read), ")"), "document");
+    }
+
+    JSON_HEDLEY_NEVER_INLINE
+    bool bson_unsupported_type_error(const std::size_t position, const char element_type) const
+    {
+        const std::string cr_str = hex_byte(static_cast<std::uint8_t>(element_type));
+        return sax->parse_error(position, cr_str,
+                                parse_error::create(exception_id::bson_unsupported_type, position, concat("Unsupported BSON record type 0x", cr_str), nullptr));
+    }
+
     /*!
     @brief reports a parse error at the last read byte
     @param[in] id_      the id of the parse_error exception
     @param[in] detail   a detailed error message
     @param[in] context  further context information
+    @param[in] tail     optional text appended to the detail, followed by "; last byte: 0x<token>"
     @return the result of the SAX parser's parse_error()
     */
-    bool last_byte_error(const exception_id id_, const std::string& detail, const std::string& context) const
+    JSON_HEDLEY_NEVER_INLINE
+    bool last_byte_error(const exception_id id_, const std::string& detail, const char* context, const char* tail = nullptr) const
     {
-        return sax->parse_error(chars_read, get_token_string(), parse_error::create(id_, chars_read, exception_message(detail, context), nullptr));
+        // with a tail, the message reads "<detail><tail>; last byte: 0x<token>"
+        return sax->parse_error(chars_read, get_token_string(), parse_error::create(id_, chars_read,
+                                exception_message(tail == nullptr ? detail : concat(detail, tail, "; last byte: 0x", get_token_string()), context), nullptr));
     }
 
     /*!
@@ -4247,7 +4301,8 @@ class binary_reader
     @param[in] context  further context information
     @return the result of the SAX parser's parse_error()
     */
-    bool unexpected_byte(const char* detail, const std::string& context) const
+    JSON_HEDLEY_NEVER_INLINE
+    bool unexpected_byte(const char* detail, const char* context) const
     {
         return last_byte_error(exception_id::unexpected_byte, concat(detail, ": 0x", get_token_string()), context);
     }
@@ -4258,6 +4313,7 @@ class binary_reader
     @param[in] context   further context information
     @return the result of the SAX parser's parse_error()
     */
+    JSON_HEDLEY_NEVER_INLINE
     bool length_type_error(const char* position, const char* context) const
     {
         const char* types = input_format == input_format_t::bjdata ? "U, i, u, I, m, l, M, L" : "U, i, I, l, L";

include/nlohmann/detail/input/json_sax.hpp

@@ -187,12 +187,19 @@ Shared by json_sax_dom_parser and json_sax_dom_callback_parser.
 @param[in] ref   the object or array that was just created
 @return whether parsing should continue (false after reporting out_of_range.408)
 */
+template<typename SAX, typename BasicJsonType>
+JSON_HEDLEY_NEVER_INLINE
+bool container_too_large_error(SAX& sax, std::size_t len, const char* kind, BasicJsonType* ref)
+{
+    return sax.parse_error(0, "", out_of_range::create(exception_id::container_too_large, concat("excessive ", kind, " size: ", std::to_string(len)), ref));
+}
+
 template<typename SAX, typename BasicJsonType>
 bool check_container_size(SAX& sax, std::size_t len, const char* kind, BasicJsonType* ref)
 {
     if (JSON_HEDLEY_UNLIKELY(len != detail::unknown_size() && len > ref->max_size()))
     {
-        return sax.parse_error(0, "", out_of_range::create(exception_id::container_too_large, concat("excessive ", kind, " size: ", std::to_string(len)), ref));
+        return container_too_large_error(sax, len, kind, ref);
     }
     return true;
 }

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -280,7 +280,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     }
 
     /// @throw invalid_iterator.214 always
-    JSON_HEDLEY_NO_RETURN void throw_cannot_get_value() const
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE void throw_cannot_get_value() const
     {
         JSON_THROW(invalid_iterator::create(exception_id::iterator_value_unavailable, "cannot get value", m_object));
     }
@@ -294,40 +294,25 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     {
         JSON_ASSERT(m_object != nullptr);
 
-        switch (m_object->m_data.m_type)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
         {
-            case value_t::object:
+            JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
-            {
+            return *m_it.array_iterator;
-                JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
-                return m_it.object_iterator->second;
-            }
-
-            case value_t::array:
-            {
-                JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
-                return *m_it.array_iterator;
-            }
-
-            case value_t::null:
-                throw_cannot_get_value();
-
-            case value_t::string:
-            case value_t::boolean:
-            case value_t::number_integer:
-            case value_t::number_unsigned:
-            case value_t::number_float:
-            case value_t::binary:
-            case value_t::discarded:
-            default:
-            {
-                if (JSON_HEDLEY_LIKELY(m_it.primitive_iterator.is_begin()))
-                {
-                    return *m_object;
-                }
-
-                throw_cannot_get_value();
-            }
         }
+
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
+        {
+            JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
+            return m_it.object_iterator->second;
+        }
+
+        // neither array nor object: a primitive value (null has no value to point to)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type != value_t::null && m_it.primitive_iterator.is_begin()))
+        {
+            return *m_object;
+        }
+
+        throw_cannot_get_value();
     }
 
     /*!
@@ -338,38 +323,25 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     {
         JSON_ASSERT(m_object != nullptr);
 
-        switch (m_object->m_data.m_type)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
         {
-            case value_t::object:
+            JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
-            {
+            return &*m_it.array_iterator;
-                JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
-                return &(m_it.object_iterator->second);
-            }
-
-            case value_t::array:
-            {
-                JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
-                return &*m_it.array_iterator;
-            }
-
-            case value_t::null:
-            case value_t::string:
-            case value_t::boolean:
-            case value_t::number_integer:
-            case value_t::number_unsigned:
-            case value_t::number_float:
-            case value_t::binary:
-            case value_t::discarded:
-            default:
-            {
-                if (JSON_HEDLEY_LIKELY(m_it.primitive_iterator.is_begin()))
-                {
-                    return m_object;
-                }
-
-                throw_cannot_get_value();
-            }
         }
+
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
+        {
+            JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
+            return &(m_it.object_iterator->second);
+        }
+
+        // neither array nor object: a primitive value
+        if (JSON_HEDLEY_LIKELY(m_it.primitive_iterator.is_begin()))
+        {
+            return m_object;
+        }
+
+        throw_cannot_get_value();
     }
 
     /*!
@@ -391,35 +363,20 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     {
         JSON_ASSERT(m_object != nullptr);
 
-        switch (m_object->m_data.m_type)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
         {
-            case value_t::object:
+            ++m_it.array_iterator;
-            {
+            return *this;
-                std::advance(m_it.object_iterator, 1);
-                break;
-            }
-
-            case value_t::array:
-            {
-                std::advance(m_it.array_iterator, 1);
-                break;
-            }
-
-            case value_t::null:
-            case value_t::string:
-            case value_t::boolean:
-            case value_t::number_integer:
-            case value_t::number_unsigned:
-            case value_t::number_float:
-            case value_t::binary:
-            case value_t::discarded:
-            default:
-            {
-                ++m_it.primitive_iterator;
-                break;
-            }
         }
 
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
+        {
+            std::advance(m_it.object_iterator, 1);
+            return *this;
+        }
+
+        // neither array nor object: a primitive value
+        ++m_it.primitive_iterator;
         return *this;
     }
 
@@ -442,35 +399,20 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     {
         JSON_ASSERT(m_object != nullptr);
 
-        switch (m_object->m_data.m_type)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
         {
-            case value_t::object:
+            --m_it.array_iterator;
-            {
+            return *this;
-                std::advance(m_it.object_iterator, -1);
-                break;
-            }
-
-            case value_t::array:
-            {
-                std::advance(m_it.array_iterator, -1);
-                break;
-            }
-
-            case value_t::null:
-            case value_t::string:
-            case value_t::boolean:
-            case value_t::number_integer:
-            case value_t::number_unsigned:
-            case value_t::number_float:
-            case value_t::binary:
-            case value_t::discarded:
-            default:
-            {
-                --m_it.primitive_iterator;
-                break;
-            }
         }
 
