Stop calling std::localeconv() on every dump()

The serializer constructor snapshotted std::localeconv() into a
locale_chars member on every dump(), even though the only reader is
dump_float(number_float_t, std::false_type)'s snprintf path, taken
only for a number_float_t that is neither IEEE single nor double.
localeconv() is not required to be thread-safe with setlocale(), so
every dump() paid for and raced on a lookup that almost never mattered.

Remove locale_chars and the locale member. Right before the
thousands-separator/decimal-point fixups in the snprintf path, read
std::localeconv() into local thousands_sep/decimal_point variables
(null-checked, first byte only, as before) - the same way
lexer::get_decimal_point() already does since #5597. Output is
unchanged unless the locale changes during a single dump(); in that
case the fixups now match what snprintf just produced, instead of a
value snapshotted before the call.

Overlaps draft PR #5608, which touches the same constructor and
dump_float() lines to move this code into a new
dump_float_snprintf(); this lands the lookup change now as #5709 asks,
and #5608 can do the lookup inside dump_float_snprintf() when it
rebases.

Verification: the full json_test_data corpus (742 files, dump(),
dump(4) and dump(-1,' ',true)) is byte-identical to before the change
under the C locale. Added a test pinning the new per-conversion
lookup: it switches LC_NUMERIC mid-dump() (via a streambuf that
switches on its first write, after the serializer's write buffer has
been flushed once but before a later float is converted) and checks
the decimal point is still normalized using the locale active at
conversion time. On a platform where long double is IEEE-754 double
(e.g. 64-bit Arm), dump_float() takes the locale-independent
to_chars() path and the test is a no-op there; it is meaningful on a
platform where long double is extended precision (most x86 targets).

Part of #5709 item 3

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

include/nlohmann/detail/output/binary_writer.hpp

@@ -29,6 +29,7 @@
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/output/output_adapters.hpp>
 #include <nlohmann/detail/string_concat.hpp>
+#include <nlohmann/detail/string_utils.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -2278,20 +2279,10 @@ class binary_writer
         const std::size_t valid = valid_utf8_prefix(data, s.size());
         if (JSON_HEDLEY_UNLIKELY(valid != s.size()))
         {
-            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", hex_byte(data[valid])), &context));
+            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", detail::hex_byte(data[valid])), &context));
         }
     }
 
-    /// @return a byte as two uppercase hexadecimal digits
-    static std::string hex_byte(const std::uint8_t byte)
-    {
-        std::string result = "00";
-        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
-        result[0] = nibble_to_hex[byte / 16];
-        result[1] = nibble_to_hex[byte % 16];
-        return result;
-    }
-
     /*!
     @brief write an integer in the shortest encoding
 

include/nlohmann/detail/output/output_adapters.hpp

@@ -8,9 +8,7 @@
 
 #pragma once
 
-#include <algorithm> // copy
 #include <cstddef> // size_t
-#include <iterator> // back_inserter
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
 #include <utility> // move
@@ -31,6 +29,10 @@ namespace detail
 template<typename CharType> struct output_adapter_protocol
 {
     virtual void write_character(CharType c) = 0;
+    /// @param[in] s      pointer to the characters to write; binary_writer legitimately
+    ///                   passes a null pointer together with length 0 for an empty
+    ///                   string or binary value, so implementations must tolerate that
+    /// @param[in] length number of characters at @a s
     virtual void write_characters(const CharType* s, std::size_t length) = 0;
     virtual ~output_adapter_protocol() = default;
 
@@ -97,7 +99,6 @@ class output_vector_adapter : public output_adapter_protocol<CharType>
         sink.write_character(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         sink.write_characters(s, length);
@@ -122,7 +123,6 @@ class output_stream_adapter : public output_adapter_protocol<CharType>
         stream.put(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         stream.write(s, static_cast<std::streamsize>(length));
@@ -147,7 +147,6 @@ class output_string_adapter : public output_adapter_protocol<CharType>
         str.push_back(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         str.append(s, length);

include/nlohmann/detail/output/serializer.hpp

@@ -3,24 +3,23 @@
 // |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
-// SPDX-FileCopyrightText: 2008, 2009 Björn Hoehrmann <bjoern@hoehrmann.de>
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
 #pragma once
 
-#include <algorithm> // reverse, remove, fill, find, none_of, min
+#include <algorithm> // remove, fill, find, none_of, min
 #include <array> // array
 #include <clocale> // localeconv, lconv
-#include <cmath> // labs, isfinite, isnan, signbit
+#include <cmath> // isfinite
 #include <cstddef> // size_t, ptrdiff_t
 #include <cstdint> // uint8_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy, memset
+#include <iterator> // next
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <type_traits> // is_same
-#include <utility> // move
 #include <vector> // vector
 
