Merge branch 'develop' into claude/deep-copy-key-comp-5649

Conflicts:
- tests/src/unit-comparison.cpp: kept both sides' helpers (key_case_less/
  key_case_json/innermost from the PR, case_insensitive_less/ci_json from
  develop's #5720) and both new test cases

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/meta/type_traits.hpp

@@ -189,6 +189,37 @@ struct actual_object_comparator
 template<typename BasicJsonType>
 using actual_object_comparator_t = typename actual_object_comparator<BasicJsonType>::type;
 
+template<typename T>
+using detect_key_comp = decltype(std::declval<const T&>().key_comp());
+
+// whether ObjectType can be constructed from a pair of Iterator together with
+// a copy of its own comparator, the way std::map can: it needs a nested
+// key_compare, a const key_comp() convertible to it, and a matching
+// (Iterator, Iterator, const key_compare&) constructor.
+//
+// used to preserve a stateful comparator when a copy is built from a range
+// past the iterative deep copy's nesting bound (see copy_object_level); an
+// object type that does not satisfy this, such as nlohmann::ordered_map
+// (which has key_compare for its std::map-like interface, but no key_comp()),
+// keeps default-constructing its comparator, just as it always has
+template<typename ObjectType, typename Iterator, typename = void>
+struct is_comparator_constructible_object_type_impl : std::false_type {};
+
+template<typename ObjectType, typename Iterator>
+struct is_comparator_constructible_object_type_impl <
+    ObjectType, Iterator, enable_if_t<is_detected<detect_key_compare, ObjectType>::value >>
+{
+    using key_compare = typename ObjectType::key_compare;
+
+    static constexpr bool value =
+        is_detected_convertible<key_compare, detect_key_comp, ObjectType>::value &&
+        std::is_constructible<ObjectType, Iterator, Iterator, const key_compare&>::value;
+};
+
+template<typename ObjectType, typename Iterator>
+struct is_comparator_constructible_object_type
+    : is_comparator_constructible_object_type_impl<ObjectType, Iterator> {};
+
 /////////////////
 // char_traits //
 /////////////////

include/nlohmann/json.hpp

@@ -1004,38 +1004,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     using copy_scratch_value_t = std::pair<typename object_t::key_type, basic_json>;
     using copy_scratch_t = std::vector<copy_scratch_value_t, AllocatorType<copy_scratch_value_t>>;
 
-    /// @brief tag selecting the constructor below; used only to build the
-    /// elements of a deep copy (@ref copy_array_level, @ref copy_object_level)
-    struct copy_construct_tag {};
-
-  public:
-    /*!
-    @brief construct a null value whose base class - and, with @ref
-           JSON_DIAGNOSTIC_POSITIONS, positions - are copied from @a src
-
-    Copy-constructing @ref json_base_class_t here, rather than default-
-    constructing the element and assigning its base class afterwards, means
-    that copying a @ref basic_json only ever requires a copy-constructible
-    base class, and never a move-assignable one as well.
-
-    @note this constructor has to be public: @ref copy_array_level and
-          @ref copy_object_level reach it through @ref array_t's or @ref
-          object_t's own emplace_back(), which constructs the element from
-          outside @ref basic_json and so cannot call a private constructor.
-          @ref copy_construct_tag is private, though, and nothing in the
-          public interface hands out a value of it, so outside code can still
-          never name it to call this constructor itself.
-    */
-    basic_json(copy_construct_tag /*unused*/, const basic_json& src)
-        : json_base_class_t(src)
-#if JSON_DIAGNOSTIC_POSITIONS
-        , start_position(src.start_position)
-        , end_position(src.end_position)
-#endif
+    /// @brief copy everything of @a src into @a dst but its type and value
+    static void copy_metadata(const basic_json& src, basic_json& dst)
     {
-    }
+        // a custom base class is only required to be copy-constructible and
+        // move-assignable, so the copy has to go through a temporary
+        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
 
-  private:
+#if JSON_DIAGNOSTIC_POSITIONS
+        dst.start_position = src.start_position;
+        dst.end_position = src.end_position;
+#endif
+    }
 
     /*!
     @brief copy the value of @a src into @a dst, which must not be structured
@@ -1099,11 +1079,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /*!
-    @brief finish the copy @a dst of @a src that a @ref copy_construct_tag
-           constructor started, other than the children of an object or array
+    @brief copy everything of @a src into the null value @a dst but the children
 
