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

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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>
This commit is contained in:
Claude
2026-09-01 05:26:52 +00:00
parent ee8b2d1699
commit 1232fa947c
3 changed files with 20 additions and 14 deletions
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include/nlohmann/json.hpp
+4 -2
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@@ -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;
}