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
+        {
+            std::advance(m_it.object_iterator, -1);
+            return *this;
+        }
+
+        // neither array nor object: a primitive value
+        --m_it.primitive_iterator;
         return *this;
     }
 
@@ -484,7 +426,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         // if objects are not the same, the comparison is undefined
         if (JSON_HEDLEY_UNLIKELY(m_object != other.m_object))
         {
-            JSON_THROW(invalid_iterator::create(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object));
+            throw_literal<invalid_iterator>(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object);
         }
 
         // value-initialized forward iterators can be compared, and must compare equal to other value-initialized iterators of the same type #4493
@@ -493,25 +435,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
             return true;
         }
 
-        switch (m_object->m_data.m_type)
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
         {
-            case value_t::object:
+            return (m_it.array_iterator == other.m_it.array_iterator);
-                return (m_it.object_iterator == other.m_it.object_iterator);
-
-            case value_t::array:
-                return (m_it.array_iterator == other.m_it.array_iterator);
-
-            case value_t::null:
-            case value_t::string:
-            case value_t::boolean:
-            case value_t::number_integer:
-            case value_t::number_unsigned:
-            case value_t::number_float:
-            case value_t::binary:
-            case value_t::discarded:
-            default:
-                return (m_it.primitive_iterator == other.m_it.primitive_iterator);
         }
+
+        if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
+        {
+            return (m_it.object_iterator == other.m_it.object_iterator);
+        }
+
+        // neither array nor object: a primitive value
+        return (m_it.primitive_iterator == other.m_it.primitive_iterator);
     }
 
     /*!
@@ -533,7 +468,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         // if objects are not the same, the comparison is undefined
         if (JSON_HEDLEY_UNLIKELY(m_object != other.m_object))
         {
-            JSON_THROW(invalid_iterator::create(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object));
+            throw_literal<invalid_iterator>(exception_id::iterators_compare_different_values, "cannot compare iterators of different containers", m_object);
         }
 
         // value-initialized forward iterators can be compared, and must compare equal to other value-initialized iterators of the same type #4493
@@ -546,7 +481,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_data.m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(exception_id::iterator_order_on_object, "cannot compare order of object iterators", m_object));
+                throw_literal<invalid_iterator>(exception_id::iterator_order_on_object, "cannot compare order of object iterators", m_object);
 
             case value_t::array:
                 return (m_it.array_iterator < other.m_it.array_iterator);
@@ -602,7 +537,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_data.m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(exception_id::iterator_arithmetic_on_object, "cannot use offsets with object iterators", m_object));
+                throw_literal<invalid_iterator>(exception_id::iterator_arithmetic_on_object, "cannot use offsets with object iterators", m_object);
 
             case value_t::array:
             {
@@ -681,7 +616,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_data.m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(exception_id::iterator_arithmetic_on_object, "cannot use offsets with object iterators", m_object));
+                throw_literal<invalid_iterator>(exception_id::iterator_arithmetic_on_object, "cannot use offsets with object iterators", m_object);
 
             case value_t::array:
                 return m_it.array_iterator - other.m_it.array_iterator;
@@ -710,7 +645,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_data.m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(exception_id::iterator_subscript_on_object, "cannot use operator[] for object iterators", m_object));
+                throw_literal<invalid_iterator>(exception_id::iterator_subscript_on_object, "cannot use operator[] for object iterators", m_object);
 
             case value_t::array:
                 return *std::next(m_it.array_iterator, n);
@@ -750,7 +685,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
             return m_it.object_iterator->first;
         }
 
-        JSON_THROW(invalid_iterator::create(exception_id::iterator_key_not_object, "cannot use key() for non-object iterators", m_object));
+        throw_literal<invalid_iterator>(exception_id::iterator_key_not_object, "cannot use key() for non-object iterators", m_object);
     }
 
     /*!

include/nlohmann/json.hpp

@@ -630,7 +630,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                     object = nullptr;  // silence warning, see #821
                     if (JSON_HEDLEY_UNLIKELY(t == value_t::null))
                     {
-                        JSON_THROW(other_error::create(detail::exception_id::internal_error, "961c151d2e87f2686a955a9be24d316f1362bf21 3.12.0", nullptr)); // LCOV_EXCL_LINE
+                        detail::throw_literal<other_error>(detail::exception_id::internal_error, "961c151d2e87f2686a955a9be24d316f1362bf21 3.12.0", nullptr); // LCOV_EXCL_LINE
                     }
                     break;
                 }
@@ -734,6 +734,34 @@ private:
             }
         }
 
+        // true if no child of the non-empty array/object v has children of
+        // its own, i.e. v can be freed in one go (its destructor then
+        // destroys scalars and empty containers only, so it recurses at most
+        // one level deep); stops at the first structured child it meets
+        static bool all_children_leaves(const basic_json& v) noexcept
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                const auto& a = *v.m_data.m_value.array;
+                for (auto it = a.rbegin(); it != a.rend(); ++it)
+                {
+                    if (!has_no_children(*it))
+                    {
+                        return false;
+                    }
+                }
+                return true;
+            }
+            for (const auto& el : *v.m_data.m_value.object)
+            {
+                if (!has_no_children(el.second))
+                {
+                    return false;
+                }
+            }
+            return true;
+        }
+
         // The walk in destroy_container() may take the children of a
         // container in any order, as long as it picks the same child again
         // while that container is not modified in between. Arrays and
@@ -789,15 +817,16 @@ private:
             }
         }
 
-        // deallocates the (already empty) array/object held by v; this is
+        // deallocates the array/object held by v, which is either empty or
-        // the same allocator-based free the old recursive implementation
+        // holds only scalars and empty containers (so its destructor
-        // used, just factored out so every level of the walk in destroy()
+        // recurses at most one level deep); this is the same allocator-based
-        // can share it
+        // free the old recursive implementation used, just factored out so
+        // every level of the walk in destroy() can share it
         static void free_container(basic_json& v) noexcept
         {
+            JSON_ASSERT(has_no_children(v) || all_children_leaves(v));
             if (v.m_data.m_type == value_t::array)
             {
-                JSON_ASSERT(v.m_data.m_value.array->empty());
                 AllocatorType<array_t> alloc;
                 std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
                 std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
@@ -805,7 +834,6 @@ private:
             else
             {
                 JSON_ASSERT(v.m_data.m_type == value_t::object);
-                JSON_ASSERT(v.m_data.m_value.object->empty());
                 AllocatorType<object_t> alloc;
                 std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
                 std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
@@ -850,7 +878,58 @@ public:
                 return;
             }
 
