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Move instead of deep-copy ordered_json values when an object grows (#5609)

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* 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>
This commit is contained in:
Niels Lohmann
2026-09-30 20:06:34 +02:00
committed by GitHub
parent d268eaa693
commit 65af260117
6 changed files with 550 additions and 10 deletions
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358
tests/src/unit-ordered_map.cpp
View File

@@ -11,6 +11,97 @@
#include <nlohmann/json.hpp>
using nlohmann::ordered_map;
#include <stdexcept>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>
// The EDG front end (Intel icpc, NVIDIA nvc++) considers the defaulted move
// constructor of std::pair<const Key, T> noexcept even if copying Key can
// throw. std::vector then moves such elements itself when it grows (and calls
// std::terminate if a key copy throws), so ordered_map leaves growing to it.
#if defined(__EDG__)
#define JSON_TEST_PAIR_MOVE_IS_NOEXCEPT
#endif
namespace
{
// number of copies made of counted values
int value_copies = 0;
// a mapped type that counts its copies; moving from it leaves -1 behind
struct counted // NOLINT(cppcoreguidelines-special-member-functions,hicpp-special-member-functions)
{
int payload = 0;
counted() = default;
explicit counted(int p) noexcept : payload(p) {}
counted(const counted& other) : payload(other.payload)
{
++value_copies;
}
counted(counted&& other) noexcept : payload(other.payload)
{
other.payload = -1;
}
counted& operator=(const counted&) = delete;
counted& operator=(counted&& other) noexcept
{
payload = other.payload;
other.payload = -1;
return *this;
}
};
#if !defined(JSON_NOEXCEPTION) && !defined(JSON_TEST_PAIR_MOVE_IS_NOEXCEPT)
// number of throwing_key copies that still succeed; the next one throws
// (a negative value means that copies never throw)
int key_copies_until_throw = -1;
// a key type whose copy constructor can be made to throw
struct throwing_key // NOLINT(cppcoreguidelines-special-member-functions,hicpp-special-member-functions)
{
int id = 0;
explicit throwing_key(int i) noexcept : id(i) {}
throwing_key(const throwing_key& other) : id(other.id)
{
if (key_copies_until_throw == 0)
{
throw std::runtime_error("key copy failed");
}
if (key_copies_until_throw > 0)
{
--key_copies_until_throw;
}
}
throwing_key& operator=(const throwing_key&) = delete;
friend bool operator==(const throwing_key& lhs, const throwing_key& rhs) noexcept
{
return lhs.id == rhs.id;
}
};
#endif
// a mapped type that cannot be default-constructed
struct no_default
{
explicit no_default(int v) noexcept : value(v) {}
int value;
};
// ordered_json must keep moving its values when an object grows
using ordered_object_t = nlohmann::ordered_json::object_t;
#if !defined(JSON_TEST_PAIR_MOVE_IS_NOEXCEPT)
static_assert(!std::is_nothrow_move_constructible<ordered_object_t::value_type>::value, "std::vector would move the elements itself");
#endif
static_assert(std::is_copy_constructible<ordered_object_t::key_type>::value, "keys must be copyable");
static_assert(std::is_default_constructible<ordered_object_t::mapped_type>::value, "values must be default-constructible");
static_assert(std::is_nothrow_move_assignable<ordered_object_t::mapped_type>::value, "values must be nothrow move-assignable");
} // namespace
TEST_CASE("ordered_map")
{
SECTION("constructor")
@@ -313,3 +404,270 @@ TEST_CASE("ordered_map")
}
}
}
TEST_CASE("ordered_map growth")
{
SECTION("values are moved, not copied, when the storage grows")
{
ordered_map<std::string, counted> om;
std::size_t growths = 0;
value_copies = 0;
// inserts 100 elements with the given function and counts the growths
const auto fill = [&om, &growths](void (*insert)(ordered_map<std::string, counted>&, int))
{
for (int i = 0; i < 100; ++i)
{
const auto old_capacity = om.capacity();
insert(om, i);
if (om.capacity() > old_capacity)
{
++growths;
}
}
};
// checks that the elements are in insertion order with their values
const auto check_contents = [&om]
{
CHECK(om.size() == 100);
int i = 0;
for (const auto& element : om)
{
CHECK(element.first == std::to_string(i));
CHECK(element.second.payload == i);
++i;
}
};
SECTION("emplace")
{
fill([](ordered_map<std::string, counted>& m, int i)
{
m.emplace(std::to_string(i), counted(i));
});
CHECK(growths >= 3);
CHECK(value_copies == 0);
check_contents();
}
SECTION("operator[]")
{
fill([](ordered_map<std::string, counted>& m, int i)
{
m[std::to_string(i)] = counted(i);
});
CHECK(growths >= 3);
CHECK(value_copies == 0);
check_contents();
}
SECTION("insert(value_type&&)")
{
fill([](ordered_map<std::string, counted>& m, int i)
