# 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. Operations address locations in the document using [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) paths. With the [`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) function, a JSON Patch is applied to the current JSON value by executing all operations from the patch, yielding the patched document as a new value.

The following steps through one operation of each kind. The first three operations are those of the example below; the other three continue on its result.

| Operation                                                 | Before                                             | After                                              |
| --------------------------------------------------------- | -------------------------------------------------- | -------------------------------------------------- |
| `{"op": "replace", "path": "/baz", "value": "boo"}`       | `{"baz": "qux", "foo": "bar"}`                     | `{"baz": "boo", "foo": "bar"}`                     |
| `{"op": "add", "path": "/hello", "value": ["world"]}`     | `{"baz": "boo", "foo": "bar"}`                     | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` |
| `{"op": "remove", "path": "/foo"}`                        | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` | `{"baz": "boo", "hello": ["world"]}`               |
| `{"op": "move", "from": "/baz", "path": "/hello/1"}`      | `{"baz": "boo", "hello": ["world"]}`               | `{"hello": ["world", "boo"]}`                      |
| `{"op": "copy", "from": "/hello/0", "path": "/greeting"}` | `{"hello": ["world", "boo"]}`                      | `{"greeting": "world", "hello": ["world", "boo"]}` |
| `{"op": "test", "path": "/greeting", "value": "world"}`   | `{"greeting": "world", "hello": ["world", "boo"]}` | unchanged (a failed test throws)                   |

Stepper through the six operations: *Next* applies the next operation, *Back* undoes it, and *Reset* returns to the original document. The current operation is highlighted in the patch list, and the lines it changes are marked in the document.

Applying a patch without copying

[`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) applies the same operations in place instead.

Duplicate member names

Duplicate member names inside an operation object are resolved by the parser before the patch is applied (the last one is kept, see [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/#behavior)), so `{"op": "add", "op": "remove", "path": "/a"}` is a `remove` operation.

Example: apply a JSON Patch

The following code shows how a JSON patch is applied to a value.

```
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using 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 << '\n';
}
```

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 with the [`diff`](https://json.nlohmann.me/api/basic_json/diff/index.md) function.

```
flowchart LR
    S["source"] -->|"diff(source, target)"| P["patch"]
    S -->|"source.patch(patch)"| T["target"]
    P -.->|"applied to source, yields"| T
```

How `diff` and `patch` relate: `diff(source, target)` yields a patch, and applying that patch to the source yields the target.

Invariant

For two JSON values *source* and *target*, the following code yields true:

```
source.patch(diff(source, target)) == target;
```

The equality holds up to [`operator==`](https://json.nlohmann.me/api/basic_json/operator_eq/index.md), with these exceptions:

- `NaN` and [discarded](https://json.nlohmann.me/api/basic_json/is_discarded/index.md) values are not equal to themselves, so the invariant cannot hold for them. `diff` emits a `replace` operation for such a value, and a `NaN` in a patch is serialized as `null`.
- Numbers that compare equal produce no operation even if their types differ: `diff(1, 1.0)`, `diff(1u, 1)`, and `diff(0.0, -0.0)` return `[]`, so the patched value keeps the number type of *source*.
- A [binary value](https://json.nlohmann.me/features/binary_values/index.md) in a patch is serialized as an object (`{"bytes":[...],"subtype":null}`), so a patch that was dumped and parsed again no longer holds a binary value.

Example: create a JSON Patch from the difference of two values

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 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 << '\n';
}
```

Output:

```
[
    {
        "op": "replace",
        "path": "/baz",
        "value": "boo"
    },
    {
        "op": "remove",
        "path": "/foo"
    },
    {
        "op": "add",
        "path": "/hello",
        "value": [
            "world"
        ]
    }
]

{
    "baz": "boo",
    "hello": [
        "world"
    ]
}
```

## See also

- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - the addressing scheme used for patch paths
- [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - a simpler, less expressive alternative patch format
- [`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) - apply a JSON Patch, returning the result as a copy
- [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) - apply a JSON Patch without copying
- [`diff`](https://json.nlohmann.me/api/basic_json/diff/index.md) - compute a JSON Patch from two values
- [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) - type to represent JSON Pointers
- [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md) - compute a JSON Merge Patch from two values
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - simple in-place modifications