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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/detail/output/serializer.hpp
+6 -5
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@@ -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
output_adapter_t<char> o = nullptr;
/// 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{{}};