Stop dump() from heap-allocating its output adapter per call

The serializer held its output sink as output_adapter_t<char>
(a std::shared_ptr<output_adapter_protocol<char>>), which dump() and
operator<< built via make_shared -- one heap allocation per call for a
sink that only wraps a reference to the caller's string or stream.

Hold the sink as a non-owning output_adapter_protocol<char>* instead and
construct the concrete adapter on the stack at the call site. The write
path (o->write_characters) is unchanged, so output is byte-for-byte
identical; a compact dump() of a small object drops from 2 heap
allocations to 1 (only the returned string remains), ~3% faster.

Completes the per-call allocation cleanup on this branch, which already
removed the indent_string buffer (both were reported in #5413).

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)))
@@ -1677,8 +1678,8 @@ 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

@@ -21043,13 +21043,14 @@ 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)))
@@ -22657,8 +22658,8 @@ 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{{}};
@@ -24360,7 +24361,8 @@ 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)
         {
@@ -27074,7 +27076,8 @@ 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;
     }