 #include <nlohmann/detail/conversions/to_chars.hpp>
@@ -28,7 +27,6 @@
 #include <nlohmann/detail/input/string_scan.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
-#include <nlohmann/detail/output/binary_writer.hpp>
 #include <nlohmann/detail/output/output_adapters.hpp>
 #include <nlohmann/detail/recursion_depth_limit.hpp>
 #include <nlohmann/detail/string_concat.hpp>
@@ -83,7 +81,6 @@ class serializer
                const std::size_t indent_step_ = 0,
                error_handler_t error_handler_ = error_handler_t::strict)
         : o(&s)
-        , locale(std::localeconv())
         , indent_char(ichar)
         , pretty_print(pretty_print_)
         , ensure_ascii(ensure_ascii_)
@@ -106,9 +103,10 @@ class serializer
     additional parameter. Arrays and objects are serialized without recursion,
     however deeply they are nested.
 
-    - strings and object keys are escaped using `escape_string()`
+    - strings and object keys are escaped using @ref dump_escaped
-    - integer numbers are converted implicitly via `operator<<`
+    - integer numbers are converted using a digit-pair lookup table (@ref dump_integer)
-    - floating-point numbers are converted to a string using `"%g"` format
+    - floating-point numbers are converted to a string using @ref dump_float, which
+      uses `to_chars` for IEEE-754 types and `snprintf` otherwise
     - binary values are serialized as objects containing the subtype and the
       byte array
 
@@ -283,128 +281,17 @@ class serializer
             }
 
             case value_t::string:
-            {
-                put_char('"');
-                dump_escaped(*val.m_data.m_value.string);
-                put_char('"');
-                return;
-            }
-
             case value_t::binary:
-            {
-                if (pretty_print)
-                {
-                    put_literal("{\n");
-
-                    // variable to hold indentation for recursive calls
-                    const auto new_indent = next_indent(current_indent, indent_step);
-
-                    put_indent(new_indent);
-
-                    put_literal("\"bytes\": [");
-
-                    if (!val.m_data.m_value.binary->empty())
-                    {
-                        for (auto i = val.m_data.m_value.binary->cbegin();
-                                i != val.m_data.m_value.binary->cend() - 1; ++i)
-                        {
-                            dump_byte(*i);
-                            put_literal(", ");
-                        }
-                        dump_byte(val.m_data.m_value.binary->back());
-                    }
-
-                    put_literal("],\n");
-                    put_indent(new_indent);
-
-                    put_literal("\"subtype\": ");
-                    if (val.m_data.m_value.binary->has_subtype())
-                    {
-                        dump_integer(val.m_data.m_value.binary->subtype());
-                    }
-                    else
-                    {
-                        put_literal("null");
-                    }
-                    put_char('\n');
-                    put_indent(current_indent);
-                    put_char('}');
-                }
-                else
-                {
-                    put_literal("{\"bytes\":[");
-
-                    if (!val.m_data.m_value.binary->empty())
-                    {
-                        for (auto i = val.m_data.m_value.binary->cbegin();
-                                i != val.m_data.m_value.binary->cend() - 1; ++i)
-                        {
-                            dump_byte(*i);
-                            put_char(',');
-                        }
-                        dump_byte(val.m_data.m_value.binary->back());
-                    }
-
-                    put_literal("],\"subtype\":");
-                    if (val.m_data.m_value.binary->has_subtype())
-                    {
-                        dump_integer(val.m_data.m_value.binary->subtype());
-                        put_char('}');
-                    }
-                    else
-                    {
-                        put_literal("null}");
-                    }
-                }
-                return;
-            }
-
             case value_t::boolean:
-            {
-                if (val.m_data.m_value.boolean)
-                {
-                    put_literal("true");
-                }
-                else
-                {
-                    put_literal("false");
-                }
-                return;
-            }
-
             case value_t::number_integer:
-            {
-                dump_integer(val.m_data.m_value.number_integer);
-                return;
-            }
-
             case value_t::number_unsigned:
-            {
-                dump_integer(val.m_data.m_value.number_unsigned);
-                return;
-            }
-
             case value_t::number_float:
-            {
-                dump_float(val.m_data.m_value.number_float);
-                return;
-            }
-
             case value_t::discarded:
-            {
-                put_literal("<discarded>");
-                return;
-            }
-
             case value_t::null:
-            {
+            default:
-                put_literal("null");
+                dump_scalar(val, current_indent);
                 return;
         }
-
-            default:            // LCOV_EXCL_LINE
-                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
-        }
     }
 
   private:
@@ -560,9 +447,9 @@ class serializer
     @brief serialize the value @a val, but not the elements of a container
 
