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>

docs/mkdocs/docs/api/ordered_map.md

@@ -28,6 +28,11 @@ A minimal map-like container that preserves insertion order for use within [`nlo
 The type uses a `std::vector` to store object elements. Therefore, adding elements can yield a reallocation in which
 case all iterators (including the `end()` iterator) and all references to the elements are invalidated.
 
+When the storage grows, the keys are copied and the mapped values are moved to the new storage. A plain `std::vector`
+would copy the whole elements instead, because their `#!cpp const` keys make them not nothrow move constructible; for
+[`ordered_json`](ordered_json.md), this would be a deep copy of every nested value. The values are only copied if
+`T` is not default constructible or not nothrow move assignable.
+
 ## Member types
 
 - **key_type** - key type (`Key`)
@@ -56,6 +61,11 @@ std::equal_to<>     // since C++14
 - **find**
 - **insert**
 
+## Exception safety
+
+**emplace**, **operator\[\]**, and **insert(value)** have the strong exception guarantee: if an exception is thrown (for
+instance, because copying a key or allocating memory fails), the contents of the container are unchanged.
+
 ## Complexity
 
 Because the elements are stored in a `std::vector` in insertion order, there is no index to look a key up by. Every
@@ -122,3 +132,4 @@ This differs from `#!cpp std::map`, where the same operations are O(log n).
 
 - Added in version 3.9.0 to implement [`nlohmann::ordered_json`](ordered_json.md).
 - Added **key_compare** member in version 3.11.0.
+- Changed in version 3.13.0: growing the storage moves the mapped values instead of copying them.

include/nlohmann/ordered_map.hpp

@@ -8,13 +8,15 @@
 
 #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 <type_traits> // enable_if, is_convertible
+#include <tuple> // forward_as_tuple
-#include <utility> // pair
+#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>
@@ -79,7 +81,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(key, std::forward<T>(t));
+        append(key, std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -94,7 +96,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(std::forward<KeyType>(key), std::forward<T>(t));
+        append(std::forward<KeyType>(key), std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -368,7 +370,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::push_back(value);
+        append(value);
         return {--this->end(), true};
     }
 
@@ -386,6 +388,64 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
     }
 
 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();
 };
 

single_include/nlohmann/json.hpp

@@ -25768,13 +25768,15 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
+#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 <type_traits> // enable_if, is_convertible
+#include <tuple> // forward_as_tuple
-#include <utility> // pair
+#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>
@@ -25841,7 +25843,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(key, std::forward<T>(t));
+        append(key, std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -25856,7 +25858,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(std::forward<KeyType>(key), std::forward<T>(t));
+        append(std::forward<KeyType>(key), std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -26130,7 +26132,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::push_back(value);
+        append(value);
         return {--this->end(), true};
     }
 
@@ -26148,6 +26150,64 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
     }
 
 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();
 };
 

tests/benchmarks/src/benchmarks.cpp

@@ -119,6 +119,39 @@ BENCHMARK_CAPTURE(ParseIndented, canada / 4,       TEST_DATA_DIRECTORY "/nativej
 BENCHMARK_CAPTURE(ParseIndented, citm_catalog / 4, TEST_DATA_DIRECTORY "/nativejson-benchmark/citm_catalog.json", 4);
 BENCHMARK_CAPTURE(ParseIndented, twitter / 4,      TEST_DATA_DIRECTORY "/nativejson-benchmark/twitter.json",      4);
 
+//////////////////////////////////////////////////////////////////////////////
+// parse JSON from string into an ordered_json
+//
+// Same as ParseString above, but with nlohmann::ordered_json, whose objects
+// keep their members in a vector: the pair of rows shows what preserving the
+// insertion order costs.
+//////////////////////////////////////////////////////////////////////////////
+
+static void ParseStringOrdered(benchmark::State& state, const char* filename)
+{
+    std::ifstream f(filename);
+    std::string str((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());
+
+    while (state.KeepRunning())
+    {
+        state.PauseTiming();
+        auto* j = new nlohmann::ordered_json();
+        state.ResumeTiming();
+
+        *j = nlohmann::ordered_json::parse(str);
+
+        state.PauseTiming();
+        delete j;
+        state.ResumeTiming();
+    }
+
+    state.SetBytesProcessed(state.iterations() * str.size());
+}
+BENCHMARK_CAPTURE(ParseStringOrdered, jeopardy,     TEST_DATA_DIRECTORY "/jeopardy/jeopardy.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, canada,       TEST_DATA_DIRECTORY "/nativejson-benchmark/canada.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, citm_catalog, TEST_DATA_DIRECTORY "/nativejson-benchmark/citm_catalog.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, twitter,      TEST_DATA_DIRECTORY "/nativejson-benchmark/twitter.json");
+
 //////////////////////////////////////////////////////////////////////////////
 // serialize JSON
 //////////////////////////////////////////////////////////////////////////////

tests/src/unit-disabled_exceptions.cpp

@@ -46,6 +46,24 @@ TEST_CASE("Tests with disabled exceptions")
         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'");
         delete sax_no_exception::error_string; // NOLINT(cppcoreguidelines-owning-memory)
     }
+
+    SECTION("growing an ordered_json object")
+    {
+        auto j = nlohmann::ordered_json::object();
+        for (int i = 0; i < 100; ++i)
+        {
+            j[std::to_string(i)] = {{"nested", i}};
+        }
+
+        CHECK(j.size() == 100);
+        int i = 0;
+        for (const auto& element : j.items())
+        {
+            CHECK(element.key() == std::to_string(i));
+            CHECK(element.value()["nested"] == i);
+            ++i;
+        }
+    }
 }
 
 DOCTEST_GCC_SUPPRESS_WARNING_POP

tests/src/unit-ordered_map.cpp

@@ -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;
+            }
+        }
+    }
+}