support std::string_view-convertible keys in operator[]

Author: khloodelhossiny

include/nlohmann/detail/meta/type_traits.hpp

@@ -624,7 +624,11 @@ using is_usable_as_basic_json_key_type = typename std::conditional <
     is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
     typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
     RequireTransparentComparator, ExcludeObjectKeyType>::value
-    && !is_json_iterator_of<BasicJsonType, KeyType>::value,
+    && !is_json_iterator_of<BasicJsonType, KeyType>::value
+#ifdef JSON_HAS_CPP_17
+    || std::is_convertible<KeyType, std::string_view>::value
+#endif
+    ,
     std::true_type,
     std::false_type >::type;
 

tests/src/unit-element_access2.cpp

@@ -1790,3 +1790,49 @@ TEST_CASE_TEMPLATE("element access 2 (additional value() tests)", Json, nlohmann
 #endif
     }
 }
+
+
+#ifdef JSON_HAS_CPP_17
+TEST_CASE("operator[] with user-defined std::string_view-convertible types")
+{
+    using json = nlohmann::json;
+
+    class TestClass
+    {
+        std::string key_data_ = "foo";
+    public:
+        operator std::string_view() const { return key_data_; }
+    };
+
+    struct TestStruct
+    {
+        operator std::string_view() const { return "bar"; }
+    };
+
+    json j = { { "foo", "from_class" }, { "bar", "from_struct" } };
+    TestClass foo_obj;
+    TestStruct bar_obj;
+
+    SECTION("read access")
+    {
+        CHECK(j[foo_obj] == "from_class");
+        CHECK(j[TestClass{}] == "from_class");
+        CHECK(j[bar_obj] == "from_struct");
+        CHECK(j[TestStruct{}] == "from_struct");
+    }
+
+    SECTION("write access")
+    {
+        j[TestClass{}] = "updated_class";
+        j[TestStruct{}] = "updated_struct";
+        CHECK(j["foo"] == "updated_class");
+        CHECK(j["bar"] == "updated_struct");
+    
+        SECTION("direct std::string_view access")
+        {
+            CHECK(j[std::string_view{"foo"}] == "updated_class");
+            CHECK(j[std::string_view{"bar"}] == "updated_struct");
+        }
+    }
+}
+#endif