-    @a dst already has @a src's base class and, with @ref
-    JSON_DIAGNOSTIC_POSITIONS, positions; only its value is still missing.
     Objects and arrays are not copied here; they are appended to @a worklist to
     be created later by @ref copy_iteratively. Until that happens, @a dst remains
     a null value, so that a partially built copy can be destroyed at any point
@@ -1111,6 +1088,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
     {
+        copy_metadata(src, dst);
+
         if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
         {
             // defer: dst stays a null value until its container exists
@@ -1131,19 +1110,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         const array_t& src_array = *src.m_data.m_value.array;
 
+        // create all elements up front: growing the array afterwards could
+        // invalidate the pointers that are handed to the worklist; resize()
+        // rather than the fill constructor, because not every array type
+        // provides the latter (e.g., ones without a matching allocator-aware
+        // fill constructor)
         dst.m_data.m_value.array = create<array_t>();
         // only now that the array exists may dst stop being a null value
         dst.m_data.m_type = value_t::array;
-
-        // create every element - its base class already copy-constructed from
-        // its counterpart in src, via the copy_construct_tag constructor -
-        // before any of their addresses are handed to worklist below: growing
-        // the array while that is going on could reallocate it and invalidate
-        // addresses taken from an earlier iteration
-        for (const auto& src_element : src_array)
-        {
-            dst.m_data.m_value.array->emplace_back(copy_construct_tag{}, src_element);
-        }
+        dst.m_data.m_value.array->resize(src_array.size());
 
         auto dst_it = dst.m_data.m_value.array->begin();
         for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
@@ -1152,6 +1127,27 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
     }
 
+    /// @brief create the object type from a range, preserving @a src_object's
+    /// comparator when the object type supports it
+    /// Enabled for object types that provide a key_comp() and a matching
+    /// range-plus-comparator constructor, such as std::map. Other object
+    /// types, such as nlohmann::ordered_map, fall back to the plain range
+    /// constructor and default-construct their comparator, just as they
+    /// always have (@ref detail::is_comparator_constructible_object_type).
+    template<typename Iterator, detail::enable_if_t<
+                 detail::is_comparator_constructible_object_type<object_t, Iterator>::value, int> = 0>
+    static object_t* create_object_with_comparator(const object_t& src_object, Iterator first, Iterator last)
+    {
+        return create<object_t>(first, last, src_object.key_comp());
+    }
+
+    template<typename Iterator, detail::enable_if_t<
+                 detail::negation<detail::is_comparator_constructible_object_type<object_t, Iterator>>::value, int> = 0>
+    static object_t* create_object_with_comparator(const object_t& /*src_object*/, Iterator first, Iterator last)
+    {
+        return create<object_t>(first, last);
+    }
+
     /// @brief create the copy of the object @a src in @a dst
     /// @note structured values are appended to @a worklist instead
     static void copy_object_level(const basic_json& src, basic_json& dst,
@@ -1161,17 +1157,16 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // build the complete key skeleton and hand it to the object's range
         // constructor: adding the keys one by one would be quadratic for object
-        // types that are backed by a vector, such as nlohmann::ordered_map; each
-        // value's base class is already copy-constructed from its counterpart
-        // in src, via the copy_construct_tag constructor
+        // types that are backed by a vector, such as nlohmann::ordered_map
         scratch.clear();
         scratch.reserve(src_object.size());
         for (const auto& element : src_object)
         {
-            scratch.emplace_back(element.first, basic_json(copy_construct_tag{}, element.second));
+            scratch.emplace_back(element.first, basic_json());
         }
 
-        dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
+        dst.m_data.m_value.object = create_object_with_comparator(src_object,
+                                    std::make_move_iterator(scratch.begin()),
                                     std::make_move_iterator(scratch.end()));
         // only now that the object exists may dst stop being a null value
         dst.m_data.m_type = value_t::object;

single_include/nlohmann/json.hpp

@@ -4197,6 +4197,37 @@ struct actual_object_comparator
 template<typename BasicJsonType>
 using actual_object_comparator_t = typename actual_object_comparator<BasicJsonType>::type;
 
