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Devirtualize binary_writer via a value-type output sink

Browse Source 
to_cbor/to_msgpack/to_ubjson/to_bjdata/to_bson wrote every byte through
output_adapter_t, a shared_ptr<output_adapter_protocol> whose
write_character/write_characters are virtual. Unlike the lexer (templated
on a concrete InputAdapterType), the binary writer never got that
treatment, so binary output paid a vtable lookup per byte and a
make_shared per call.

Template binary_writer on an OutputSinkType and give it two concrete,
non-virtual sinks:

- output_vector_sink: appends straight into a std::vector (push_back /
  insert), used by the vector-returning to_* convenience functions. No
  vtable, no shared_ptr; the writes inline.
- output_adapter_sink: forwards to a type-erased output_adapter_t, so the
  existing to_*(j, output_adapter) overloads (streams, strings, custom
  adapters) keep working exactly as before -- one virtual call each,
  unchanged.

binary_writer keeps a convenience constructor taking output_adapter_t
(building the default output_adapter_sink), so the adapter overloads are
untouched; only the convenience functions switch to the vector sink. The
friend declaration and the basic_json binary_writer alias gain the new
(defaulted) template parameter.

Output is byte-for-byte identical: verified across ~3000 randomized
values plus curated edge cases (all scalar widths, strings with invalid
UTF-8, binary, nested arrays/objects) for CBOR, MessagePack, UBJSON (both
size/type settings), BJData, and BSON, plus the output_adapter path, in
C++11/17/20. Warning-clean under clang -Weverything and the gcc pedantic
set; clang-tidy clean on the changed headers; make check-amalgamation
clean.

Throughput (g++ -O3, vs develop): scalar-dense binary output such as
integer arrays ~1.4x; many small to_cbor calls ~1.04x (DOM traversal
bound); string/blob-heavy output unchanged (already bulk-bound). No
workload regressed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAYM1qhSA2FDaDcGfPW3fG
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-20 23:27:51 +00:00
co-authored by Claude Opus 4.8
parent 3565f40229
commit ebb3abba41
4 changed files with 486 additions and 324 deletions
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include/nlohmann/json.hpp
+13 -7
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@@ -140,7 +140,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
friend ::nlohmann::detail::serializer<basic_json>;
template<typename BasicJsonType>
friend class ::nlohmann::detail::iter_impl;
template<typename BasicJsonType, typename CharType>
template<typename BasicJsonType, typename CharType, typename OutputSinkType>
friend class ::nlohmann::detail::binary_writer;
template<typename BasicJsonType, typename InputType, typename SAX>
friend class ::nlohmann::detail::binary_reader;
@@ -186,7 +186,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
template<typename InputType>
using binary_reader = ::nlohmann::detail::binary_reader<basic_json, InputType>;
template<typename CharType> using binary_writer = ::nlohmann::detail::binary_writer<basic_json, CharType>;
template<typename CharType, typename OutputSinkType = ::nlohmann::detail::output_adapter_sink<CharType>>
using binary_writer = ::nlohmann::detail::binary_writer<basic_json, CharType, OutputSinkType>;
JSON_PRIVATE_UNLESS_TESTED:
using serializer = ::nlohmann::detail::serializer<basic_json>;
@@ -4327,7 +4328,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static std::vector<std::uint8_t> to_cbor(const basic_json& j)
{
std::vector<std::uint8_t> result;
to_cbor(j, result);
binary_writer<std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
detail::output_vector_sink<std::uint8_t>(result)).write_cbor(j);
return result;
}
@@ -4350,7 +4352,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static std::vector<std::uint8_t> to_msgpack(const basic_json& j)
{
std::vector<std::uint8_t> result;
to_msgpack(j, result);
binary_writer<std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
detail::output_vector_sink<std::uint8_t>(result)).write_msgpack(j);
return result;
}
@@ -4375,7 +4378,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const bool use_type = false)
{
std::vector<std::uint8_t> result;
to_ubjson(j, result, use_size, use_type);
binary_writer<std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
detail::output_vector_sink<std::uint8_t>(result)).write_ubjson(j, use_size, use_type);
return result;
}
@@ -4403,7 +4407,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const bjdata_version_t version = bjdata_version_t::draft2)
{
std::vector<std::uint8_t> result;
to_bjdata(j, result, use_size, use_type, version);
binary_writer<std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
detail::output_vector_sink<std::uint8_t>(result)).write_ubjson(j, use_size, use_type, true, true, version);
return result;
}
@@ -4430,7 +4435,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static std::vector<std::uint8_t> to_bson(const basic_json& j)
{
std::vector<std::uint8_t> result;
to_bson(j, result);
binary_writer<std::uint8_t, detail::output_vector_sink<std::uint8_t>>(
detail::output_vector_sink<std::uint8_t>(result)).write_bson(j);
return result;
}