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https://github.com/nlohmann/json.git
synced 2026-10-01 06:14:58 +01:00
Niels Lohmann
65af260117
* Move instead of deep-copy ordered_json values when an object grows ordered_map keeps its elements in a std::vector<std::pair<const Key, T>>. With a std::string key, that pair is not nothrow move constructible (the const key has to be copied), so std::vector copies every element when it reallocates. For ordered_json, this deep-copies every member value an object already holds, including whole nested subtrees, on each growth step. Grow the storage in ordered_map instead, copying the keys and moving the values. This happens in two phases, so the strong exception guarantee is kept without try/catch. The first phase may throw, but only touches a temporary buffer: it copies the keys, value-initializes the values, and constructs the new element. The second phase moves the values (noexcept) and swaps the buffers. Because the new element is constructed before any value is moved, arguments that refer to elements of the container stay valid, as with std::vector. Types that cannot take this path keep the std::vector behavior. Parsing into ordered_json (ParseStringOrdered, Apple M1 Max, clang -O3): twitter 3.20 -> 1.70 ms, citm_catalog 7.73 -> 3.67 ms, jeopardy 219 -> 177 ms, canada unchanged. The number of allocations for twitter and citm_catalog drops by two thirds. Also add ParseStringOrdered rows to the benchmarks, and document the growth behavior and the exception safety of ordered_map. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix CI: skip std::pair noexcept assumptions on EDG-based compilers ci_icpc and ci_nvhpc failed to compile unit-ordered_map.cpp: the static assertion that std::pair<const std::string, ordered_json> is not nothrow move-constructible fails there. The EDG front end (Intel icpc 2021.10, NVIDIA nvc++ 25.5) considers the defaulted move constructor of std::pair<const Key, T> noexcept even if copying Key can throw. With these compilers, std::vector already moves such elements itself when it grows, and ordered_map correctly leaves growing to it. The same misjudgement makes std::vector call std::terminate when a key copy throws during growth, so the exception-safety test with throwing_key would abort on these compilers as well. Skip the static assertion and the exception-safety section when __EDG__ is defined. Verified with icpc 2021.10 (-std=gnu++11) and nvc++ 25.5 (C++11 and C++17) on Compiler Explorer: unit-ordered_map and unit-disabled_exceptions build and pass. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
70 lines
2.3 KiB
C++
70 lines
2.3 KiB
C++
// __ _____ _____ _____
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// __| | __| | | | JSON for Modern C++ (supporting code)
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// | | |__ | | | | | | version 3.12.0
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// |_____|_____|_____|_|___| https://github.com/nlohmann/json
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//
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// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
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// SPDX-License-Identifier: MIT
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#include "doctest_compatibility.h"
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// disable -Wnoexcept as exceptions are switched off for this test suite
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DOCTEST_GCC_SUPPRESS_WARNING_PUSH
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DOCTEST_GCC_SUPPRESS_WARNING("-Wnoexcept")
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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/////////////////////////////////////////////////////////////////////
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// for #2824
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/////////////////////////////////////////////////////////////////////
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class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>
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{
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public:
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explicit sax_no_exception(json& j) : nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>(j, false) {}
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static bool parse_error(std::size_t /*position*/, const std::string& /*last_token*/, const json::exception& ex)
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{
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error_string = new std::string(ex.what()); // NOLINT(cppcoreguidelines-owning-memory)
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return false;
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}
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static std::string* error_string;
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};
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std::string* sax_no_exception::error_string = nullptr;
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TEST_CASE("Tests with disabled exceptions")
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{
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SECTION("issue #2824 - encoding of json::exception::what()")
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{
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json j;
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sax_no_exception sax(j);
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CHECK (!json::sax_parse("xyz", &sax));
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CHECK(*sax_no_exception::error_string == "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: 'x'");
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delete sax_no_exception::error_string; // NOLINT(cppcoreguidelines-owning-memory)
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}
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SECTION("growing an ordered_json object")
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{
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auto j = nlohmann::ordered_json::object();
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for (int i = 0; i < 100; ++i)
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{
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j[std::to_string(i)] = {{"nested", i}};
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}
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CHECK(j.size() == 100);
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int i = 0;
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for (const auto& element : j.items())
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{
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CHECK(element.key() == std::to_string(i));
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CHECK(element.value()["nested"] == i);
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++i;
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}
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}
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}
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DOCTEST_GCC_SUPPRESS_WARNING_POP
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