-            // Destroy the tree without recursing per nesting level and
+            // This json_value is not itself a basic_json, so the top-level
+            // container is first moved into a local stand-in ("cur"); a
+            // default-constructed basic_json has a null pointer in its
+            // m_value (see data::m_value's initializer), so swapping it
+            // with *this leaves this union's own pointer null, and it is
+            // never looked at or freed a second time.
+            basic_json cur;
+            cur.m_data.m_type = t;
+            using std::swap;
+            swap(cur.m_data.m_value, *this);
+
+            destroy_bounded(cur, 0);
+        }
+
+        // Destroy the array/object v, whose children are destroyed first.
+        // The first recursion_depth_limit() levels are destroyed by plain
+        // recursion, which is the fastest way and cannot throw; anything
+        // nested deeper is handed to destroy_walk(), which uses no call stack.
+        static void destroy_bounded(basic_json& v, std::size_t depth) noexcept
+        {
+            if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
+            {
+                destroy_walk(v);
+                return;
+            }
+
+            if (v.m_data.m_type == value_t::array)
+            {
+                for (auto& child : *v.m_data.m_value.array)
+                {
+                    if (child.m_data.m_type == value_t::array || child.m_data.m_type == value_t::object)
+                    {
+                        destroy_bounded(child, depth + 1);
+                    }
+                }
+            }
+            else
+            {
+                for (auto& child : *v.m_data.m_value.object)
+                {
+                    if (child.second.m_data.m_type == value_t::array || child.second.m_data.m_type == value_t::object)
+                    {
+                        destroy_bounded(child.second, depth + 1);
+                    }
+                }
+            }
+
+            // only leaves and already destroyed (null) children remain
+            free_container(v);
+        }
+
+            // Destroy the tree below top without recursing per nesting level and
             // without any heap allocation: a heap-allocated flattening
             // stack (the previous implementation) can itself throw
             // bad_alloc, which would escape this noexcept destructor and
@@ -874,16 +953,24 @@ public:
             // pointer in its m_value (see data::m_value's initializer),
             // so swapping it with *this leaves this union's own pointer
             // null, and it is never looked at or freed a second time.
+        static void destroy_walk(basic_json& top) noexcept
+        {
             basic_json cur;
-            cur.m_data.m_type = t;
+            take(cur, top);
-            using std::swap;
-            swap(cur.m_data.m_value, *this);
 
             basic_json prev; // value_t::null: no parent
 
+            // true right after cur was entered: its children have not been
+            // looked at yet. If none of them has children of its own, cur is
+            // freed in one go instead of child by child; this is tried once
+            // per container, so the walk stays linear.
+            bool fresh = true;
+
             while (true)
             {
-                if (has_no_children(cur))
+                const bool fresh_now = fresh;
+                fresh = false;
+                if (has_no_children(cur) || (fresh_now && all_children_leaves(cur)))
                 {
                     if (prev.m_data.m_type == value_t::null)
                     {
@@ -922,6 +1009,7 @@ public:
                 take(cur_child_ref, prev);
                 take(prev, cur);
                 take(cur, tmp);
+                fresh = true;
             }
         }
 
@@ -2348,7 +2436,7 @@ public:
             // if an object is wanted but impossible, throw an exception
             if (JSON_HEDLEY_UNLIKELY(manual_type == value_t::object && !is_an_object))
             {
-                JSON_THROW(type_error::create(detail::exception_id::object_from_non_pairs, "cannot create object from initializer list", nullptr));
+                detail::throw_literal<type_error>(detail::exception_id::object_from_non_pairs, "cannot create object from initializer list", nullptr);
             }
         }
 
@@ -2468,7 +2556,7 @@ public:
         // make sure the iterator fits the current value
         if (JSON_HEDLEY_UNLIKELY(first.m_object != last.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterators_incompatible, "iterators are not compatible", nullptr));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterators_incompatible, "iterators are not compatible", nullptr);
         }
 
         // copy type from the first iterator
@@ -2487,7 +2575,7 @@ public:
                 if (JSON_HEDLEY_UNLIKELY(!first.m_it.primitive_iterator.is_begin()
                                          || !last.m_it.primitive_iterator.is_end()))
                 {
-                    JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_range_out_of_range, "iterators out of range", first.m_object));
+                    detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_range_out_of_range, "iterators out of range", first.m_object);
                 }
                 break;
             }
@@ -2555,7 +2643,7 @@ public:
             case value_t::null:
             case value_t::discarded:
             default:
-                JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_range_of_null, detail::concat("cannot construct with iterators from ", first.m_object->type_name()), first.m_object));
+                throw_iterator_range_of_null(first.m_object);
         }
 
         set_parents();
@@ -2936,6 +3024,40 @@ public:
         return is_binary() ? m_data.m_value.binary : nullptr;
     }
 
+    /// out-of-line throw helpers (message construction kept out of the hot paths)
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_iterator_range_of_null(const basic_json* obj)
+    {
+        JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_range_of_null, detail::concat("cannot construct with iterators from ", obj->type_name()), obj));
+    }
+
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_incompatible_reference_type(const basic_json& obj)
+    {
+        JSON_THROW(type_error::create(detail::exception_id::incompatible_reference_type, detail::concat("incompatible ReferenceType for get_ref, actual type is ", obj.type_name()), &obj));
+    }
+
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_array_index_exceeds_size_type(size_type idx)
+    {
+        JSON_THROW(std::length_error(detail::concat("array index ", std::to_string(idx), " exceeds size_type")));
+    }
+
+    /// @throw out_of_range.401
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_array_index_out_of_range(size_type idx, const basic_json& j)
+    {
+        JSON_THROW(out_of_range::create(detail::exception_id::array_index_out_of_range, detail::concat("array index ", std::to_string(idx), " is out of range"), &j));
+    }
+
+    /// @throw out_of_range.403
+    template<typename KeyType>
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_key_not_found(const KeyType& key, const basic_json& j)
+    {
+        JSON_THROW(out_of_range::create(detail::exception_id::key_not_found, detail::concat("key '", key, "' not found"), &j));
+    }
+
     /*!
     @brief helper function to implement get_ref()
 
@@ -2958,7 +3080,7 @@ public:
             return *ptr;
         }
 
-        JSON_THROW(type_error::create(detail::exception_id::incompatible_reference_type, detail::concat("incompatible ReferenceType for get_ref, actual type is ", obj.type_name()), &obj));
+        throw_incompatible_reference_type(obj);
     }
 
   public:
@@ -3406,7 +3528,7 @@ public:
             // std::map or ordered_map) never moves from its argument, so key is still
             // valid here regardless of whether KeyType was deduced as an rvalue reference
             // NOLINTNEXTLINE(bugprone-use-after-move,hicpp-invalid-access-moved)
-            JSON_THROW(out_of_range::create(detail::exception_id::key_not_found, detail::concat("key '", key_for_message(key), "' not found"), &j));
+            throw_key_not_found(key_for_message(key), j);
         }
         return it->second;
     }
@@ -3443,7 +3565,7 @@ public:
 
         if (JSON_HEDLEY_UNLIKELY(idx >= j.m_data.m_value.array->size()))
         {
-            JSON_THROW(out_of_range::create(detail::exception_id::array_index_out_of_range, detail::concat("array index ", std::to_string(idx), " is out of range"), &j));
+            throw_array_index_out_of_range(idx, j);
         }
 
         return (*j.m_data.m_value.array)[idx];
@@ -3561,7 +3683,7 @@ public:
                 // resize() rejects other indices that are too large to represent
                 if (JSON_HEDLEY_UNLIKELY(idx == (std::numeric_limits<size_type>::max)()))
                 {
-                    JSON_THROW(std::length_error(detail::concat("array index ", std::to_string(idx), " exceeds size_type")));
+                    throw_array_index_exceeds_size_type(idx);
                 }
 #if JSON_DIAGNOSTICS
                 // remember array size & capacity before resizing
@@ -3899,7 +4021,7 @@ public:
         // make sure the iterator fits the current value
         if (JSON_HEDLEY_UNLIKELY(this != pos.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
         }
 
         IteratorType result = end();
@@ -3915,7 +4037,7 @@ public:
             {
                 if (JSON_HEDLEY_UNLIKELY(!pos.m_it.primitive_iterator.is_begin()))
                 {
-                    JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_out_of_range, "iterator out of range", this));
+                    detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_out_of_range, "iterator out of range", this);
                 }
 
                 m_data.m_value.destroy(m_data.m_type);
@@ -3957,7 +4079,7 @@ public:
         // make sure the iterator fits the current value
         if (JSON_HEDLEY_UNLIKELY(this != first.m_object || this != last.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_range_from_other_value, "iterators do not fit current value", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_range_from_other_value, "iterators do not fit current value", this);
         }
 