     An object or array with elements is opened and pushed onto @a stack for
-    @ref dump_internal to walk; everything else - including a binary value,
+    @ref dump_iteratively to walk; everything else - including a binary value,
     which looks like an object but has no elements to descend into - is written
-    out here in full.
+    out in full by @ref dump_scalar.
     */
     void dump_value(const BasicJsonType& val,
                     const std::size_t current_indent,
@@ -620,6 +507,35 @@ class serializer
                 return;
             }
 
+            case value_t::string:
+            case value_t::binary:
+            case value_t::boolean:
+            case value_t::number_integer:
+            case value_t::number_unsigned:
+            case value_t::number_float:
+            case value_t::discarded:
+            case value_t::null:
+            default:
+                dump_scalar(val, current_indent);
+                return;
+        }
+    }
+
+    /*!
+    @brief serialize the value @a val, which is neither an object nor an array
+
+    Shared by @ref dump_internal and @ref dump_value, so that a value is written
+    the same way however deeply it is nested. A binary value is written out here
+    in full: it looks like an object, but has no elements to descend into.
+
+    @param[in] val             value to serialize; not an object or array
+    @param[in] current_indent  the indentation of @a val, used for a
+                               pretty-printed binary value
+    */
+    void dump_scalar(const BasicJsonType& val, const std::size_t current_indent)
+    {
+        switch (val.m_data.m_type)
+        {
             case value_t::string:
             {
                 put_char('"');
@@ -740,6 +656,8 @@ class serializer
                 return;
             }
 
+            case value_t::object: // LCOV_EXCL_LINE
+            case value_t::array:  // LCOV_EXCL_LINE
             default:            // LCOV_EXCL_LINE
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
@@ -768,7 +686,7 @@ class serializer
     Escape a string by replacing certain special characters by a sequence of an
     escape character (backslash) and another character and other control
     characters by a sequence of "\u" followed by a four-digit hex
-    representation. The escaped string is written to output stream @a o.
+    representation. The escaped string is appended to @ref write_buffer.
 
     @param[in] s  the string to escape
 
@@ -962,7 +880,7 @@ class serializer
                     {
                         case error_handler_t::strict:
                         {
-                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", hex_bytes(byte | 0)), nullptr));
+                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", detail::hex_byte(byte)), nullptr));
                         }
 
                         case error_handler_t::ignore:
@@ -995,9 +913,9 @@ class serializer
                                 }
                                 else
                                 {
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xEF');
+                                    string_buffer[bytes++] = '\xEF';
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xBF');
+                                    string_buffer[bytes++] = '\xBF';
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xBD');
+                                    string_buffer[bytes++] = '\xBD';
                                 }
 
                                 // write buffer and reset index; there must be 13 bytes
@@ -1054,7 +972,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s[s.size() - 1] | 0))), nullptr));
+                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", detail::hex_byte(static_cast<std::uint8_t>(s[s.size() - 1]))), nullptr));
                 }
 
                 case error_handler_t::ignore:
@@ -1275,20 +1193,6 @@ class serializer
         }
     }
 
-    /*!
-     * @brief convert a byte to a uppercase hex representation
-     * @param[in] byte byte to represent
-     * @return representation ("00".."FF")
-     */
-    static std::string hex_bytes(std::uint8_t byte)
-    {
-        std::string result = "FF";
-        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
-        result[0] = nibble_to_hex[byte / 16];
-        result[1] = nibble_to_hex[byte % 16];
-        return result;
-    }
-
     /*!
      * @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
      *
@@ -1402,7 +1306,7 @@ class serializer
     /*!
     @brief dump an integer
 
-    Dump a given integer to output stream @a o. Works internally with
+    Dump a given integer, appending it to @ref write_buffer. Works internally with
     @a number_buffer.
 
     @param[in] x  integer number (signed or unsigned) to dump
@@ -1439,7 +1343,7 @@ class serializer
         }
 
         // use a pointer to fill the buffer
-        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)
 
         number_unsigned_t abs_value;
 
@@ -1493,7 +1397,7 @@ class serializer
     /*!
     @brief dump a floating-point number
 
-    Dump a given floating-point number to output stream @a o. Works internally
+    Dump a given floating-point number, appending it to @ref write_buffer. Works internally
     with @a number_buffer.
 