+template<typename T>
+using detect_key_comp = decltype(std::declval<const T&>().key_comp());
+
+// whether ObjectType can be constructed from a pair of Iterator together with
+// a copy of its own comparator, the way std::map can: it needs a nested
+// key_compare, a const key_comp() convertible to it, and a matching
+// (Iterator, Iterator, const key_compare&) constructor.
+//
+// used to preserve a stateful comparator when a copy is built from a range
+// past the iterative deep copy's nesting bound (see copy_object_level); an
+// object type that does not satisfy this, such as nlohmann::ordered_map
+// (which has key_compare for its std::map-like interface, but no key_comp()),
+// keeps default-constructing its comparator, just as it always has
+template<typename ObjectType, typename Iterator, typename = void>
+struct is_comparator_constructible_object_type_impl : std::false_type {};
+
+template<typename ObjectType, typename Iterator>
+struct is_comparator_constructible_object_type_impl <
+    ObjectType, Iterator, enable_if_t<is_detected<detect_key_compare, ObjectType>::value >>
+{
+    using key_compare = typename ObjectType::key_compare;
+
+    static constexpr bool value =
+        is_detected_convertible<key_compare, detect_key_comp, ObjectType>::value &&
+        std::is_constructible<ObjectType, Iterator, Iterator, const key_compare&>::value;
+};
+
+template<typename ObjectType, typename Iterator>
+struct is_comparator_constructible_object_type
+    : is_comparator_constructible_object_type_impl<ObjectType, Iterator> {};
+
 /////////////////
 // char_traits //
 /////////////////
@@ -27931,38 +27962,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     using copy_scratch_value_t = std::pair<typename object_t::key_type, basic_json>;
     using copy_scratch_t = std::vector<copy_scratch_value_t, AllocatorType<copy_scratch_value_t>>;
 
-    /// @brief tag selecting the constructor below; used only to build the
-    /// elements of a deep copy (@ref copy_array_level, @ref copy_object_level)
-    struct copy_construct_tag {};
-
-  public:
-    /*!
-    @brief construct a null value whose base class - and, with @ref
-           JSON_DIAGNOSTIC_POSITIONS, positions - are copied from @a src
-
-    Copy-constructing @ref json_base_class_t here, rather than default-
-    constructing the element and assigning its base class afterwards, means
-    that copying a @ref basic_json only ever requires a copy-constructible
-    base class, and never a move-assignable one as well.
-
-    @note this constructor has to be public: @ref copy_array_level and
-          @ref copy_object_level reach it through @ref array_t's or @ref
-          object_t's own emplace_back(), which constructs the element from
-          outside @ref basic_json and so cannot call a private constructor.
-          @ref copy_construct_tag is private, though, and nothing in the
-          public interface hands out a value of it, so outside code can still
-          never name it to call this constructor itself.
-    */
-    basic_json(copy_construct_tag /*unused*/, const basic_json& src)
-        : json_base_class_t(src)
-#if JSON_DIAGNOSTIC_POSITIONS
-        , start_position(src.start_position)
-        , end_position(src.end_position)
-#endif
+    /// @brief copy everything of @a src into @a dst but its type and value
+    static void copy_metadata(const basic_json& src, basic_json& dst)
     {
-    }
+        // a custom base class is only required to be copy-constructible and
+        // move-assignable, so the copy has to go through a temporary
+        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
 
-  private:
+#if JSON_DIAGNOSTIC_POSITIONS
+        dst.start_position = src.start_position;
+        dst.end_position = src.end_position;
+#endif
+    }
 
     /*!
     @brief copy the value of @a src into @a dst, which must not be structured
@@ -28026,11 +28037,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /*!
-    @brief finish the copy @a dst of @a src that a @ref copy_construct_tag
-           constructor started, other than the children of an object or array
+    @brief copy everything of @a src into the null value @a dst but the children
 
-    @a dst already has @a src's base class and, with @ref
-    JSON_DIAGNOSTIC_POSITIONS, positions; only its value is still missing.
     Objects and arrays are not copied here; they are appended to @a worklist to
     be created later by @ref copy_iteratively. Until that happens, @a dst remains
     a null value, so that a partially built copy can be destroyed at any point
@@ -28038,6 +28046,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
     {
+        copy_metadata(src, dst);
+
         if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
         {
             // defer: dst stays a null value until its container exists
@@ -28058,19 +28068,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         const array_t& src_array = *src.m_data.m_value.array;
 
