Niels Lohmann ebb3abba41 Devirtualize binary_writer via a value-type output sink ...

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>

2026-07-20 23:27:51 +00:00

..

nlohmann

Devirtualize binary_writer via a value-type output sink

2026-07-20 23:27:51 +00:00