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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
2026-09-10 19:00:23 +02:00
committed by Niels Lohmann
parent 772e44ff97
commit c5e2a95ff1
4 changed files with 24 additions and 17 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/output/serializer.hpp
+6 -5
View File
@@ -62,7 +62,8 @@ 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] pretty_print_ whether the output shall be pretty-printed
@param[in] ensure_ascii_ If @a ensure_ascii_ is true, all non-ASCII
@@ -76,12 +77,12 @@ class serializer
being threaded through every call to @ref dump, @ref dump_internal and
@ref dump_iteratively.
*/
serializer(output_adapter_t<char> s, const char ichar,
serializer(output_adapter_protocol<char>* s, const char ichar,
const bool pretty_print_ = false,
const bool ensure_ascii_ = false,
const std::size_t indent_step_ = 0,
error_handler_t error_handler_ = error_handler_t::strict)
: o(std::move(s))
: o(s)
, locale(std::localeconv())
, indent_char(ichar)
, pretty_print(pretty_print_)
@@ -1678,8 +1679,8 @@ class serializer
const char decimal_point;
};
/// 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{{}};
include/nlohmann/json.hpp
+5 -3
View File
@@ -1343,16 +1343,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const error_handler_t error_handler = error_handler_t::strict) const
{
string_t result;
detail::output_string_adapter<char, string_t> string_adapter(result);
if (indent >= 0)
{
serializer s(detail::output_adapter<char, string_t>(result), indent_char,
serializer s(&string_adapter, indent_char,
true, ensure_ascii, static_cast<std::size_t>(indent), error_handler);
s.dump(*this);
}
else
{
serializer s(detail::output_adapter<char, string_t>(result), indent_char,
serializer s(&string_adapter, indent_char,
false, ensure_ascii, 0, error_handler);
s.dump(*this);
}
@@ -4083,7 +4084,8 @@ 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(),
pretty_print, false, static_cast<std::size_t>(indentation));
s.dump(j);
return o;
single_include/nlohmann/json.hpp
+11 -8
View File
@@ -21410,7 +21410,8 @@ 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] pretty_print_ whether the output shall be pretty-printed
@param[in] ensure_ascii_ If @a ensure_ascii_ is true, all non-ASCII
@@ -21424,12 +21425,12 @@ class serializer
being threaded through every call to @ref dump, @ref dump_internal and
@ref dump_iteratively.
*/
serializer(output_adapter_t<char> s, const char ichar,
serializer(output_adapter_protocol<char>* s, const char ichar,
const bool pretty_print_ = false,
const bool ensure_ascii_ = false,
const std::size_t indent_step_ = 0,
error_handler_t error_handler_ = error_handler_t::strict)
: o(std::move(s))
: o(s)
, locale(std::localeconv())
, indent_char(ichar)
, pretty_print(pretty_print_)
@@ -23026,8 +23027,8 @@ class serializer
const char decimal_point;
};
/// 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{{}};
@@ -24738,16 +24739,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const error_handler_t error_handler = error_handler_t::strict) const
{
string_t result;
detail::output_string_adapter<char, string_t> string_adapter(result);
if (indent >= 0)
{
serializer s(detail::output_adapter<char, string_t>(result), indent_char,
serializer s(&string_adapter, indent_char,
true, ensure_ascii, static_cast<std::size_t>(indent), error_handler);
s.dump(*this);
}
else
{
serializer s(detail::output_adapter<char, string_t>(result), indent_char,
serializer s(&string_adapter, indent_char,
false, ensure_ascii, 0, error_handler);
s.dump(*this);
}
@@ -27478,7 +27480,8 @@ 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(),
pretty_print, false, static_cast<std::size_t>(indentation));
s.dump(j);
return o;
tests/src/unit-convenience.cpp
+2 -1
View File
@@ -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), ' ', false, ensure_ascii);
nlohmann::detail::output_stream_adapter<char> adapter(ss);
json::serializer s(&adapter, ' ', false, ensure_ascii);
s.dump_escaped(original);
s.flush(); // dump_escaped writes into the serializer's internal buffer
CHECK(ss.str() == escaped);