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json/include/nlohmann/ordered_map.hpp
T
Niels Lohmann 4b2d2244c3 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>
2026-09-28 18:48:05 +02:00

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// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
#pragma once
#include <algorithm> // max, min
#include <functional> // equal_to, less
#include <initializer_list> // initializer_list
#include <iterator> // input_iterator_tag, iterator_traits
#include <memory> // allocator
#include <stdexcept> // for out_of_range
#include <tuple> // forward_as_tuple
#include <type_traits> // enable_if, integral_constant, is_convertible, is_nothrow_move_constructible
#include <utility> // forward, move, pair, piecewise_construct
#include <vector> // vector
#include <nlohmann/detail/macro_scope.hpp>
#include <nlohmann/detail/meta/type_traits.hpp>
NLOHMANN_JSON_NAMESPACE_BEGIN
/// ordered_map: a minimal map-like container that preserves insertion order
/// for use within nlohmann::basic_json<ordered_map>
template <class Key, class T, class IgnoredLess = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T>>>
struct ordered_map : std::vector<std::pair<const Key, T>, Allocator>
{
using key_type = Key;
using mapped_type = T;
using Container = std::vector<std::pair<const Key, T>, Allocator>;
using iterator = typename Container::iterator;
using const_iterator = typename Container::const_iterator;
using size_type = typename Container::size_type;
using value_type = typename Container::value_type;
#ifdef JSON_HAS_CPP_14
using key_compare = std::equal_to<>;
#else
using key_compare = std::equal_to<Key>;
#endif
// Explicit constructors instead of `using Container::Container`
// otherwise older compilers choke on it (GCC <= 5.5, xcode <= 9.4)
ordered_map() noexcept(noexcept(Container())) : Container{} {}
explicit ordered_map(const Allocator& alloc) noexcept(noexcept(Container(alloc))) : Container{alloc} {}
template <class It>
ordered_map(It first, It last, const Allocator& alloc = Allocator())
: Container{first, last, alloc} {}
ordered_map(std::initializer_list<value_type> init, const Allocator& alloc = Allocator() )
: Container{init, alloc} {}
ordered_map(const ordered_map&) = default;
ordered_map(ordered_map&&) noexcept(std::is_nothrow_move_constructible<Container>::value) = default;
~ordered_map() = default;
ordered_map& operator=(const ordered_map& other)
{
if (this != &other)
{
ordered_map tmp(other);
Container::operator=(std::move(static_cast<Container&>(tmp)));
}
return *this;
}
ordered_map& operator=(ordered_map&& other) noexcept(std::is_nothrow_move_assignable<Container>::value)
{
Container::operator=(std::move(static_cast<Container&>(other)));
return *this;
}
std::pair<iterator, bool> emplace(const key_type& key, T&& t)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return {it, false};
}
}
append(key, std::forward<T>(t));
return {std::prev(this->end()), true};
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
std::pair<iterator, bool> emplace(KeyType && key, T && t)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return {it, false};
}
}
append(std::forward<KeyType>(key), std::forward<T>(t));
return {std::prev(this->end()), true};
}
T& operator[](const key_type& key)
{
return emplace(key, T{}).first->second;
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
T & operator[](KeyType && key)
{
return emplace(std::forward<KeyType>(key), T{}).first->second;
}
const T& operator[](const key_type& key) const
{
return at(key);
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
const T & operator[](KeyType && key) const
{
return at(std::forward<KeyType>(key));
}
T& at(const key_type& key)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it->second;
}
}
JSON_THROW(std::out_of_range("key not found"));
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
T & at(KeyType && key) // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it->second;
}
}
JSON_THROW(std::out_of_range("key not found"));
}
const T& at(const key_type& key) const
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it->second;
}
}
JSON_THROW(std::out_of_range("key not found"));
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
const T & at(KeyType && key) const // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it->second;
}
}
JSON_THROW(std::out_of_range("key not found"));
}
size_type erase(const key_type& key)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
// Since we cannot move const Keys, re-construct them in place
for (auto next = it; ++next != this->end(); ++it)
{
it->~value_type(); // Destroy but keep allocation
new (&*it) value_type{std::move(*next)};
}
Container::pop_back();
return 1;
}
}
return 0;
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
size_type erase(KeyType && key) // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
// Since we cannot move const Keys, re-construct them in place
for (auto next = it; ++next != this->end(); ++it)
{
it->~value_type(); // Destroy but keep allocation
new (&*it) value_type{std::move(*next)};
}
Container::pop_back();
return 1;
}
}
return 0;
}
iterator erase(iterator pos)
{
return erase(pos, std::next(pos));
}
iterator erase(iterator first, iterator last)
{
if (first == last)
{
return first;
}
const auto elements_affected = std::distance(first, last);
const auto offset = std::distance(Container::begin(), first);
// This is the start situation. We need to delete elements_affected
// elements (3 in this example: e, f, g), and need to return an
// iterator past the last deleted element (h in this example).