     @param[in] x  floating-point number to dump
@@ -1554,21 +1458,28 @@ class serializer
         // check if the buffer was large enough
         JSON_ASSERT(static_cast<std::size_t>(len) < number_buffer.size());
 
+        // look up the locale's thousands separator and decimal point now,
+        // matching what snprintf_float() just used (see lexer::get_decimal_point())
+        const auto* loc = std::localeconv();
+        JSON_ASSERT(loc != nullptr);
+        const char thousands_sep = (loc->thousands_sep == nullptr) ? '\0' : *loc->thousands_sep;
+        const char decimal_point = (loc->decimal_point == nullptr) ? '\0' : *loc->decimal_point;
+
         // erase thousands separators
-        if (locale.thousands_sep != '\0')
+        if (thousands_sep != '\0')
         {
             // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::remove returns an iterator, see https://github.com/nlohmann/json/issues/3081
-            const auto end = std::remove(number_buffer.begin(), number_buffer.begin() + len, locale.thousands_sep);
+            const auto end = std::remove(number_buffer.begin(), number_buffer.begin() + len, thousands_sep);
             std::fill(end, number_buffer.end(), '\0');
             JSON_ASSERT((end - number_buffer.begin()) <= len);
             len = (end - number_buffer.begin());
         }
 
         // convert decimal point to '.'
-        if (locale.decimal_point != '\0' && locale.decimal_point != '.')
+        if (decimal_point != '\0' && decimal_point != '.')
         {
             // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::find returns an iterator, see https://github.com/nlohmann/json/issues/3081
-            const auto dec_pos = std::find(number_buffer.begin(), number_buffer.end(), locale.decimal_point);
+            const auto dec_pos = std::find(number_buffer.begin(), number_buffer.end(), decimal_point);
             if (dec_pos != number_buffer.end())
             {
                 *dec_pos = '.';
@@ -1613,34 +1524,17 @@ class serializer
      */
     number_unsigned_t remove_sign(number_integer_t x) noexcept
     {
-        JSON_ASSERT(x < 0 && x < (std::numeric_limits<number_integer_t>::max)()); // NOLINT(misc-redundant-expression)
+        JSON_ASSERT(x < 0);
         return static_cast<number_unsigned_t>(-(x + 1)) + 1;
     }
 
   private:
-    /// the locale's thousand separator and decimal point characters
-    struct locale_chars
-    {
-        explicit locale_chars(const std::lconv* loc) noexcept
-            : thousands_sep(loc->thousands_sep == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->thousands_sep)))
-            , decimal_point(loc->decimal_point == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->decimal_point)))
-        {}
-
-        const char thousands_sep;
-        const char decimal_point;
-    };
-
     /// the output of the serializer (non-owning; the adapter lives at the call site)
     output_adapter_protocol<char>* o = nullptr;
 
     /// a (hopefully) large enough character buffer
     std::array<char, 64> number_buffer{{}};
 
-    /// computed once from std::localeconv() at construction; @ref
-    /// locale_chars keeps std::localeconv()'s pointer from having to be held
-    /// past the constructor, while still letting these stay const
-    const locale_chars locale;
-
     /// string buffer
     std::array<char, 512> string_buffer{{}};
 

include/nlohmann/detail/string_utils.hpp

@@ -3,6 +3,7 @@
 // |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
+// SPDX-FileCopyrightText: 2008, 2009 Björn Hoehrmann <bjoern@hoehrmann.de>
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
@@ -36,6 +37,16 @@ StringType to_string(std::size_t value)
     return result;
 }
 
+/// @return a byte as two uppercase hexadecimal digits
+inline std::string hex_byte(const std::uint8_t byte)
+{
+    std::string result = "00";
+    constexpr const char* nibble_to_hex = "0123456789ABCDEF";
+    result[0] = nibble_to_hex[byte / 16];
+    result[1] = nibble_to_hex[byte % 16];
+    return result;
+}
+
 ///////////////////
 // UTF-8 decoding //
 ///////////////////

single_include/nlohmann/json.hpp

@@ -6199,6 +6199,7 @@ NLOHMANN_JSON_NAMESPACE_END
 // |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
+// SPDX-FileCopyrightText: 2008, 2009 Björn Hoehrmann <bjoern@hoehrmann.de>
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
@@ -6234,6 +6235,16 @@ StringType to_string(std::size_t value)
     return result;
 }
 
+/// @return a byte as two uppercase hexadecimal digits
+inline std::string hex_byte(const std::uint8_t byte)
+{
+    std::string result = "00";
+    constexpr const char* nibble_to_hex = "0123456789ABCDEF";
+    result[0] = nibble_to_hex[byte / 16];
+    result[1] = nibble_to_hex[byte % 16];
+    return result;
+}
+
 ///////////////////
 // UTF-8 decoding //
 ///////////////////
@@ -20146,9 +20157,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
-#include <algorithm> // copy
 #include <cstddef> // size_t
-#include <iterator> // back_inserter
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
 #include <utility> // move
@@ -20170,6 +20179,10 @@ namespace detail
 template<typename CharType> struct output_adapter_protocol
 {
     virtual void write_character(CharType c) = 0;
+    /// @param[in] s      pointer to the characters to write; binary_writer legitimately
+    ///                   passes a null pointer together with length 0 for an empty
+    ///                   string or binary value, so implementations must tolerate that
+    /// @param[in] length number of characters at @a s
     virtual void write_characters(const CharType* s, std::size_t length) = 0;
     virtual ~output_adapter_protocol() = default;
 
