Merge branch 'develop' into claude/fix-issue-2649-738aba

Conflicts:
- tests/src/unit-alt-string.cpp: kept both new sections (this PR's
  "conversion between basic_json specializations (#2649)" and develop's
  "strict enum" regression test for #5667)

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

docs/mkdocs/docs/api/basic_json/basic_json.md

@@ -272,6 +272,15 @@ basic_json(basic_json&& other) noexcept;
         When used without parentheses around an empty initializer list, `basic_json()` is called instead of this
         function, yielding the JSON `#!json null` value.
 
+- Overload 4:
+
+    !!! info "Implicit conversion"
+
+        The conversion is implicit unless [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
+        is defined to `0` and `BasicJsonType::string_t` differs from `string_t`. In that case, the constructor is
+        `explicit`, so a JSON value with a different string type is no longer silently converted, for example when it is
+        passed to a function taking `#!cpp const json&`. Write `#!cpp json(other)` or `#!cpp other.get<json>()` instead.
+
 - Overload 7:
 
     !!! info "Preconditions"
@@ -420,7 +429,8 @@ basic_json(basic_json&& other) noexcept;
 1. Since version 1.0.0.
 2. Since version 1.0.0.
 3. Since version 2.1.0.
-4. Since version 3.2.0.
+4. Since version 3.2.0. Explicit for different string types if `JSON_USE_IMPLICIT_CONVERSIONS` is `0` since
+   version 3.13.0.
 5. Since version 1.0.0.
 6. Since version 1.0.0.
 7. Since version 1.0.0. Fixed in version 3.13.0 to also check the iterator range for binary values; before, a range

docs/mkdocs/docs/api/macros/json_use_implicit_conversions.md

@@ -5,7 +5,9 @@
 ```
 
 When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on. The
-value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md).
+value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md) and the
+[converting constructor](../basic_json/basic_json.md) from a `basic_json` specialization with a different string
+type (overload 4).
 
 ## Default definition
 
@@ -57,6 +59,25 @@ By default, implicit conversions are enabled.
     auto s = j.get<std::string>();
     ```
 
+??? example "Conversion between `basic_json` specializations"
+
+    A `basic_json` specialization with a different string type is also no longer converted implicitly when
+    `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`:
+
+    ```cpp
+    using wjson = nlohmann::basic_json<std::map, std::vector, std::wstring>;
+
+    void load(const nlohmann::json& j);
+
+    wjson wj = /* ... */;
+    load(wj);                            // error: no implicit conversion
+    load(nlohmann::json(wj));            // OK: explicit conversion
+    load(wj.get<nlohmann::json>());      // OK: explicit conversion
+    ```
+
+    Specializations that share the same string type, such as `json` and `ordered_json`, remain implicitly
+    convertible.
+
 ## See also
 
 - [**operator ValueType**](../basic_json/operator_ValueType.md) - get a value (implicit)
@@ -66,3 +87,4 @@ By default, implicit conversions are enabled.
 ## Version history
 
 - Added in version 3.9.0.
+- Also affects the conversion between `basic_json` specializations with different string types since version 3.13.0.

include/nlohmann/detail/conversions/to_json.hpp

@@ -291,7 +291,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {

include/nlohmann/detail/meta/type_traits.hpp

@@ -189,37 +189,6 @@ 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

@@ -1127,27 +1127,6 @@ 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,
@@ -1165,8 +1144,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             scratch.emplace_back(element.first, basic_json());
         }
 
-        dst.m_data.m_value.object = create_object_with_comparator(src_object,
-                                    std::make_move_iterator(scratch.begin()),
+        dst.m_data.m_value.object = create<object_t>(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;
@@ -1628,12 +1606,42 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+          (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+          || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
@@ -1689,6 +1697,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -2455,7 +2464,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -2594,7 +2603,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

single_include/nlohmann/json.hpp

@@ -4197,37 +4197,6 @@ 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 //
 /////////////////
@@ -6853,7 +6822,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {
@@ -28085,27 +28056,6 @@ 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,
@@ -28123,8 +28073,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             scratch.emplace_back(element.first, basic_json());
         }
 
-        dst.m_data.m_value.object = create_object_with_comparator(src_object,
-                                    std::make_move_iterator(scratch.begin()),
+        dst.m_data.m_value.object = create<object_t>(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;
@@ -28586,12 +28535,42 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+        (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+        || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
@@ -28647,6 +28626,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -29413,7 +29393,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -29552,7 +29532,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

tests/src/unit-alt-string.cpp

@@ -13,6 +13,7 @@
 
 #include <cstdint>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -419,6 +420,41 @@ TEST_CASE("alternative string type")
         CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
     }
 
+    SECTION("conversion between basic_json specializations (#2649)")
+    {
+        // explicit conversions are always possible
+        CHECK(std::is_constructible<nlohmann::json, alt_json>::value);
+        CHECK(std::is_constructible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_constructible<nlohmann::json, nlohmann::ordered_json>::value);
+        CHECK(std::is_constructible<nlohmann::ordered_json, nlohmann::json>::value);
+
+        // specializations with the same string type are implicitly convertible
+        CHECK(std::is_convertible<nlohmann::ordered_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, nlohmann::ordered_json>::value);
+
+        // specializations with different string types are only implicitly convertible
+        // if implicit conversions are enabled
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        CHECK(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, alt_json>::value);
+#else
+        CHECK_FALSE(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK_FALSE(std::is_convertible<nlohmann::json, alt_json>::value);
+#endif
+
+        // get<BasicJsonType>() works in either case
+        const nlohmann::json j = {{"foo", 1}, {"bar", true}};
+        CHECK(j.get<nlohmann::ordered_json>() == nlohmann::ordered_json(j));
+        // (only a number is converted here, as objects and strings are affected by #3425)
+        CHECK(nlohmann::json(42).get<alt_json>() == 42);
+        CHECK(alt_json(nlohmann::json(42)) == 42);
+
+        // get_to() also works in either case
+        alt_json a;
+        nlohmann::json(42).get_to(a);
+        CHECK(a == 42);
+    }
+
     SECTION("strict enum")
     {
         // regression test for #5667: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT's from_json

tests/src/unit-comparison.cpp

@@ -827,46 +827,6 @@ 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
@@ -931,47 +891,6 @@ 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,