         IteratorType result = end();
@@ -3974,7 +4096,7 @@ public:
                 if (JSON_HEDLEY_UNLIKELY(!first.m_it.primitive_iterator.is_begin()
                                          || !last.m_it.primitive_iterator.is_end()))
                 {
-                    JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_range_out_of_range, "iterators out of range", this));
+                    detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_range_out_of_range, "iterators out of range", this);
                 }
 
                 m_data.m_value.destroy(m_data.m_type);
@@ -4073,7 +4195,7 @@ public:
         {
             if (JSON_HEDLEY_UNLIKELY(idx >= size()))
             {
-                JSON_THROW(out_of_range::create(detail::exception_id::array_index_out_of_range, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
+                throw_array_index_out_of_range(idx, *this);
             }
 
             m_data.m_value.array->erase(m_data.m_value.array->begin() + static_cast<difference_type>(idx));
@@ -4761,7 +4883,7 @@ public:
             // check if iterator pos fits to this JSON value
             if (JSON_HEDLEY_UNLIKELY(pos.m_object != this))
             {
-                JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+                detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
             }
 
             // insert to array and return iterator
@@ -4781,7 +4903,7 @@ public:
             // check if iterator pos fits to this JSON value
             if (JSON_HEDLEY_UNLIKELY(pos.m_object != this))
             {
-                JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+                detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
             }
 
             // moving into a local first keeps this safe even if val aliases
@@ -4803,7 +4925,7 @@ public:
             // check if iterator pos fits to this JSON value
             if (JSON_HEDLEY_UNLIKELY(pos.m_object != this))
             {
-                JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+                detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
             }
 
             // insert to array and return iterator
@@ -4826,24 +4948,24 @@ public:
         // check if iterator pos fits to this JSON value
         if (JSON_HEDLEY_UNLIKELY(pos.m_object != this))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
         }
 
         // check if range iterators belong to the same JSON object
         if (JSON_HEDLEY_UNLIKELY(first.m_object != last.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::insert_range_incompatible, "iterators do not fit", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::insert_range_incompatible, "iterators do not fit", this);
         }
 
         if (JSON_HEDLEY_UNLIKELY(first.m_object == this))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::insert_range_into_itself, "passed iterators may not belong to container", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::insert_range_into_itself, "passed iterators may not belong to container", this);
         }
 
         // passed iterators must belong to arrays
         if (JSON_HEDLEY_UNLIKELY(!first.m_object->is_array()))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterators first and last must point to arrays", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterators first and last must point to arrays", this);
         }
 
         // insert to array and return iterator
@@ -4863,7 +4985,7 @@ public:
         // check if iterator pos fits to this JSON value
         if (JSON_HEDLEY_UNLIKELY(pos.m_object != this))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterator does not fit current value", this);
         }
 
         // copy the values first: ilist may refer to elements of this array
@@ -4891,13 +5013,13 @@ public:
         // check if range iterators belong to the same JSON object
         if (JSON_HEDLEY_UNLIKELY(first.m_object != last.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::insert_range_incompatible, "iterators do not fit", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::insert_range_incompatible, "iterators do not fit", this);
         }
 
         // passed iterators must belong to objects
         if (JSON_HEDLEY_UNLIKELY(!first.m_object->is_object()))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::iterator_from_other_value, "iterators first and last must point to objects", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::iterator_from_other_value, "iterators first and last must point to objects", this);
         }
 
         m_data.m_value.object->insert(first.m_it.object_iterator, last.m_it.object_iterator);
@@ -4932,7 +5054,7 @@ public:
         // check if range iterators belong to the same JSON object
         if (JSON_HEDLEY_UNLIKELY(first.m_object != last.m_object))
         {
-            JSON_THROW(invalid_iterator::create(detail::exception_id::insert_range_incompatible, "iterators do not fit", this));
+            detail::throw_literal<invalid_iterator>(detail::exception_id::insert_range_incompatible, "iterators do not fit", this);
         }
 
         // passed iterators must belong to objects
@@ -6832,7 +6954,7 @@ public:
                         if (JSON_HEDLEY_UNLIKELY(idx > parent.size()))
                         {
                             // avoid undefined behavior
-                            JSON_THROW(out_of_range::create(detail::exception_id::array_index_out_of_range, detail::concat("array index ", std::to_string(idx), " is out of range"), &parent));
+                            throw_array_index_out_of_range(idx, parent);
                         }
 
                         // default case: insert add offset
@@ -6851,7 +6973,7 @@ public:
                 case value_t::binary:
                 case value_t::discarded:
                 default:
-                    JSON_THROW(out_of_range::create(detail::exception_id::patch_add_parent_not_container, detail::concat("cannot add value: the JSON Patch 'add' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
+                    detail::throw_patch_add_parent(parent);
             }
         };
 
@@ -6875,7 +6997,7 @@ public:
                 }
                 else
                 {
-                    JSON_THROW(out_of_range::create(detail::exception_id::key_not_found, detail::concat("key '", last_path, "' not found"), this));
+                    throw_key_not_found(last_path, *this);
                 }
             }
             else if (parent.is_array())
@@ -6887,7 +7009,7 @@ public:
             {
                 // the parent of a "remove" target must be an object or array
                 // (see #5396)
-                JSON_THROW(out_of_range::create(detail::exception_id::patch_remove_parent_not_container, detail::concat("cannot remove value: the JSON Patch 'remove' target's parent is of type ", parent.type_name(), ", but must be an object or array"), &parent));
+                detail::throw_patch_remove_parent(parent);
             }
         };
 
@@ -6918,7 +7040,7 @@ public:
         // type check: top level value must be an array
         if (JSON_HEDLEY_UNLIKELY(!json_patch.is_array()))
         {
-            JSON_THROW(parse_error::create(detail::exception_id::patch_not_an_array, 0, "JSON patch must be an array of objects", &json_patch));
+            detail::throw_patch_not_an_array(json_patch);
         }
 
         // iterate and apply the operations
@@ -6932,21 +7054,16 @@ public:
                 // find value
                 auto it = val.m_data.m_value.object->find(member);
 
-                // context-sensitive error message
-                const auto error_msg = (op == "op") ? "operation" : detail::concat("operation '", op, '\''); // NOLINT(bugprone-unused-local-non-trivial-variable)
-
                 // check if the desired value is present
                 if (JSON_HEDLEY_UNLIKELY(it == val.m_data.m_value.object->end()))
                 {
-                    // NOLINTNEXTLINE(performance-inefficient-string-concatenation)
+                    detail::throw_patch_missing_member(op, member, false, val);
-                    JSON_THROW(parse_error::create(detail::exception_id::patch_invalid_operation, 0, detail::concat(error_msg, " must have member '", member, "'"), &val));
                 }
 
                 // check if the result is of type string
                 if (JSON_HEDLEY_UNLIKELY(string_type && !it->second.is_string()))
                 {
-                    // NOLINTNEXTLINE(performance-inefficient-string-concatenation)
+                    detail::throw_patch_missing_member(op, member, true, val);
-                    JSON_THROW(parse_error::create(detail::exception_id::patch_invalid_operation, 0, detail::concat(error_msg, " must have string member '", member, "'"), &val));
                 }
 
                 // no error: return value
@@ -6956,7 +7073,7 @@ public:
             // type check: every element of the array must be an object
             if (JSON_HEDLEY_UNLIKELY(!val.is_object()))
             {
-                JSON_THROW(parse_error::create(detail::exception_id::patch_not_an_array, 0, "JSON patch must be an array of objects", &val));
+                detail::throw_patch_not_an_array(val);
             }
 
             // collect mandatory members
@@ -6992,7 +7109,7 @@ public:
 
                     if (JSON_HEDLEY_UNLIKELY(is_proper_prefix(from_ptr, ptr)))
                     {
-                        JSON_THROW(out_of_range::create(detail::exception_id::patch_move_into_child, detail::concat("cannot move value: 'from' path '", from_path, "' is a proper prefix of 'path' '", path, "'"), &result));
+                        detail::throw_patch_move_into_child(from_path, path, result);
                     }
 