@@ -20236,7 +20249,6 @@ class output_vector_adapter : public output_adapter_protocol<CharType>
         sink.write_character(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         sink.write_characters(s, length);
@@ -20261,7 +20273,6 @@ class output_stream_adapter : public output_adapter_protocol<CharType>
         stream.put(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         stream.write(s, static_cast<std::streamsize>(length));
@@ -20286,7 +20297,6 @@ class output_string_adapter : public output_adapter_protocol<CharType>
         str.push_back(c);
     }
 
-    JSON_HEDLEY_NON_NULL(2)
     void write_characters(const CharType* s, std::size_t length) override
     {
         str.append(s, length);
@@ -20359,6 +20369,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/string_concat.hpp>
 
+// #include <nlohmann/detail/string_utils.hpp>
+
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -22608,20 +22620,10 @@ class binary_writer
         const std::size_t valid = valid_utf8_prefix(data, s.size());
         if (JSON_HEDLEY_UNLIKELY(valid != s.size()))
         {
-            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", hex_byte(data[valid])), &context));
+            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", detail::hex_byte(data[valid])), &context));
         }
     }
 
-    /// @return a byte as two uppercase hexadecimal digits
-    static std::string hex_byte(const std::uint8_t byte)
-    {
-        std::string result = "00";
-        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
-        result[0] = nibble_to_hex[byte / 16];
-        result[1] = nibble_to_hex[byte % 16];
-        return result;
-    }
-
     /*!
     @brief write an integer in the shortest encoding
 
@@ -22954,24 +22956,23 @@ NLOHMANN_JSON_NAMESPACE_END
 // |  |  |__   |  |  | | | |  version 3.12.0
 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
 //
-// SPDX-FileCopyrightText: 2008, 2009 Björn Hoehrmann <bjoern@hoehrmann.de>
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
 
 
-#include <algorithm> // reverse, remove, fill, find, none_of, min
+#include <algorithm> // remove, fill, find, none_of, min
 #include <array> // array
 #include <clocale> // localeconv, lconv
-#include <cmath> // labs, isfinite, isnan, signbit
+#include <cmath> // isfinite
 #include <cstddef> // size_t, ptrdiff_t
 #include <cstdint> // uint8_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy, memset
+#include <iterator> // next
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <type_traits> // is_same
-#include <utility> // move
 #include <vector> // vector
 
 // #include <nlohmann/detail/conversions/to_chars.hpp>
@@ -24103,8 +24104,6 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/meta/cpp_future.hpp>
 
-// #include <nlohmann/detail/output/binary_writer.hpp>
-
 // #include <nlohmann/detail/output/output_adapters.hpp>
 
 // #include <nlohmann/detail/recursion_depth_limit.hpp>
@@ -24164,7 +24163,6 @@ class serializer
                const std::size_t indent_step_ = 0,
                error_handler_t error_handler_ = error_handler_t::strict)
         : o(&s)
-        , locale(std::localeconv())
         , indent_char(ichar)
         , pretty_print(pretty_print_)
         , ensure_ascii(ensure_ascii_)
@@ -24187,9 +24185,10 @@ class serializer
     additional parameter. Arrays and objects are serialized without recursion,
     however deeply they are nested.
 
-    - strings and object keys are escaped using `escape_string()`
+    - strings and object keys are escaped using @ref dump_escaped
-    - integer numbers are converted implicitly via `operator<<`
+    - integer numbers are converted using a digit-pair lookup table (@ref dump_integer)
-    - floating-point numbers are converted to a string using `"%g"` format
+    - floating-point numbers are converted to a string using @ref dump_float, which
+      uses `to_chars` for IEEE-754 types and `snprintf` otherwise
     - binary values are serialized as objects containing the subtype and the
       byte array
 
