From 755c54700318ec0eddd31f9d0639bc91d567c19b Mon Sep 17 00:00:00 2001 From: Niels Lohmann Date: Sun, 6 Sep 2026 16:41:35 +0200 Subject: [PATCH] Return the parsed value by move from from_cbor() and friends 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 --- include/nlohmann/json.hpp | 98 +++++++++++++++++++++++--------- single_include/nlohmann/json.hpp | 98 +++++++++++++++++++++++--------- 2 files changed, 140 insertions(+), 56 deletions(-) diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp index be4ccb90f..71f54e357 100644 --- a/include/nlohmann/json.hpp +++ b/include/nlohmann/json.hpp @@ -4473,8 +4473,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -4516,8 +4522,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -4571,8 +4586,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -4626,8 +4650,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -4712,8 +4751,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict)) // cppcheck-suppress[accessMoved] + { + result = value_t::discarded; + } + return result; } /// @} diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp index 31bfb869d..3ff248ddf 100644 --- a/single_include/nlohmann/json.hpp +++ b/single_include/nlohmann/json.hpp @@ -25901,8 +25901,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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) @@ -25918,8 +25921,11 @@ 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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -25944,8 +25950,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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 @@ -25959,8 +25968,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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) @@ -25975,8 +25987,11 @@ 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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -25999,8 +26014,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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 @@ -26014,8 +26032,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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) @@ -26030,8 +26051,11 @@ 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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -26054,8 +26078,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(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 @@ -26069,8 +26096,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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) @@ -26085,8 +26115,11 @@ 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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -26100,8 +26133,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec basic_json result; auto ia = detail::input_adapter(std::forward(i)); detail::json_sax_dom_parser sdp(result, allow_exceptions); - const bool res = binary_reader(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(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) @@ -26116,8 +26152,11 @@ 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 sdp(result, allow_exceptions); - const bool res = binary_reader(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(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 @@ -26140,8 +26179,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec auto ia = i.get(); detail::json_sax_dom_parser sdp(result, allow_exceptions); // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg) - const bool res = binary_reader(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(std::move(ia), input_format_t::bson).sax_parse(input_format_t::bson, &sdp, strict)) // cppcheck-suppress[accessMoved] + { + result = value_t::discarded; + } + return result; } /// @}