+        // create all elements up front: growing the array afterwards could
+        // invalidate the pointers that are handed to the worklist; resize()
+        // rather than the fill constructor, because not every array type
+        // provides the latter (e.g., ones without a matching allocator-aware
+        // fill constructor)
         dst.m_data.m_value.array = create<array_t>();
         // only now that the array exists may dst stop being a null value
         dst.m_data.m_type = value_t::array;
-
-        // create every element - its base class already copy-constructed from
-        // its counterpart in src, via the copy_construct_tag constructor -
-        // before any of their addresses are handed to worklist below: growing
-        // the array while that is going on could reallocate it and invalidate
-        // addresses taken from an earlier iteration
-        for (const auto& src_element : src_array)
-        {
-            dst.m_data.m_value.array->emplace_back(copy_construct_tag{}, src_element);
-        }
+        dst.m_data.m_value.array->resize(src_array.size());
 
         auto dst_it = dst.m_data.m_value.array->begin();
         for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
@@ -28079,6 +28085,27 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
     }
 
+    /// @brief create the object type from a range, preserving @a src_object's
+    /// comparator when the object type supports it
+    /// Enabled for object types that provide a key_comp() and a matching
+    /// range-plus-comparator constructor, such as std::map. Other object
+    /// types, such as nlohmann::ordered_map, fall back to the plain range
+    /// constructor and default-construct their comparator, just as they
+    /// always have (@ref detail::is_comparator_constructible_object_type).
+    template<typename Iterator, detail::enable_if_t<
+                 detail::is_comparator_constructible_object_type<object_t, Iterator>::value, int> = 0>
+    static object_t* create_object_with_comparator(const object_t& src_object, Iterator first, Iterator last)
+    {
+        return create<object_t>(first, last, src_object.key_comp());
+    }
+
+    template<typename Iterator, detail::enable_if_t<
+                 detail::negation<detail::is_comparator_constructible_object_type<object_t, Iterator>>::value, int> = 0>
+    static object_t* create_object_with_comparator(const object_t& /*src_object*/, Iterator first, Iterator last)
+    {
+        return create<object_t>(first, last);
+    }
+
     /// @brief create the copy of the object @a src in @a dst
     /// @note structured values are appended to @a worklist instead
     static void copy_object_level(const basic_json& src, basic_json& dst,
@@ -28088,17 +28115,16 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // build the complete key skeleton and hand it to the object's range
         // constructor: adding the keys one by one would be quadratic for object
-        // types that are backed by a vector, such as nlohmann::ordered_map; each
-        // value's base class is already copy-constructed from its counterpart
-        // in src, via the copy_construct_tag constructor
+        // types that are backed by a vector, such as nlohmann::ordered_map
         scratch.clear();
         scratch.reserve(src_object.size());
         for (const auto& element : src_object)
         {
-            scratch.emplace_back(element.first, basic_json(copy_construct_tag{}, element.second));
+            scratch.emplace_back(element.first, basic_json());
         }
 
-        dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
+        dst.m_data.m_value.object = create_object_with_comparator(src_object,
+                                    std::make_move_iterator(scratch.begin()),
                                     std::make_move_iterator(scratch.end()));
         // only now that the object exists may dst stop being a null value
         dst.m_data.m_type = value_t::object;

tests/src/unit-comparison.cpp

@@ -827,6 +827,46 @@ Json nest(Json j, const std::size_t depth)
     return j;
 }
 
+// a std::map comparator with state: case-insensitive, unless constructed
+// case-sensitive. Used to check that copying an object copies the original's
+// comparator rather than default-constructing a new one (see #5649).
+struct key_case_less
+{
+    key_case_less() = default;
+    explicit key_case_less(const bool cs) noexcept : case_sensitive(cs) {}
+
+    bool operator()(const std::string& a, const std::string& b) const
+    {
+        if (case_sensitive)
+        {
+            return a < b;
+        }
+        return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end(),
+                                            [](unsigned char x, unsigned char y)
+        {
+            return std::tolower(x) < std::tolower(y);
+        });
+    }
+
+    bool case_sensitive = false;
+};
+
+template<class Key, class Value, class /*Compare*/, class Allocator>
+using key_case_map = std::map<Key, Value, key_case_less, Allocator>;
+using key_case_json = nlohmann::basic_json<key_case_map>;
+
+// the innermost value of a chain of single-element arrays
+template<typename Json>
+const Json& innermost(const Json& j)
+{
+    const Json* p = &j;
+    while (p->is_array())
+    {
+        p = &(*p)[0];
+    }
+    return *p;
+}
+
 // orders keys case-insensitively, so "key" and "KEY" compare equivalent
 // (neither less than the other) although they are not equal
 struct case_insensitive_less
@@ -891,6 +931,47 @@ TEST_CASE("equality of objects whose entries have no fixed order")
     }
 }
 
