Merge remote-tracking branch 'origin/develop' into HEAD

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

docs/mkdocs/docs/api/macros/json_brace_init_copy_semantics.md

@@ -50,9 +50,11 @@ The default value is `0` (disabled — existing behavior is preserved).
     ```
 
     Code that relies on these producing arrays must use `json::array()` instead (see below). Lists with more than one
-    element, and a single `[string, value]` pair such as `{{"key", "value"}}`, which still creates an object, are not
-    affected. The library's own conversions are not affected either: for example, `std::tuple<int>{5}` still becomes
-    `[5]`.
+    element, and a single `[string, value]` pair *written as a braced list*, such as `{{"key", "value"}}`, which still
+    creates an object, are not affected. This exception is based on how the pair is written, not on the shape of its
+    value: an existing JSON value that happens to be a two-element array with a string as its first element, such as
+    `json arr = {"key", 42};`, is still copied by `json j{arr};` rather than turned into an object. The library's own
+    conversions are not affected either: for example, `std::tuple<int>{5}` still becomes `[5]`.
 
 !!! note "ABI compatibility"
 

include/nlohmann/detail/json_ref.hpp

@@ -34,6 +34,7 @@ class json_ref
 
     json_ref(std::initializer_list<json_ref> init)
         : owned_value(init)
+        , braced_list(true)
     {}
 
     template <
@@ -69,9 +70,17 @@ class json_ref
         return &** this;
     }
 
+    /// whether the value was written as a braced list, such as {"key", 1},
+    /// rather than given as a value
+    bool is_braced_list() const noexcept
+    {
+        return braced_list;
+    }
+
   private:
     mutable value_type owned_value = nullptr;
     value_type const* value_ref = nullptr;
+    bool braced_list = false;
 };
 
 }  // namespace detail

include/nlohmann/json.hpp

@@ -2026,6 +2026,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                bool type_deduction = true,
                value_t manual_type = value_t::array)
     {
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+        // a single element that is a value rather than a braced list is
+        // copied or moved as is, whatever its content looks like
+        if (type_deduction && init.size() == 1 && !init.begin()->is_braced_list())
+        {
+            *this = init.begin()->moved_or_copied();
+            set_parents();
+            assert_invariant();
+            return;
+        }
+#endif
+
         // check if each element is an array with two elements whose first
         // element is a string
         bool is_an_object = std::all_of(init.begin(), init.end(),

single_include/nlohmann/json.hpp

@@ -20865,6 +20865,7 @@ class json_ref
 
     json_ref(std::initializer_list<json_ref> init)
         : owned_value(init)
+        , braced_list(true)
     {}
 
     template <
@@ -20900,9 +20901,17 @@ class json_ref
         return &** this;
     }
 
+    /// whether the value was written as a braced list, such as {"key", 1},
+    /// rather than given as a value
+    bool is_braced_list() const noexcept
+    {
+        return braced_list;
+    }
+
   private:
     mutable value_type owned_value = nullptr;
     value_type const* value_ref = nullptr;
+    bool braced_list = false;
 };
 
 }  // namespace detail
@@ -28975,6 +28984,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                bool type_deduction = true,
                value_t manual_type = value_t::array)
     {
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+        // a single element that is a value rather than a braced list is
+        // copied or moved as is, whatever its content looks like
+        if (type_deduction && init.size() == 1 && !init.begin()->is_braced_list())
+        {
+            *this = init.begin()->moved_or_copied();
+            set_parents();
+            assert_invariant();
+            return;
+        }
+#endif
+
         // check if each element is an array with two elements whose first
         // element is a string
         bool is_an_object = std::all_of(init.begin(), init.end(),

tests/src/unit-brace-init-copy-semantics.cpp

@@ -72,6 +72,28 @@ TEST_CASE("JSON_BRACE_INIT_COPY_SEMANTICS")
         CHECK(j7 == json::array({1, 2}));
     }
 
+    SECTION("single-element brace initialization copies a pair-shaped array value (#5662)")
+    {
+        // a JSON value that happens to be a 2-element array whose first
+        // element is a string must still be copied, not turned into an
+        // object; only a braced list written in the source, such as the
+        // inner {"key", "value"} of {{"key", "value"}}, describes an object
+        json const pair_shaped = json::array({"key", 42});
+
+        json const j1{pair_shaped};
+        CHECK(j1.is_array());
+        CHECK(j1 == pair_shaped);
+
+        json const j2 = {pair_shaped};
+        CHECK(j2.is_array());
+        CHECK(j2 == pair_shaped);
+
+        // the same holds for an rvalue of the same shape
+        json const j3{json::array({"key", 42})};
+        CHECK(j3.is_array());
+        CHECK(j3 == pair_shaped);
+    }
+
     SECTION("what the macro does not change")
     {
         // lists with more than one element are unaffected

tools/generate_natvis/nlohmann_json.natvis.j2

@@ -36,31 +36,5 @@
         </Expand>
     </Type>
 
-    <!-- Fallback for when the basic_json entry above does not match: json_default_base is the (empty) default
-         base class of basic_json, and base class visualizers are inherited by derived types and evaluated
-         against the derived object, so m_data is accessible here. The class lives in {{ ns }} after
-         3.12.0 (#5238) and in {{ ns }}::detail up to 3.12.0, so both names are listed. -->
-{% for default_base in ['json_default_base', 'detail::json_default_base'] %}
-    <Type Name="{{ ns }}::{{ default_base }}">
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::null">null</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::object">{*(m_data.m_value.object)}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::array">{*(m_data.m_value.array)}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::string">{*(m_data.m_value.string)}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::boolean">{m_data.m_value.boolean}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_integer">{m_data.m_value.number_integer}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_unsigned">{m_data.m_value.number_unsigned}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_float">{m_data.m_value.number_float}</DisplayString>
-        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::discarded">discarded</DisplayString>
-        <Expand>
-            <ExpandedItem Condition="m_data.m_type == {{ ns }}::detail::value_t::object">
-                *(m_data.m_value.object),view(simple)
-            </ExpandedItem>
-            <ExpandedItem Condition="m_data.m_type == {{ ns }}::detail::value_t::array">
-                *(m_data.m_value.array),view(simple)
-            </ExpandedItem>
-        </Expand>
-    </Type>
-
-{% endfor %}
 {% endfor %}
 </AutoVisualizer>