                     // the "from" location must exist - use at()
@@ -7038,7 +7155,7 @@ public:
                     // throw an exception if the test fails
                     if (JSON_HEDLEY_UNLIKELY(!success))
                     {
-                        JSON_THROW(other_error::create(detail::exception_id::patch_test_failed, detail::concat("unsuccessful: ", val.dump()), &val));
+                        detail::throw_patch_test_failed(val);
                     }
 
                     break;
@@ -7049,7 +7166,7 @@ public:
                 {
                     // op must be "add", "remove", "replace", "move", "copy", or
                     // "test"
-                    JSON_THROW(parse_error::create(detail::exception_id::patch_invalid_operation, 0, detail::concat("operation value '", op, "' is invalid"), &val));
+                    detail::throw_patch_invalid_operation(op, val);
                 }
             }
         }
@@ -7103,12 +7220,11 @@ public:
         std::size_t index = 0;
         /// objects: the next member of source to look at
         const_iterator member{}; // NOLINT(readability-redundant-member-init)
-        /// objects: the keys common to both, in source's order
+        /// objects: for each member of source (in order), its value in
-        std::vector<typename object_t::key_type> common_keys{}; // NOLINT(readability-redundant-member-init)
+        /// target, or nullptr if target has no such key; `index` is the next
-        /// objects: the next entry of common_keys
+        std::vector<const basic_json*> target_values{}; // NOLINT(readability-redundant-member-init)
-        std::size_t next_common = 0;
+        /// objects: the members only target has, in target's order
-        /// objects: the "add" operations for keys only target has
+        std::vector<const_iterator> added{}; // NOLINT(readability-redundant-member-init)
-        basic_json added_ops{}; // NOLINT(readability-redundant-member-init)
     };
 
     // The operations of a diff are built by the functions below rather than
@@ -7164,111 +7280,113 @@ public:
         }
     }
 
+    /// @brief appends the "add" operations for the members @a added to @a result
+    static void diff_added(basic_json& result, const std::vector<const_iterator>& added, const string_t& path)
+    {
+        for (const auto& it : added)
+        {
+            diff_add(result, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
+        }
+    }
+
     /*!
     @brief compare the keys of objects @a source and @a target
 
     If object_t does not keep its members in insertion order, or if the keys
     both objects have are in the same order in both, and the keys only
-    @a target has come after them, stores the keys common to both in
+    @a target has come after them, stores in @a target_values, for each member
-    source's order in @a common_keys, stores the "add" operations for the keys
+    of source in order, its value in target (nullptr for a key target does not
-    only @a target has in @a added_ops, and returns true: the caller then diffs
+    have), stores the members only target has in @a added, and returns true:
-    the objects member by member. Otherwise, appends operations that remove
+    the caller then diffs the objects member by member. Otherwise, appends
-    every member of @a source and add every member of @a target to @a result,
+    operations that remove every member of @a source and add every member of
-    and returns false.
+    @a target to @a result, and returns false.
     */
     static bool diff_object_keys(basic_json& result, const basic_json& source, const basic_json& target,
-                                 const string_t& path, std::vector<typename object_t::key_type>& common_keys,
+                                 const string_t& path, std::vector<const basic_json*>& target_values,
-                                 basic_json& added_ops)
+                                 std::vector<const_iterator>& added)
     {
-        // first pass: record, for every source key, whether it is
+        // first pass: one lookup per source key
-        // common to both objects (in source's iteration order) or
+        target_values.reserve(source.size());
-        // was deleted (i.e., in source but not in target) -- this is
+        std::size_t common = 0;
-        // a by-product of the target.find() call already needed to
-        // tell the two cases apart, so it adds no extra lookups. The
-        // "remove" ops themselves are emitted later, interleaved
-        // with the per-key diffs in the caller's fast path, to match
-        // source's original iteration order (as the original,
-        // pre-reordering-aware implementation did) instead of
-        // grouping all removes before all per-key diffs.
-        std::vector<typename object_t::key_type> common_keys_source_order;
         for (auto it = source.cbegin(); it != source.cend(); ++it)
         {
-            if (target.find(it.key()) != target.end())
+            const auto found = target.find(it.key());
+            if (found != target.cend())
             {
-                common_keys_source_order.push_back(it.key());
+                target_values.push_back(&found.value());
-            }
+                ++common;
-        }
-
-        // second pass: find keys that were added (i.e., in target but
-        // not in source), and record the keys common to both, in
-        // target's iteration order -- again a by-product of the
-        // source.find() call already needed to detect added keys. At
-        // the same time, determine whether every added key comes
-        // after every common key in target's order (a precondition
-        // for the fast path, which only ever appends new keys
-        // at the very end). Both are only needed for an object_t that
-        // keeps its members in insertion order, such as the one
-        // backing `ordered_json`; for any other object_t, the fast
-        // path is always taken and they are not computed.
-        // The patch ops for keys that were added (i.e., in target but not
-        // in source) are built here so the fast path can reuse
-        // them without a second source.find() per target key. Only
-        // used by the fast path -- the slow (reordering) path
-        // rebuilds "add" ops for every key itself.
-        std::vector<typename object_t::key_type> common_keys_target_order;
-        bool new_keys_form_suffix = true;
-        bool seen_new_key = false;
-        for (auto it = target.cbegin(); it != target.cend(); ++it)
-        {
-            if (source.find(it.key()) == source.end())
-            {
-                seen_new_key = true;
-                diff_add(added_ops, detail::concat<string_t>(path, '/', detail::escape(it.key())), it.value());
             }
             else
             {
-#ifdef JSON_HEDLEY_MSVC_VERSION
+                target_values.push_back(nullptr);
-#pragma warning(push )
-#pragma warning(disable : 4127) // ignore warning to replace if with if constexpr
-#endif
-                if (detail::is_ordered_map<object_t>::value)
-                {
-                    common_keys_target_order.push_back(it.key());
-                    if (seen_new_key)
-                    {
-                        new_keys_form_suffix = false;
-                    }
-                }
-#ifdef JSON_HEDLEY_MSVC_VERSION
-#pragma warning( pop )
-#endif
             }
         }
 
-        // Only an object type that keeps its members in insertion
+        // second pass: find the keys only target has. An object type that
-        // order, such as nlohmann::ordered_map, can need reordering:
+        // keeps its members in insertion order, such as the one backing
-        // patch() appends a new member at the end of such an object.
+        // ordered_json, also needs the common keys to be in the same order
-        // Any other object type places its members itself - std::map
+        // in both objects, and the new keys to come after them, as patch()
-        // in key order, a hash map in an order its operator== ignores -
+        // appends a new member at the end of such an object. Any other
-        // so a member-by-member diff always reproduces target there.
+        // object type places its members itself, so a member-by-member diff
+        // always reproduces target there.
 #ifdef JSON_HEDLEY_MSVC_VERSION
 #pragma warning(push )
 #pragma warning(disable : 4127) // ignore warning to replace if with if constexpr
 #endif
-        if (!detail::is_ordered_map<object_t>::value
+        bool fast_path = true;
-                || (common_keys_source_order == common_keys_target_order && new_keys_form_suffix))
+        if (detail::is_ordered_map<object_t>::value)
         {
-            // fast path: order of common keys already matches (or the
+            // walk the common keys of source in lockstep with target: a key
-            // object_t's iteration order does not depend on
+            // of target that is not the next one is either new or out of order
-            // insertion history), so a plain per-key diff is correct
+            auto src = source.cbegin();
-            // and minimal, as before
+            std::size_t next = 0;
-            common_keys = std::move(common_keys_source_order);
+            bool seen_new = false;
-            return true;
+            for (auto it = target.cbegin(); it != target.cend() && fast_path; ++it)
+            {
+                while (next < target_values.size() && target_values[next] == nullptr)
+                {
+                    ++src;
+                    ++next;
+                }
+
+                if (next < target_values.size() && src.key() == it.key())
+                {
+                    // a common key after a new one: new keys are no suffix
+                    fast_path = !seen_new;
+                    ++src;
+                    ++next;
+                }
+                else if (source.find(it.key()) == source.cend())
+                {
+                    seen_new = true;
+                    added.push_back(it);
+                }
+                else
+                {
+                    // a common key out of order
+                    fast_path = false;
+                }
+            }
+        }
+        else if (target.size() != common)
+        {
+            for (auto it = target.cbegin(); it != target.cend(); ++it)
+            {
+                if (source.find(it.key()) == source.cend())
+                {
+                    added.push_back(it);
+                }
+            }
         }
 #ifdef JSON_HEDLEY_MSVC_VERSION
 #pragma warning( pop )
 #endif
 