@@ -24364,128 +24363,17 @@ class serializer
             }
 
             case value_t::string:
-            {
-                put_char('"');
-                dump_escaped(*val.m_data.m_value.string);
-                put_char('"');
-                return;
-            }
-
             case value_t::binary:
-            {
-                if (pretty_print)
-                {
-                    put_literal("{\n");
-
-                    // variable to hold indentation for recursive calls
-                    const auto new_indent = next_indent(current_indent, indent_step);
-
-                    put_indent(new_indent);
-
-                    put_literal("\"bytes\": [");
-
-                    if (!val.m_data.m_value.binary->empty())
-                    {
-                        for (auto i = val.m_data.m_value.binary->cbegin();
-                                i != val.m_data.m_value.binary->cend() - 1; ++i)
-                        {
-                            dump_byte(*i);
-                            put_literal(", ");
-                        }
-                        dump_byte(val.m_data.m_value.binary->back());
-                    }
-
-                    put_literal("],\n");
-                    put_indent(new_indent);
-
-                    put_literal("\"subtype\": ");
-                    if (val.m_data.m_value.binary->has_subtype())
-                    {
-                        dump_integer(val.m_data.m_value.binary->subtype());
-                    }
-                    else
-                    {
-                        put_literal("null");
-                    }
-                    put_char('\n');
-                    put_indent(current_indent);
-                    put_char('}');
-                }
-                else
-                {
-                    put_literal("{\"bytes\":[");
-
-                    if (!val.m_data.m_value.binary->empty())
-                    {
-                        for (auto i = val.m_data.m_value.binary->cbegin();
-                                i != val.m_data.m_value.binary->cend() - 1; ++i)
-                        {
-                            dump_byte(*i);
-                            put_char(',');
-                        }
-                        dump_byte(val.m_data.m_value.binary->back());
-                    }
-
-                    put_literal("],\"subtype\":");
-                    if (val.m_data.m_value.binary->has_subtype())
-                    {
-                        dump_integer(val.m_data.m_value.binary->subtype());
-                        put_char('}');
-                    }
-                    else
-                    {
-                        put_literal("null}");
-                    }
-                }
-                return;
-            }
-
             case value_t::boolean:
-            {
-                if (val.m_data.m_value.boolean)
-                {
-                    put_literal("true");
-                }
-                else
-                {
-                    put_literal("false");
-                }
-                return;
-            }
-
             case value_t::number_integer:
-            {
-                dump_integer(val.m_data.m_value.number_integer);
-                return;
-            }
-
             case value_t::number_unsigned:
-            {
-                dump_integer(val.m_data.m_value.number_unsigned);
-                return;
-            }
-
             case value_t::number_float:
-            {
-                dump_float(val.m_data.m_value.number_float);
-                return;
-            }
-
             case value_t::discarded:
-            {
-                put_literal("<discarded>");
-                return;
-            }
-
             case value_t::null:
-            {
+            default:
-                put_literal("null");
+                dump_scalar(val, current_indent);
                 return;
         }
-
-            default:            // LCOV_EXCL_LINE
-                JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
-        }
     }
 
   private:
@@ -24641,9 +24529,9 @@ class serializer
     @brief serialize the value @a val, but not the elements of a container
 
     An object or array with elements is opened and pushed onto @a stack for
-    @ref dump_internal to walk; everything else - including a binary value,
+    @ref dump_iteratively to walk; everything else - including a binary value,
     which looks like an object but has no elements to descend into - is written
-    out here in full.
+    out in full by @ref dump_scalar.
     */
     void dump_value(const BasicJsonType& val,
                     const std::size_t current_indent,
@@ -24701,6 +24589,35 @@ class serializer
                 return;
             }
 
+            case value_t::string:
+            case value_t::binary:
+            case value_t::boolean:
+            case value_t::number_integer:
+            case value_t::number_unsigned:
+            case value_t::number_float:
+            case value_t::discarded:
+            case value_t::null:
+            default:
+                dump_scalar(val, current_indent);
+                return;
+        }
+    }
+
+    /*!
+    @brief serialize the value @a val, which is neither an object nor an array
+
+    Shared by @ref dump_internal and @ref dump_value, so that a value is written
+    the same way however deeply it is nested. A binary value is written out here
+    in full: it looks like an object, but has no elements to descend into.
+
+    @param[in] val             value to serialize; not an object or array
+    @param[in] current_indent  the indentation of @a val, used for a
+                               pretty-printed binary value
+    */
+    void dump_scalar(const BasicJsonType& val, const std::size_t current_indent)
+    {
+        switch (val.m_data.m_type)
+        {
             case value_t::string:
             {
                 put_char('"');
@@ -24821,6 +24738,8 @@ class serializer
                 return;
             }
 
+            case value_t::object: // LCOV_EXCL_LINE
+            case value_t::array:  // LCOV_EXCL_LINE
             default:            // LCOV_EXCL_LINE
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
@@ -24849,7 +24768,7 @@ class serializer
     Escape a string by replacing certain special characters by a sequence of an
     escape character (backslash) and another character and other control
     characters by a sequence of "\u" followed by a four-digit hex
-    representation. The escaped string is written to output stream @a o.
+    representation. The escaped string is appended to @ref write_buffer.
 
