json/docs/mkdocs/docs/features/json_patch.md

JSON Patch and Diff

Patches

JSON Patch (RFC 6902) 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 paths. With the patch 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.

!!! tip "Applying a patch without copying"

[`patch`](../api/basic_json/patch.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`](../api/basic_json/patch_inplace.md) applies the same operations in place instead.

!!! note "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`](../api/basic_json/object_t.md#behavior)), so
`#!json {"op": "add", "op": "remove", "path": "/a"}` is a `remove` operation.

??? example "Example: apply a JSON Patch"

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

```cpp
--8<-- "examples/patch.cpp"
```

Output:

```json
--8<-- "examples/patch.output"
```

Diff

The library can also calculate a JSON patch (i.e., a diff) given two JSON values with the diff 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.

!!! success "Invariant"

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

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

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

- `NaN` and [discarded](../api/basic_json/is_discarded.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 `#!json 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 `#!json []`, so the patched value keeps the number type of *source*.
- A [binary value](binary_values.md) in a patch is serialized as an object
  (`#!json {"bytes":[...],"subtype":null}`), so a patch that was dumped and parsed again no longer holds a binary
  value.

??? example "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.

```cpp
--8<-- "examples/diff.cpp"
```

Output:

```json
--8<-- "examples/diff.output"
```

See also

• JSON Pointer - the addressing scheme used for patch paths
• JSON Merge Patch - a simpler, less expressive alternative patch format
• patch - apply a JSON Patch, returning the result as a copy
• patch_inplace - apply a JSON Patch without copying
• diff - compute a JSON Patch from two values
• json_pointer - type to represent JSON Pointers
• merge_diff - compute a JSON Merge Patch from two values
• Modifying values - simple in-place modifications