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2026-07-09 12:05:10 +00:00 · 2026-07-08 18:19:46 +00:00
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+# JSON Patch and Diff
+
+## Patches
+
+JSON Patch ([RFC 6902](https://tools.ietf.org/html/rfc6902)) defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With the `patch` function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.
+
+Example
+
+The following code shows how a JSON patch is applied to a value.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // the original document
+    json doc = R"(
+        {
+          "baz": "qux",
+          "foo": "bar"
+        }
+    )"_json;
+
+    // the patch
+    json patch = R"(
+        [
+          { "op": "replace", "path": "/baz", "value": "boo" },
+          { "op": "add", "path": "/hello", "value": ["world"] },
+          { "op": "remove", "path": "/foo"}
+        ]
+    )"_json;
+
+    // apply the patch
+    json patched_doc = doc.patch(patch);
+
+    // output original and patched document
+    std::cout << std::setw(4) << doc << "\n\n"
+              << std::setw(4) << patched_doc << std::endl;
+}
+```
+
+Output:
+
+```
+{
+    "baz": "qux",
+    "foo": "bar"
+}
+
+{
+    "baz": "boo",
+    "hello": [
+        "world"
+    ]
+}
+```
+
+## Diff
+
+The library can also calculate a JSON patch (i.e., a **diff**) given two JSON values.
+
+Invariant
+
+For two JSON values *source* and *target*, the following code yields always true:
+
+```
+source.patch(diff(source, target)) == target;
+```
+
+Example
+
+The following code shows how a JSON patch is created as a diff for two JSON values.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // the source document
+    json source = R"(
+        {
+            "baz": "qux",
+            "foo": "bar"
+        }
+    )"_json;
+
+    // the target document
+    json target = R"(
+        {
+            "baz": "boo",
+            "hello": [
+                "world"
+            ]
+        }
+    )"_json;
+
+    // create the patch
+    json patch = json::diff(source, target);
+
+    // roundtrip
+    json patched_source = source.patch(patch);
+
+    // output patch and roundtrip result
+    std::cout << std::setw(4) << patch << "\n\n"
+              << std::setw(4) << patched_source << std::endl;
+}
+```
+
+Output:
+
+```
+[
+    {
+        "op": "replace",
+        "path": "/baz",
+        "value": "boo"
+    },
+    {
+        "op": "remove",
+        "path": "/foo"
+    },
+    {
+        "op": "add",
+        "path": "/hello",
+        "value": [
+            "world"
+        ]
+    }
+]
+
+{
+    "baz": "boo",
+    "hello": [
+        "world"
+    ]
+}
+```