{
m.insert({std::to_string(i), counted(i)});
});
CHECK(growths >= 3);
CHECK(value_copies == 0);
check_contents();
}
SECTION("insert(const value_type&)")
{
fill([](ordered_map<std::string, counted>& m, int i)
{
const std::pair<const std::string, counted> value(std::to_string(i), counted(i));
m.insert(value);
});
CHECK(growths >= 3);
// only the inserted values are copied
CHECK(value_copies == 100);
check_contents();
}
SECTION("insert(first, last)")
{
std::vector<std::pair<const std::string, counted>> values;
values.reserve(100);
for (int i = 0; i < 100; ++i)
{
values.emplace_back(std::to_string(i), counted(i));
}
value_copies = 0;
om.insert(values.cbegin(), values.cend());
// only the inserted values are copied
CHECK(value_copies == 100);
check_contents();
}
}
SECTION("elements keep their order and values over many growths")
{
ordered_map<std::string, counted> om;
for (int i = 0; i < 1000; ++i)
{
om.emplace(std::to_string(i), counted(i));
}
CHECK(om.size() == 1000);
int i = 0;
for (const auto& element : om)
{
CHECK(element.first == std::to_string(i));
CHECK(element.second.payload == i);
++i;
}
}
SECTION("arguments may refer to elements of the full container")
{
SECTION("moving a value out of the container")
{
ordered_map<std::string, counted> om;
om.reserve(4);
while (om.size() < om.capacity())
{
const auto i = static_cast<int>(om.size());
om.emplace(std::to_string(i), counted(i));
}
const auto size = om.size();
om.emplace("new", std::move(om.at("0")));
CHECK(om.size() == size + 1);
CHECK(om.at("new").payload == 0);
CHECK(om.at("0").payload == -1);
}
SECTION("using a value as key")
{
ordered_map<std::string, std::string> om;
om.reserve(4);
while (om.size() < om.capacity())
{
const auto i = std::to_string(om.size());
om.emplace("k" + i, "v" + i);
}
const auto size = om.size();
om.emplace(om.at("k0"), std::string("x"));
CHECK(om.size() == size + 1);
CHECK(om.at("k0") == "v0");
CHECK(om.at("v0") == "x");
}
SECTION("ordered_json")
{
auto j = nlohmann::ordered_json::object();
auto& object = j.get_ref<nlohmann::ordered_json::object_t&>();
object.reserve(4);
while (object.size() < object.capacity())
{
const auto i = std::to_string(object.size());
j[i] = "a value that is too long for the small string optimization " + i;
}
const auto size = j.size();
j.emplace("new", std::move(j["0"]));
CHECK(j.size() == size + 1);
CHECK(j["new"] == "a value that is too long for the small string optimization 0");
CHECK(j["0"].is_null());
}
}
#if !defined(JSON_NOEXCEPTION) && !defined(JSON_TEST_PAIR_MOVE_IS_NOEXCEPT)
SECTION("the container is unchanged if growing it throws")
{
ordered_map<throwing_key, counted> om;
om.reserve(4);
while (om.size() < om.capacity())
{
const auto i = static_cast<int>(om.size());
om.emplace(throwing_key(i), counted(i));
}
const auto size = om.size();
const auto capacity = om.capacity();
// checks that the elements are unchanged
const auto check_unchanged = [&om, size, capacity]
{
CHECK(om.size() == size);
CHECK(om.capacity() == capacity);
int i = 0;
for (const auto& element : om)
{
CHECK(element.first.id == i);
CHECK(element.second.payload == i);
++i;
}
};
SECTION("emplace")
{
// growing copies the existing keys and then the new one; let each of these copies throw
for (std::size_t k = 0; k <= size; ++k)
{
counted value(100);
key_copies_until_throw = static_cast<int>(k);
CHECK_THROWS_AS(om.emplace(throwing_key(100), std::move(value)), std::runtime_error);
key_copies_until_throw = -1;
check_unchanged();
CHECK(value.payload == 100); // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved)
}
om.emplace(throwing_key(100), counted(100));
CHECK(om.size() == size + 1);
CHECK(om.capacity() > capacity);
CHECK(om.at(throwing_key(100)).payload == 100);
}
SECTION("insert(const value_type&)")
{
const std::pair<const throwing_key, counted> value(throwing_key(100), counted(100));
value_copies = 0;
key_copies_until_throw = static_cast<int>(size / 2);
CHECK_THROWS_AS(om.insert(value), std::runtime_error);
key_copies_until_throw = -1;
check_unchanged();
CHECK(value_copies == 0);
}
}
#endif
SECTION("elements that std::vector moves, or that cannot be moved back")
{
SECTION("nothrow move-constructible elements")
{
ordered_map<int, counted> om;
value_copies = 0;
for (int i = 0; i < 100; ++i)
{
om.emplace(i, counted(i));
}
CHECK(om.size() == 100);
CHECK(value_copies == 0);
}
SECTION("mapped type without default constructor")
{
ordered_map<std::string, no_default> om;
for (int i = 0; i < 100; ++i)
{
om.emplace(std::to_string(i), no_default(i));
}
CHECK(om.size() == 100);
int i = 0;
for (const auto& element : om)
{
CHECK(element.first == std::to_string(i));
CHECK(element.second.value == i);
++i;
}
}
}
}