Un-nest the run-length ternary in the string escaper

clang-tidy's readability-avoid-nested-conditional-operator (newly enforced
by the ci_clang_tidy job's clang) rejected the nested ?: that picked the
run length: EnsureAscii ? (is_ascii_copyable(...) ? scan : 0) : bulk.

Compute it in an immediately-invoked lambda instead. The value stays const
and the laziness is unchanged - the ascii scan still runs only on the
ensure_ascii path when the first byte is copyable, and string_bulk_run only
on the other path (EnsureAscii is a template bool, so the dead branch is
folded away). Output is byte-for-byte identical; single_include regenerated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L1oJ2ggRHS37zeVe94QTA1
Signed-off-by: Claude <noreply@anthropic.com>

include/nlohmann/detail/output/serializer.hpp

@@ -63,13 +63,14 @@ class serializer
 
   public:
     /*!
-    @param[in] s  output stream to serialize to
+    @param[in] s  output adapter to serialize to; not owned by the serializer,
+                  so it must outlive it (it lives at the call site)
     @param[in] ichar  indentation character to use
     @param[in] error_handler_  how to react on decoding errors
     */
-    serializer(output_adapter_t<char> s, const char ichar,
+    serializer(output_adapter_protocol<char>& s, const char ichar,
                error_handler_t error_handler_ = error_handler_t::strict)
-        : o(std::move(s))
+        : o(&s)
         , loc(std::localeconv())
         , 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)))
@@ -844,9 +845,21 @@ class serializer
             if (state == UTF8_ACCEPT)
             {
                 const auto* const data = reinterpret_cast<const unsigned char*>(s.data());
-                const std::size_t run = EnsureAscii
+                // A run can only be non-empty when the very first byte is one
-                                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                // the scanner may copy, so test that single byte before paying
-                                        : string_bulk_run(data + i, s.size() - i);
+                // for the scan. Without it, text whose characters all have to be
+                // escaped - CJK under ensure_ascii, where every byte is >= 0x80 -
+                // runs the scanner once per character only to be told zero.
+                const std::size_t run = [&]() -> std::size_t
+                {
+                    if (EnsureAscii)
+                    {
+                        return is_ascii_copyable(data[i])
+                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                        : 0;
+                    }
+                    return string_bulk_run(data + i, s.size() - i);
+                }();
                 if (run != 0)
                 {
                     // emit any bytes still pending in string_buffer first to
@@ -1677,8 +1690,8 @@ class serializer
     }
 
   private:
-    /// the output of the serializer
+    /// the output of the serializer (non-owning; the adapter lives at the call site)
-    output_adapter_t<char> o = nullptr;
+    output_adapter_protocol<char>* o = nullptr;
 
     /// a (hopefully) large enough character buffer
     std::array<char, 64> number_buffer{{}};

include/nlohmann/json.hpp

@@ -1341,7 +1341,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                   const error_handler_t error_handler = error_handler_t::strict) const
     {
         string_t result;
-        serializer s(detail::output_adapter<char, string_t>(result), indent_char, error_handler);
+        detail::output_string_adapter<char, string_t> string_adapter(result);
+        serializer s(string_adapter, indent_char, error_handler);
 
         if (indent >= 0)
         {
@@ -4055,7 +4056,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         o.width(0);
 
         // do the actual serialization
-        serializer s(detail::output_adapter<char>(o), o.fill());
+        detail::output_stream_adapter<char> stream_adapter(o);
+        serializer s(stream_adapter, o.fill());
         s.dump(j, pretty_print, false, static_cast<unsigned int>(indentation));
         return o;
     }

single_include/nlohmann/json.hpp

@@ -21123,13 +21123,14 @@ class serializer
 
   public:
     /*!
-    @param[in] s  output stream to serialize to
+    @param[in] s  output adapter to serialize to; not owned by the serializer,
+                  so it must outlive it (it lives at the call site)
     @param[in] ichar  indentation character to use
     @param[in] error_handler_  how to react on decoding errors
     */
-    serializer(output_adapter_t<char> s, const char ichar,
+    serializer(output_adapter_protocol<char>& s, const char ichar,
                error_handler_t error_handler_ = error_handler_t::strict)
-        : o(std::move(s))
+        : o(&s)
         , loc(std::localeconv())
         , 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)))
@@ -21904,9 +21905,21 @@ class serializer
             if (state == UTF8_ACCEPT)
             {
                 const auto* const data = reinterpret_cast<const unsigned char*>(s.data());
-                const std::size_t run = EnsureAscii
+                // A run can only be non-empty when the very first byte is one
-                                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                // the scanner may copy, so test that single byte before paying
-                                        : string_bulk_run(data + i, s.size() - i);
+                // for the scan. Without it, text whose characters all have to be
+                // escaped - CJK under ensure_ascii, where every byte is >= 0x80 -
+                // runs the scanner once per character only to be told zero.
+                const std::size_t run = [&]() -> std::size_t
+                {
+                    if (EnsureAscii)
+                    {
+                        return is_ascii_copyable(data[i])
+                        ? find_ascii_copyable_run(data + i, s.size() - i)
+                        : 0;
+                    }
+                    return string_bulk_run(data + i, s.size() - i);
+                }();
                 if (run != 0)
                 {
                     // emit any bytes still pending in string_buffer first to
@@ -22737,8 +22750,8 @@ class serializer
     }
 
   private:
-    /// the output of the serializer
+    /// the output of the serializer (non-owning; the adapter lives at the call site)
-    output_adapter_t<char> o = nullptr;
+    output_adapter_protocol<char>* o = nullptr;
 
     /// a (hopefully) large enough character buffer
     std::array<char, 64> number_buffer{{}};
@@ -24440,7 +24453,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                   const error_handler_t error_handler = error_handler_t::strict) const
     {
         string_t result;
-        serializer s(detail::output_adapter<char, string_t>(result), indent_char, error_handler);
+        detail::output_string_adapter<char, string_t> string_adapter(result);
+        serializer s(string_adapter, indent_char, error_handler);
 
         if (indent >= 0)
         {
@@ -27154,7 +27168,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         o.width(0);
 
         // do the actual serialization
-        serializer s(detail::output_adapter<char>(o), o.fill());
+        detail::output_stream_adapter<char> stream_adapter(o);
+        serializer s(stream_adapter, o.fill());
         s.dump(j, pretty_print, false, static_cast<unsigned int>(indentation));
         return o;
     }

tests/src/unit-convenience.cpp

@@ -98,7 +98,8 @@ void check_escaped(const char* original, const char* escaped = "", bool ensure_a
 void check_escaped(const char* original, const char* escaped, const bool ensure_ascii)
 {
     std::stringstream ss;
-    json::serializer s(nlohmann::detail::output_adapter<char>(ss), ' ');
+    nlohmann::detail::output_stream_adapter<char> adapter(ss);
+    json::serializer s(adapter, ' ');
     s.dump_escaped(original, ensure_ascii);
     s.flush(); // dump_escaped writes into the serializer's internal buffer
     CHECK(ss.str() == escaped);