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Return the parsed value by move from from_cbor() and friends

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The binary entry points end with

    return res ? result : basic_json(value_t::discarded);

The condition operator's second operand is an lvalue, so this is not a case
where the return value can be elided or implicitly moved from: every
successful from_cbor(), from_msgpack(), from_ubjson(), from_bjdata() and
from_bson() call deep-copies the value it just parsed, and then destroys the
original.

The copy is not cheap, and it is not incidental: basic_json's copy
constructor walks the whole value. Parsing a 2 MB CBOR document with 60,000
objects, median of 25 runs, clang 17 -O3:

    from_cbor      26.99 ms  ->  14.65 ms
    from_msgpack   26.82 ms  ->  14.82 ms

Moving instead of copying is the entire change; the parsed value is not used
again after the return expression is evaluated.

There is a second reason to prefer the move. The copy constructor recurses
once per nesting level, so the copy is also a stack-overflow path on the
return side, on a value the reader has already accepted. That is currently
masked because the readers themselves recurse and overflow first (#5104), but
it has to be fixed for making them iterative to have any effect.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-07 07:50:09 +02:00
parent 09b6b6b5ba
commit 755c547003
2 changed files with 140 additions and 56 deletions
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include/nlohmann/json.hpp
+70 -28
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@@ -4473,8 +4473,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::forward<InputType>(i));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in CBOR format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -4490,8 +4493,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::move(first), std::move(last));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
template<typename T>
@@ -4516,8 +4522,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = i.get();
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::cbor).sax_parse(input_format_t::cbor, &sdp, strict, tag_handler)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in MessagePack format
@@ -4531,8 +4540,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::forward<InputType>(i));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in MessagePack format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -4547,8 +4559,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::move(first), std::move(last));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
template<typename T>
@@ -4571,8 +4586,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = i.get();
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::msgpack).sax_parse(input_format_t::msgpack, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in UBJSON format
@@ -4586,8 +4604,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::forward<InputType>(i));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in UBJSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -4602,8 +4623,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::move(first), std::move(last));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
template<typename T>
@@ -4626,8 +4650,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = i.get();
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::ubjson).sax_parse(input_format_t::ubjson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in BJData format
@@ -4641,8 +4668,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::forward<InputType>(i));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::bjdata).sax_parse(input_format_t::bjdata, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::bjdata).sax_parse(input_format_t::bjdata, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in BJData format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -4657,8 +4687,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::move(first), std::move(last));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::bjdata).sax_parse(input_format_t::bjdata, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::bjdata).sax_parse(input_format_t::bjdata, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in BSON format
@@ -4672,8 +4705,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::forward<InputType>(i));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @brief create a JSON value from an input in BSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -4688,8 +4724,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
basic_json result;
auto ia = detail::input_adapter(std::move(first), std::move(last));
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
template<typename T>
@@ -4712,8 +4751,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = i.get();
detail::json_sax_dom_parser<basic_json, decltype(ia)> sdp(result, allow_exceptions);
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
const bool res = binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict); // cppcheck-suppress[accessMoved]
return res ? result : basic_json(value_t::discarded);
if (!binary_reader<decltype(ia)>(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict)) // cppcheck-suppress[accessMoved]
{
result = value_t::discarded;
}
return result;
}
/// @}