+        if (fast_path)
+        {
+            return true;
+        }
+
         // slow path: the common keys are in a different relative
         // order in source and target (only possible for a
         // reorderable object_t like ordered_map). Building a
@@ -7318,6 +7436,14 @@ public:
             return;
         }
 
+        diff_unequal(result, source, target, path, depth);
+    }
+
+    /// @brief @ref diff_recursively for values known to be unequal; callers
+    ///        compare first so they build no path for equal ones
+    static void diff_unequal(basic_json& result, const basic_json& source, const basic_json& target,
+                             const string_t& path, const std::size_t depth)
+    {
         if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
         {
             diff_iteratively(result, source, target, path);
@@ -7340,7 +7466,10 @@ public:
                 while (i < source.size() && i < target.size())
                 {
                     // recursive call to compare array values at index i
-                    diff_recursively(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
+                    if (!(source[i] == target[i]))
+                    {
+                        diff_unequal(result, source[i], target[i], detail::concat<string_t>(path, '/', detail::to_string<string_t>(i)), depth + 1);
+                    }
                     ++i;
                 }
 
@@ -7352,25 +7481,24 @@ public:
 
             case value_t::object:
             {
-                std::vector<typename object_t::key_type> common_keys;
+                std::vector<const basic_json*> target_values;
-                basic_json added_ops(value_t::array);
+                std::vector<const_iterator> added;
-                if (diff_object_keys(result, source, target, path, common_keys, added_ops))
+                if (diff_object_keys(result, source, target, path, target_values, added))
                 {
-                    // fast path: common_keys is, by construction, the
+                    // fast path: target_values holds, for every member of
-                    // subsequence of source's keys that are common to both
+                    // source in order, its value in target -- so no further
-                    // objects, in source's iteration order -- so it can be
+                    // lookup is needed. Deleted keys are interleaved here
-                    // walked in lockstep with `source` using a cheap key
+                    // too, in source's original order, to match the
-                    // comparison instead of another lookup. Deleted keys
-                    // (those source keys not in common_keys) are interleaved
-                    // here too, in source's original order, to match the
                     // historical (pre-reordering-aware) output order.
-                    auto common_it = common_keys.cbegin();
+                    std::size_t i = 0;
-                    for (auto it = source.cbegin(); it != source.cend(); ++it)
+                    for (auto it = source.cbegin(); it != source.cend(); ++it, ++i)
                     {
-                        if (common_it != common_keys.cend() && it.key() == *common_it)
+                        if (target_values[i] != nullptr)
                         {
-                            diff_recursively(result, it.value(), target[it.key()], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
+                            if (!(it.value() == *target_values[i]))
-                            ++common_it;
+                            {
+                                diff_unequal(result, it.value(), *target_values[i], detail::concat<string_t>(path, '/', detail::escape(it.key())), depth + 1);
+                            }
                         }
                         else
                         {
@@ -7379,10 +7507,8 @@ public:
                         }
                     }
 
-                    // append the "add" ops for brand-new keys collected by
+                    // append the "add" operations for brand-new keys
-                    // diff_object_keys -- no second source.find() per target
+                    diff_added(result, added, path);
-                    // key needed
-                    result.insert(result.end(), added_ops.begin(), added_ops.end());
                 }
                 break;
             }
@@ -7455,17 +7581,17 @@ public:
 
                 case value_t::object:
                 {
-                    std::vector<typename object_t::key_type> common_keys;
+                    std::vector<const basic_json*> target_values;
-                    basic_json added_ops(value_t::array);
+                    std::vector<const_iterator> added;
-                    if (diff_object_keys(result, s, t, current_path, common_keys, added_ops))
+                    if (diff_object_keys(result, s, t, current_path, target_values, added))
                     {
                         // fast path: the frame walks source in lockstep with
                         // common_keys, as diff_recursively does, and appends
                         // added_ops once all members are done
                         stack.emplace_back(&s, &t, current_path.size());
                         stack.back().member = s.cbegin();
-                        stack.back().common_keys = std::move(common_keys);
+                        stack.back().target_values = std::move(target_values);
-                        stack.back().added_ops = std::move(added_ops);
+                        stack.back().added = std::move(added);
                     }
                     return;
                 }
@@ -7527,11 +7653,10 @@ public:
                 if (it != s->cend())
                 {
                     ++stack.back().member;
-                    const std::size_t next_common = stack.back().next_common;
+                    const basic_json* const target_value_ptr = stack.back().target_values[stack.back().index++];
-                    if (next_common < stack.back().common_keys.size() && it.key() == stack.back().common_keys[next_common])
+                    if (target_value_ptr != nullptr)
                     {
-                        ++stack.back().next_common;
+                        const basic_json& target_value = *target_value_ptr;
-                        const basic_json& target_value = (*t)[it.key()];
                         detail::concat_into(current_path, '/', detail::escape(it.key()));
                         enter(it.value(), target_value);
                         if (stack.size() == depth)
@@ -7549,7 +7674,7 @@ public:
 
                 // append the "add" ops for brand-new keys collected when the
                 // object was entered
-                result.insert(result.end(), stack.back().added_ops.begin(), stack.back().added_ops.end());
+                diff_added(result, stack.back().added, current_path);
             }
 
             // this array or object is done: continue with the one it is in

include/nlohmann/ordered_map.hpp

@@ -92,13 +92,19 @@ private:
 
     /// @brief shared implementation of the const and non-const at() overloads
     /// @throw std::out_of_range if @a key is not found
+    JSON_HEDLEY_NO_RETURN JSON_HEDLEY_NEVER_INLINE
+    static void throw_key_not_found()
+    {
+        JSON_THROW(std::out_of_range("key not found"));
+    }
+
     template<typename Self, typename KeyType>
     static auto at_impl(Self& self, const KeyType& key) -> decltype((self.begin()->second))
     {
         const auto it = find_impl(self, key);
         if (it == self.end())
         {
-            JSON_THROW(std::out_of_range("key not found"));
+            throw_key_not_found();
         }
         return it->second;
     }

tests/src/unit-constructor2.cpp

@@ -323,3 +323,57 @@ TEST_CASE("copy constructor and destructor across value types")
         CHECK(w.size() == 9);
     }
 }
+
+TEST_CASE_TEMPLATE("destructor of nested containers", Json, nlohmann::json, nlohmann::ordered_json)
+{
+    SECTION("empty containers")
+    {
+        {
+            Json j = Json::array();
+        }
+        {
+            Json j = Json::object();
+        }
+        {
+            Json j = Json::array({Json::array(), Json::object(), Json::array({Json::array()})});
+        }
+    }
+
+    SECTION("leaves mixed with nested containers")
+    {
+        Json j = Json::object();
+        j["leaf"] = 1;
+        j["str"] = "text";
+        j["empty_a"] = Json::array();
+        j["empty_o"] = Json::object();
+        j["arr"] = Json::array({1, "a", Json::array({1, 2}), Json::object({{"k", Json::array({Json::object()})}}), nullptr});
+        j["obj"] = Json::object({{"x", Json::object({{"y", Json::array({1, Json::object({{"z", 1}})})}})}, {"w", 2}});
+        Json copy = j;
+        CHECK(copy == j);
+        j = Json::array({std::move(copy), j, 3, Json::array()});
+    }
+
+    SECTION("deep nesting")
+    {
+        for (int kind = 0; kind < 3; ++kind)
+        {
+            Json j = 1;
+            for (int i = 0; i < 5000; ++i)
+            {
+                switch (kind)
+                {
+                    case 0:
+                        j = Json::array({std::move(j)});
+                        break;
+                    case 1:
+                        j = Json::object({{"k", std::move(j)}});
+                        break;
+                    default:
+                        // siblings next to the nested child
+                        j = (i % 2 == 0) ? Json::array({1, Json::array(), std::move(j)}) : Json::object({{"a", 1}, {"b", Json::object()}, {"c", std::move(j)}});
+                        break;
+                }
+            }
+        }
+    }
+}

tests/src/unit-iterators1.cpp

@@ -10,6 +10,8 @@
 
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
+#include <string>
+#include <vector>
 using nlohmann::json;
 