     @param[in] s  the string to escape
 
@@ -25043,7 +24962,7 @@ class serializer
                     {
                         case error_handler_t::strict:
                         {
-                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", hex_bytes(byte | 0)), nullptr));
+                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", detail::hex_byte(byte)), nullptr));
                         }
 
                         case error_handler_t::ignore:
@@ -25076,9 +24995,9 @@ class serializer
                                 }
                                 else
                                 {
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xEF');
+                                    string_buffer[bytes++] = '\xEF';
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xBF');
+                                    string_buffer[bytes++] = '\xBF';
-                                    string_buffer[bytes++] = detail::binary_writer<BasicJsonType, char>::to_char_type('\xBD');
+                                    string_buffer[bytes++] = '\xBD';
                                 }
 
                                 // write buffer and reset index; there must be 13 bytes
@@ -25135,7 +25054,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s[s.size() - 1] | 0))), nullptr));
+                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", detail::hex_byte(static_cast<std::uint8_t>(s[s.size() - 1]))), nullptr));
                 }
 
                 case error_handler_t::ignore:
@@ -25356,20 +25275,6 @@ class serializer
         }
     }
 
-    /*!
-     * @brief convert a byte to a uppercase hex representation
-     * @param[in] byte byte to represent
-     * @return representation ("00".."FF")
-     */
-    static std::string hex_bytes(std::uint8_t byte)
-    {
-        std::string result = "FF";
-        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
-        result[0] = nibble_to_hex[byte / 16];
-        result[1] = nibble_to_hex[byte % 16];
-        return result;
-    }
-
     /*!
      * @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
      *
@@ -25483,7 +25388,7 @@ class serializer
     /*!
     @brief dump an integer
 
-    Dump a given integer to output stream @a o. Works internally with
+    Dump a given integer, appending it to @ref write_buffer. Works internally with
     @a number_buffer.
 
     @param[in] x  integer number (signed or unsigned) to dump
@@ -25520,7 +25425,7 @@ class serializer
         }
 
         // use a pointer to fill the buffer
-        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)
 
         number_unsigned_t abs_value;
 
@@ -25574,7 +25479,7 @@ class serializer
     /*!
     @brief dump a floating-point number
 
-    Dump a given floating-point number to output stream @a o. Works internally
+    Dump a given floating-point number, appending it to @ref write_buffer. Works internally
     with @a number_buffer.
 
     @param[in] x  floating-point number to dump
@@ -25635,21 +25540,28 @@ class serializer
         // check if the buffer was large enough
         JSON_ASSERT(static_cast<std::size_t>(len) < number_buffer.size());
 
+        // look up the locale's thousands separator and decimal point now,
+        // matching what snprintf_float() just used (see lexer::get_decimal_point())
+        const auto* loc = std::localeconv();
+        JSON_ASSERT(loc != nullptr);
+        const char thousands_sep = (loc->thousands_sep == nullptr) ? '\0' : *loc->thousands_sep;
+        const char decimal_point = (loc->decimal_point == nullptr) ? '\0' : *loc->decimal_point;
+
         // erase thousands separators
-        if (locale.thousands_sep != '\0')
+        if (thousands_sep != '\0')
         {
             // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::remove returns an iterator, see https://github.com/nlohmann/json/issues/3081
-            const auto end = std::remove(number_buffer.begin(), number_buffer.begin() + len, locale.thousands_sep);
+            const auto end = std::remove(number_buffer.begin(), number_buffer.begin() + len, thousands_sep);
             std::fill(end, number_buffer.end(), '\0');
             JSON_ASSERT((end - number_buffer.begin()) <= len);
             len = (end - number_buffer.begin());
         }
 
         // convert decimal point to '.'
-        if (locale.decimal_point != '\0' && locale.decimal_point != '.')
+        if (decimal_point != '\0' && decimal_point != '.')
         {
             // NOLINTNEXTLINE(readability-qualified-auto,llvm-qualified-auto): std::find returns an iterator, see https://github.com/nlohmann/json/issues/3081
-            const auto dec_pos = std::find(number_buffer.begin(), number_buffer.end(), locale.decimal_point);
+            const auto dec_pos = std::find(number_buffer.begin(), number_buffer.end(), decimal_point);
             if (dec_pos != number_buffer.end())
             {
                 *dec_pos = '.';
@@ -25694,34 +25606,17 @@ class serializer
      */
     number_unsigned_t remove_sign(number_integer_t x) noexcept
     {
-        JSON_ASSERT(x < 0 && x < (std::numeric_limits<number_integer_t>::max)()); // NOLINT(misc-redundant-expression)
+        JSON_ASSERT(x < 0);
         return static_cast<number_unsigned_t>(-(x + 1)) + 1;
     }
 
   private:
-    /// the locale's thousand separator and decimal point characters
-    struct locale_chars
-    {
-        explicit locale_chars(const std::lconv* loc) noexcept
-            : thousands_sep(loc->thousands_sep == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->thousands_sep)))
-            , decimal_point(loc->decimal_point == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->decimal_point)))
-        {}
-
-        const char thousands_sep;
-        const char decimal_point;
-    };
-
     /// the output of the serializer (non-owning; the adapter lives at the call site)
     output_adapter_protocol<char>* o = nullptr;
 
