Merge pull request #2054 from nlohmann/feature/issue1983

Fix bug in diff function

include/nlohmann/json.hpp

@@ -8347,7 +8347,7 @@ class basic_json
                     result.push_back(
                     {
                         {"op", "add"},
-                        {"path", path + "/" + std::to_string(i)},
+                        {"path", path + "/-"},
                         {"value", target[i]}
                     });
                     ++i;

single_include/nlohmann/json.hpp

@@ -23833,7 +23833,7 @@ class basic_json
                     result.push_back(
                     {
                         {"op", "add"},
-                        {"path", path + "/" + std::to_string(i)},
+                        {"path", path + "/-"},
                         {"value", target[i]}
                     });
                     ++i;

test/src/unit-regression.cpp

@@ -1896,6 +1896,14 @@ TEST_CASE("regression tests")
         static_assert(!std::is_constructible<json, std::tuple<NotSerializableData, std::string>>::value, "");
         static_assert(std::is_constructible<json, std::tuple<int, std::string>>::value, "");
     }
+
+    SECTION("issue #1983 - JSON patch diff for op=add formation is not as per standard (RFC 6902)")
+    {
+        const auto source = R"({ "foo": [ "1", "2" ] })"_json;
+        const auto target = R"({"foo": [ "1", "2", "3" ]})"_json;
+        const auto result = json::diff(source, target);
+        CHECK(result.dump() == R"([{"op":"add","path":"/foo/-","value":"3"}])");
+    }
 }
 
 #if not defined(JSON_NOEXCEPTION)