 TEST_CASE("iterators 1")
@@ -1628,3 +1630,108 @@ TEST_CASE("iterators 1")
         }
     }
 }
+
+TEST_CASE_TEMPLATE("reverse iteration and member access", Json, nlohmann::json, nlohmann::ordered_json)
+{
+    SECTION("array: -- and rbegin/rend")
+    {
+        Json j = {1, 2, 3, 4};
+
+        auto it = j.end();
+        std::vector<int> seen;
+        while (it != j.begin())
+        {
+            --it;
+            seen.push_back(it->template get<int>());
+        }
+        CHECK(seen == std::vector<int> {4, 3, 2, 1});
+
+        auto it2 = j.end();
+        auto old = it2--;
+        CHECK(old == j.end());
+        CHECK(*it2 == 4);
+
+        seen.clear();
+        for (auto r = j.rbegin(); r != j.rend(); ++r)
+        {
+            seen.push_back(r->template get<int>());
+        }
+        CHECK(seen == std::vector<int> {4, 3, 2, 1});
+
+        seen.clear();
+        for (auto r = j.crbegin(); r != j.crend(); ++r)
+        {
+            seen.push_back(*r);
+        }
+        CHECK(seen == std::vector<int> {4, 3, 2, 1});
+    }
+
+    SECTION("object: -- and rbegin/rend")
+    {
+        Json j = {{"a", 1}, {"b", 2}, {"c", 3}};
+        // order of keys depends on the object type; use the forward order as reference
+        std::vector<std::string> fwd;
+        for (auto it = j.begin(); it != j.end(); ++it)
+        {
+            fwd.push_back(it.key());
+        }
+        std::vector<std::string> rev(fwd.rbegin(), fwd.rend());
+
+        std::vector<std::string> seen;
+        auto it = j.end();
+        while (it != j.begin())
+        {
+            --it;
+            seen.push_back(it.key());
+        }
+        CHECK(seen == rev);
+
+        auto it2 = j.end();
+        auto old = it2--;
+        CHECK(old == j.end());
+        CHECK(it2.key() == rev.front());
+
+        seen.clear();
+        for (auto r = j.rbegin(); r != j.rend(); ++r)
+        {
+            seen.push_back(r.key());
+        }
+        CHECK(seen == rev);
+    }
+
+    SECTION("object: operator-> reads and writes the mapped value")
+    {
+        Json j = {{"a", 1}, {"s", "str"}};
+        auto it = j.find("s");
+        REQUIRE(it != j.end());
+        CHECK(it->is_string());
+        CHECK(it->size() == 1);
+        *it = Json::array({1, 2});
+        CHECK(it->is_array());
+        it->push_back(3);
+        CHECK(j["s"].size() == 3);
+
+        const Json& cj = j;
+        auto cit = cj.find("a");
+        REQUIRE(cit != cj.end());
+        CHECK(cit->is_number_integer());
+        CHECK(cit->template get<int>() == 1);
+    }
+
+    SECTION("primitives: -- and ->")
+    {
+        Json j = 42;
+        auto it = j.end();
+        --it;
+        CHECK(it == j.begin());
+        CHECK(it->template get<int>() == 42);
+    }
+
+    SECTION("comparing iterators of different containers throws")
+    {
+        Json a = {1, 2};
+        Json b = {1, 2};
+        CHECK_THROWS_WITH_AS(a.begin() == b.begin(), "[json.exception.invalid_iterator.212] cannot compare iterators of different containers", nlohmann::json::invalid_iterator);
+        CHECK_THROWS_WITH_AS(a.begin() < b.begin(), "[json.exception.invalid_iterator.212] cannot compare iterators of different containers", nlohmann::json::invalid_iterator);
+    }
+}

tests/src/unit-json_patch.cpp

@@ -2066,3 +2066,40 @@ TEST_CASE("JSON patch - every operation on ordered_json")
         }
     }
 }
+
+TEST_CASE_TEMPLATE("JSON patch - diff round trip on hand-made documents", Json, nlohmann::json, nlohmann::ordered_json)
+{
+    const auto roundtrip = [](const Json & a, const Json & b)
+    {
+        CAPTURE(a.dump())
+        CAPTURE(b.dump())
+        const Json p = Json::diff(a, b);
+        CHECK(a.patch(p) == b);
+        CHECK(Json::diff(b, b).empty());
+    };
+
+    SECTION("reordered keys")
+    {
+        const Json a = Json::parse(R"({"x":1,"y":2,"z":{"p":1,"q":2}})");
+        const Json b = Json::parse(R"({"z":{"q":2,"p":1},"y":2,"x":1})");
+        roundtrip(a, b);
+        roundtrip(b, a);
+    }
+
+    SECTION("nested add, remove and replace")
+    {
+        const Json a = Json::parse(R"({"a":[1,2,{"k":"v","gone":true}],"b":{"c":{"d":1}},"e":"s"})");
+        const Json b = Json::parse(R"({"a":[1,3,{"k":"w","new":[]}],"b":{"c":{"d":[1,2]}},"f":null})");
+        roundtrip(a, b);
+        roundtrip(b, a);
+    }
+
+    SECTION("arrays of different length and type changes")
+    {
+        roundtrip(Json::parse("[1,2,3,4,5]"), Json::parse("[1,3]"));
+        roundtrip(Json::parse("[1]"), Json::parse("[1,[2],{\"a\":3}]"));
+        roundtrip(Json::parse("[]"), Json::parse("{}"));
+        roundtrip(Json::parse("{\"a\":[]}"), Json::parse("{\"a\":{}}"));
+        roundtrip(Json(1), Json::parse("[1]"));
+    }
+}