     /// a (hopefully) large enough character buffer
     std::array<char, 64> number_buffer{{}};
 
-    /// computed once from std::localeconv() at construction; @ref
-    /// locale_chars keeps std::localeconv()'s pointer from having to be held
-    /// past the constructor, while still letting these stay const
-    const locale_chars locale;
-
     /// string buffer
     std::array<char, 512> string_buffer{{}};
 

tests/src/unit-locale-cpp.cpp

@@ -14,7 +14,10 @@ using nlohmann::json;
 
 #include <array>
 #include <clocale>
+#include <limits>
 #include <map>
+#include <ostream>
+#include <streambuf>
 #include <string>
 #include <utility>
 #include <vector>
@@ -385,3 +388,92 @@ TEST_CASE("locale with a multi-byte decimal point")
 
     CHECK(std::setlocale(LC_NUMERIC, "C") != nullptr);
 }
+
+namespace
+{
+// a streambuf that switches LC_NUMERIC the first time anything is written to
+// it, so a dump() in progress can be made to change locale mid-flight: after
+// the serializer was constructed (and, before #5709 item 3, after it had
+// cached std::localeconv() for the whole call) but before a later float is
+// converted
+struct LocaleSwitchingStreambuf final : std::streambuf
+{
+    explicit LocaleSwitchingStreambuf(const char* switch_to)
+        : locale_after_first_write(switch_to)
+    {}
+
+    std::string data {}; // NOLINT(readability-redundant-member-init)
+    std::string locale_after_first_write;
+    bool switched = false;
+
+    std::streamsize xsputn(const char* s, std::streamsize n) override
+    {
+        if (!switched)
+        {
+            switched = std::setlocale(LC_NUMERIC, locale_after_first_write.c_str()) != nullptr;
+        }
+        data.append(s, static_cast<std::size_t>(n));
+        return n;
+    }
+};
+} // namespace
+
+TEST_CASE("locale changes during a single dump() (#5709 item 3)")
+{
+    // dump_float() only reads the locale on the snprintf path, taken for a
+    // number_float_t that is not an IEEE-754 single or double, i.e. not
+    // (is_iec559 && digits == 24 && max_exponent == 128) and not (is_iec559
+    // && digits == 53 && max_exponent == 1024) - see dump_float(). Checking
+    // is_iec559 alone is not enough: on x86_64, long double is a 64-bit
+    // (80-bit extended) format for which is_iec559 is also true, so it still
+    // takes the snprintf path this test means to exercise. Only a
+    // number_float_t whose digits/max_exponent match float or double (e.g.
+    // long double on 64-bit Arm, where it is IEEE-754 double) takes the
+    // locale-independent to_chars() path instead, and this test is a no-op
+    // there.
+    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+    using ld_limits = std::numeric_limits<long_double_json::number_float_t>;
+    const bool is_ieee_single_or_double =
+        (ld_limits::is_iec559 && ld_limits::digits == 24 && ld_limits::max_exponent == 128) ||
+        (ld_limits::is_iec559 && ld_limits::digits == 53 && ld_limits::max_exponent == 1024);
+    if (is_ieee_single_or_double)
+    {
+        MESSAGE("long double is IEEE-754 single or double on this platform; dump_float()'s snprintf/locale path is not exercised here");
+    }
+
+    const char* de_DE_name = "de_DE.UTF-8";
+    if (std::setlocale(LC_NUMERIC, de_DE_name) == nullptr)
+    {
+        de_DE_name = "de_DE";
+        if (std::setlocale(LC_NUMERIC, de_DE_name) == nullptr)
+        {
+            MESSAGE("locale de_DE is not usable");
+            return;
+        }
+    }
+    const std::string decimal_point = std::localeconv()->decimal_point;
+    REQUIRE(std::setlocale(LC_NUMERIC, "C") != nullptr);
+    if (decimal_point != ",")
+    {
+        MESSAGE("de_DE's decimal point is not ',' on this platform, skipping");
+        return;
+    }
+
+    // a string long enough to overflow the serializer's internal write
+    // buffer, so that it is flushed to the output adapter - and the locale
+    // switched - before the number after it is converted
+    const std::string padding(5000, 'a');
+    const long_double_json j = { padding, 1234.5L };
+
+    LocaleSwitchingStreambuf buf(de_DE_name);
+    std::ostream os(&buf);
+    os << j;
+    CHECK(std::setlocale(LC_NUMERIC, "C") != nullptr);
+
+    REQUIRE(buf.switched);
+    // whatever locale was in effect when the float was actually converted,
+    // the output is normalized to use '.' as the decimal point: it must be
+    // looked up at conversion time, not once for the whole dump() - the same
+    // fix #5597 made on the parser side
+    CHECK(buf.data == "[\"" + padding + "\",1234.5]");
+}