+TEST_CASE("copying an object preserves its comparator's state")
+{
+    // Past the iterative deep copy's nesting bound, an object copy used to be
+    // built with a default-constructed comparator instead of a copy of the
+    // original's. For an object type whose comparator carries state - here, a
+    // std::map that compares keys case-sensitively only when created that way
+    // - this reordered the copy's keys and could even drop entries that the
+    // original's comparator kept distinct (see #5649).
+    key_case_json object = key_case_json::object_t(key_case_less(true)); // case-sensitive
+    object["b"] = 1;
+    object["B"] = 2;
+    object["a"] = 3;
+    REQUIRE(object.dump() == R"({"B":2,"a":3,"b":1})");
+
+    for (const std::size_t depth : std::vector<std::size_t> {0, 127, 128, 200})
+    {
+        CAPTURE(depth);
+
+        key_case_json original = object;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            original = key_case_json::array({std::move(original)});
+        }
+
+        {
+            const key_case_json copy = original; // NOLINT(performance-unnecessary-copy-initialization)
+            CHECK(innermost(copy).size() == 3);
+            CHECK(innermost(copy).dump() == R"({"B":2,"a":3,"b":1})");
+            CHECK(copy == original);
+        }
+
+        {
+            key_case_json copy = key_case_json::array();
+            copy = original;
+            CHECK(innermost(copy).size() == 3);
+            CHECK(innermost(copy).dump() == R"({"B":2,"a":3,"b":1})");
+            CHECK(copy == original);
+        }
+    }
+}
+
 TEST_CASE("equality of an object whose comparator treats different keys as equivalent")
 {
     // https://github.com/nlohmann/json/issues/5655: past the nesting bound,

tests/src/unit-custom-base-class.cpp

@@ -405,73 +405,3 @@ TEST_CASE("JSON Visit Node")
         );
     CHECK(expected.empty());
 }
-
-// A custom base class with a const member: copy-constructible (initializing a
-// const member works fine), but not copy-/move-assignable (assigning one does
-// not). Used to check that copy construction never requires more than that.
-struct const_member_base
-{
-    const int id = 7; // NOLINT(misc-non-private-member-variables-in-classes)
-};
-
-using json_with_const_base = nlohmann::basic_json <
-                             std::map,
-                             std::vector,
-                             std::string,
-                             bool,
-                             std::int64_t,
-                             std::uint64_t,
-                             double,
-                             std::allocator,
-                             nlohmann::adl_serializer,
-                             std::vector<std::uint8_t>,
-                             const_member_base
-                             >;
-
-// build an array nested @a depth levels deep, with the innermost value 1;
-// every level is constructed (never assigned), since const_member_base does
-// not support assignment
-static json_with_const_base make_nested_array(std::size_t depth)
-{
-    if (depth == 0)
-    {
-        return json_with_const_base(1);
-    }
-    return json_with_const_base::array({make_nested_array(depth - 1)});
-}
-
-TEST_CASE("Regression test for issue #5674 - copy construction must not require an assignable base class")
-{
-    SECTION("depth 0")
-    {
-        // as in the original bug report: copy construction only, no assignment
-        const json_with_const_base j = {1, 2};
-        const json_with_const_base copy = j; // NOLINT(performance-unnecessary-copy-initialization)
-
-        CHECK(copy.size() == 2);
-        CHECK(copy.id == 7);
-    }
-
-    SECTION("nested deeper than the copy constructor's descent bound")
-    {
-        // beyond nesting_depth_limit() (128) levels, the copy constructor
-        // copies without the call stack (copy_iteratively / copy_array_level),
-        // which used to assign the base class of every element it created
-        const std::size_t depth = 300;
-
-        const json_with_const_base j = make_nested_array(depth);
-        const json_with_const_base copy = j; // NOLINT(performance-unnecessary-copy-initialization)
-
-        const json_with_const_base* c = &copy;
-        for (std::size_t level = 0; level <= depth; ++level)
-        {
-            CAPTURE(level)
-            REQUIRE(c->id == 7);
-            if (level < depth)
-            {
-                c = &c->at(0);
-            }
-        }
-        CHECK(*c == 1);
-    }
-}