tests/src/unit-throw_messages.cpp

@@ -0,0 +1,135 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include <string>
+
+// The exceptions in hot core functions are constructed out of line. These tests pin the
+// exact what() texts of the sites that moved, so the refactoring cannot change them.
+
+#define CHECK_MSG(expr, msg) CHECK_THROWS_WITH(expr, msg)
+
+TEST_CASE("exception messages of out-of-line throw sites")
+{
+    json arr = {1, 2, 3};
+    json arr2 = {4, 5};
+    json obj = {{"a", 1}, {"b", 2}};
+    json obj2 = {{"c", 3}};
+    json num = 1;
+
+    SECTION("iterators")
+    {
+        CHECK_MSG((void)(arr.begin() == arr2.begin()), "[json.exception.invalid_iterator.212] cannot compare iterators of different containers");
+        CHECK_MSG((void)(arr.begin() < arr2.begin()), "[json.exception.invalid_iterator.212] cannot compare iterators of different containers");
+        CHECK_MSG((void)(obj.begin() < obj.end()), "[json.exception.invalid_iterator.213] cannot compare order of object iterators");
+        CHECK_MSG(obj.begin() += 1, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+        CHECK_MSG((void)(obj.begin() - 1), "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+        CHECK_MSG((void)obj.begin()[0], "[json.exception.invalid_iterator.208] cannot use operator[] for object iterators");
+        CHECK_MSG((void)arr.begin().key(), "[json.exception.invalid_iterator.207] cannot use key() for non-object iterators");
+        json null;
+        CHECK_MSG((void)*null.end(), "[json.exception.invalid_iterator.214] cannot get value");
+    }
+
+    SECTION("constructors")
+    {
+        CHECK_MSG(json(arr.begin(), arr2.end()), "[json.exception.invalid_iterator.201] iterators are not compatible");
+        CHECK_MSG(json(num.begin() + 1, num.end() + 1), "[json.exception.invalid_iterator.204] iterators out of range");
+        CHECK_MSG(json(json::initializer_list_t{1, 2}, false, json::value_t::object), "[json.exception.type_error.301] cannot create object from initializer list");
+    }
+
+    SECTION("get_ref and operator[]")
+    {
+        CHECK_MSG((void)num.get_ref<std::string&>(), "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number");
+        CHECK_MSG((void)num[0], "[json.exception.type_error.305] cannot use operator[] with a numeric argument with number");
+    }
+
+    SECTION("at")
+    {
+        const json carr = arr;
+        const json cobj = obj;
+        CHECK_MSG((void)arr.at(static_cast<json::size_type>(10)), "[json.exception.out_of_range.401] array index 10 is out of range");
+        CHECK_MSG((void)carr.at(static_cast<json::size_type>(3)), "[json.exception.out_of_range.401] array index 3 is out of range");
+        CHECK_MSG((void)num.at(static_cast<json::size_type>(0)), "[json.exception.type_error.304] cannot use at() with number");
+        CHECK_MSG((void)obj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
+        CHECK_MSG((void)cobj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
+        CHECK_MSG((void)obj.at(std::string("zz")), "[json.exception.out_of_range.403] key 'zz' not found");
+        CHECK_MSG((void)num.at("a"), "[json.exception.type_error.304] cannot use at() with number");
+
+        nlohmann::ordered_json oobj = {{"a", 1}};
+        const nlohmann::ordered_json coobj = oobj;
+        CHECK_MSG((void)oobj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
+        CHECK_MSG((void)coobj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
+        CHECK_MSG((void)coobj.at(std::string("zz")), "[json.exception.out_of_range.403] key 'zz' not found");
+    }
+
+    SECTION("erase")
+    {
+        CHECK_MSG(arr.erase(arr2.begin()), "[json.exception.invalid_iterator.202] iterator does not fit current value");
+        CHECK_MSG(num.erase(num.end()), "[json.exception.invalid_iterator.205] iterator out of range");
+        CHECK_MSG(arr.erase(arr.begin(), arr2.end()), "[json.exception.invalid_iterator.203] iterators do not fit current value");
+        CHECK_MSG(num.erase(num.begin() + 1, num.end()), "[json.exception.invalid_iterator.204] iterators out of range");
+        CHECK_MSG(arr.erase(static_cast<json::size_type>(10)), "[json.exception.out_of_range.401] array index 10 is out of range");
+    }
+
+    SECTION("insert")
+    {
+        CHECK_MSG(arr.insert(arr2.begin(), 1), "[json.exception.invalid_iterator.202] iterator does not fit current value");
+        CHECK_MSG(arr.insert(arr2.begin(), 2, 1), "[json.exception.invalid_iterator.202] iterator does not fit current value");
+        CHECK_MSG(arr.insert(arr2.begin(), arr2.begin(), arr2.end()), "[json.exception.invalid_iterator.202] iterator does not fit current value");
+        CHECK_MSG(arr.insert(arr.begin(), arr.begin(), arr2.end()), "[json.exception.invalid_iterator.210] iterators do not fit");
+        CHECK_MSG(arr.insert(arr.begin(), arr.begin(), arr.end()), "[json.exception.invalid_iterator.211] passed iterators may not belong to container");
+        CHECK_MSG(arr.insert(arr.begin(), obj.begin(), obj.end()), "[json.exception.invalid_iterator.202] iterators first and last must point to arrays");
+        CHECK_MSG(arr.insert(arr2.begin(), {1, 2}), "[json.exception.invalid_iterator.202] iterator does not fit current value");
+        CHECK_MSG(obj.insert(obj2.begin(), arr.end()), "[json.exception.invalid_iterator.210] iterators do not fit");
+        CHECK_MSG(obj.insert(arr.begin(), arr.end()), "[json.exception.invalid_iterator.202] iterators first and last must point to objects");
+        CHECK_MSG(obj.update(arr.begin(), arr2.end()), "[json.exception.invalid_iterator.210] iterators do not fit");
+    }
+
+    SECTION("patch")
+    {
+        const auto patch_msg = [](const char* doc, const char* patch)
+        {
+            try
+            {
+                json d = json::parse(doc);
+                d.patch(json::parse(patch));
+            }
+            catch (const json::exception& e)
+            {
+                // with JSON_DIAGNOSTICS, what() names the offending value like "(/0) "; drop it
+                std::string msg = e.what();
+                const auto pos = msg.find(" (/");
+                if (pos != std::string::npos)
+                {
+                    msg.erase(pos + 1, msg.find(") ", pos) - pos + 1);
+                }
+                return msg;
+            }
+            return std::string();
+        };
+
+        CHECK(patch_msg("{}", "{}") == "[json.exception.parse_error.104] parse error: JSON patch must be an array of objects");
+        CHECK(patch_msg("{}", "[1]") == "[json.exception.parse_error.104] parse error: JSON patch must be an array of objects");
+        CHECK(patch_msg("{}", "[{}]") == "[json.exception.parse_error.105] parse error: operation must have member 'op'");
+        CHECK(patch_msg("{}", "[{\"op\":1}]") == "[json.exception.parse_error.105] parse error: operation must have string member 'op'");
+        CHECK(patch_msg("{}", "[{\"op\":\"add\"}]") == "[json.exception.parse_error.105] parse error: operation 'add' must have member 'path'");
+        CHECK(patch_msg("{}", "[{\"op\":\"add\",\"path\":1}]") == "[json.exception.parse_error.105] parse error: operation 'add' must have string member 'path'");
+        CHECK(patch_msg("{}", "[{\"op\":\"foo\",\"path\":\"\"}]") == "[json.exception.parse_error.105] parse error: operation value 'foo' is invalid");
+        CHECK(patch_msg("{\"a\":1}", "[{\"op\":\"remove\",\"path\":\"/b\"}]") == "[json.exception.out_of_range.403] key 'b' not found");
+        CHECK(patch_msg("[1]", "[{\"op\":\"add\",\"path\":\"/5\",\"value\":0}]") == "[json.exception.out_of_range.401] array index 5 is out of range");
+        CHECK(patch_msg("{\"a\":1}", "[{\"op\":\"add\",\"path\":\"/a/b\",\"value\":0}]") == "[json.exception.out_of_range.411] cannot add value: the JSON Patch 'add' target's parent is of type number, but must be an object or array");
+        CHECK(patch_msg("{\"a\":1}", "[{\"op\":\"remove\",\"path\":\"/a/b\"}]") == "[json.exception.out_of_range.413] cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array");
+        CHECK(patch_msg("{\"a\":{}}", "[{\"op\":\"move\",\"from\":\"/a\",\"path\":\"/a/b\"}]") == "[json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'");
+        CHECK(patch_msg("{\"a\":1}", "[{\"op\":\"test\",\"path\":\"/a\",\"value\":2}]") == "[json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/a\",\"value\":2}");
+        CHECK(patch_msg("{\"a\":1}", "[{\"op\":\"move\",\"path\":\"/a\"}]") == "[json.exception.parse_error.105] parse error: operation 'move' must have member 'from'");
+    }
+}