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Return the parsed value by move from from_cbor() and friends (#5501)

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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
2026-09-10 17:15:34 +02:00
committed by GitHub
parent 3e2de32226
commit d69fb8654d
2 changed files with 140 additions and 56 deletions
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single_include/nlohmann/json.hpp
+70 -28
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@@ -26080,8 +26080,11 @@ 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)
@@ -26097,8 +26100,11 @@ 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>
@@ -26123,8 +26129,11 @@ 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
@@ -26138,8 +26147,11 @@ 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)
@@ -26154,8 +26166,11 @@ 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>
@@ -26178,8 +26193,11 @@ 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
@@ -26193,8 +26211,11 @@ 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)
@@ -26209,8 +26230,11 @@ 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>
@@ -26233,8 +26257,11 @@ 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
@@ -26248,8 +26275,11 @@ 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)
@@ -26264,8 +26294,11 @@ 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
@@ -26279,8 +26312,11 @@ 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)
@@ -26295,8 +26331,11 @@ 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>
@@ -26319,8 +26358,11 @@ 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;
}
/// @}