// Note that offset is the distance from the start of the vector
// to first. We will need this later.
// [ a, b, c, d, e, f, g, h, i, j ]
// ^ ^
// first last
// Since we cannot move const Keys, we re-construct them in place.
// We start at first and re-construct (viz. copy) the elements from
// the back of the vector. Example for the first iteration:
// ,--------.
// v | destroy e and re-construct with h
// [ a, b, c, d, e, f, g, h, i, j ]
// ^ ^
// it it + elements_affected
for (auto it = first; std::next(it, elements_affected) != Container::end(); ++it)
{
it->~value_type(); // destroy but keep allocation
new (&*it) value_type{std::move(*std::next(it, elements_affected))}; // "move" next element to it
}
// [ a, b, c, d, h, i, j, h, i, j ]
// ^ ^
// first last
// remove the unneeded elements at the end of the vector
Container::resize(this->size() - static_cast<size_type>(elements_affected));
// [ a, b, c, d, h, i, j ]
// ^ ^
// first last
// first is now pointing past the last deleted element, but we cannot
// use this iterator, because it may have been invalidated by the
// resize call. Instead, we can return begin() + offset.
return Container::begin() + offset;
}
size_type count(const key_type& key) const
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return 1;
}
}
return 0;
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
size_type count(KeyType && key) const // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return 1;
}
}
return 0;
}
iterator find(const key_type& key)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it;
}
}
return Container::end();
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
iterator find(KeyType && key) // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it;
}
}
return Container::end();
}
const_iterator find(const key_type& key) const
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it;
}
}
return Container::end();
}
template<class KeyType, detail::enable_if_t<
detail::is_usable_as_key_type<key_compare, key_type, KeyType>::value, int> = 0>
const_iterator find(KeyType && key) const // NOLINT(cppcoreguidelines-missing-std-forward)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, key))
{
return it;
}
}
return Container::end();
}
std::pair<iterator, bool> insert( value_type&& value )
{
return emplace(value.first, std::move(value.second));
}
std::pair<iterator, bool> insert( const value_type& value )
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (m_compare(it->first, value.first))
{
return {it, false};
}
}
append(value);
return {--this->end(), true};
}
template<typename InputIt>
using require_input_iter = typename std::enable_if<std::is_convertible<typename std::iterator_traits<InputIt>::iterator_category,
std::input_iterator_tag>::value>::type;
template<typename InputIt, typename = require_input_iter<InputIt>>
void insert(InputIt first, InputIt last)
{
for (auto it = first; it != last; ++it)
{
insert(*it);
}
}
private:
/*!
@brief add an element whose key is not yet contained at the end
A std::vector copies all elements when it grows, because their const keys
make them not nothrow move constructible. For ordered_json, this is a deep
copy of every value. Where the strong exception guarantee can be kept, grow
the storage here instead, copying only the keys and moving the values.
*/
template<typename... Args>
void append(Args&& ... args)
{
// evaluated here rather than at class scope, because T is still
// incomplete when basic_json instantiates its object_t
using move_values = std::integral_constant<bool, detail::conjunction<
detail::negation<std::is_nothrow_move_constructible<value_type>>,
std::is_copy_constructible<key_type>,
detail::is_default_constructible<mapped_type>,
std::is_nothrow_move_assignable<mapped_type>>::value>;
append_impl(move_values{}, std::forward<Args>(args)...);
}
template<typename... Args>
void append_impl(std::true_type /*unused*/, Args&& ... args)
{
if (this->size() < this->capacity())
{
Container::emplace_back(std::forward<Args>(args)...);
return;
}
// 1. May throw, but only changes tmp: copy the keys, value-initialize
// the values, and add the new element. The arguments may refer to
// elements of this container, so they are used before any value is
// moved out of it.
Container tmp(this->get_allocator()); // equal allocators, so swap() is valid
tmp.reserve((std::min)(this->max_size(), (std::max)(size_type{1}, 2 * this->size())));
for (const auto& element : *this)
{
tmp.emplace_back(std::piecewise_construct, std::forward_as_tuple(element.first), std::forward_as_tuple());
}
tmp.emplace_back(std::forward<Args>(args)...);
// 2. Cannot throw: move the values over and adopt the new storage.
auto it = tmp.begin();
for (auto& element : *this)
{
it->second = std::move(element.second);
++it;
}
Container::swap(tmp);
}
template<typename... Args>
void append_impl(std::false_type /*unused*/, Args&& ... args)
{
Container::emplace_back(std::forward<Args>(args)...);
}
JSON_NO_UNIQUE_ADDRESS key_compare m_compare = key_compare();
};
NLOHMANN_JSON_NAMESPACE_END
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