# <small>nlohmann::basic_json::</small>patch

```cpp
basic_json patch(const basic_json& json_patch) const;
```

[JSON Patch](http://jsonpatch.com) defines a JSON document structure for expressing a sequence of operations to apply to
a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from
the patch.

## Parameters

`json_patch` (in)
:   JSON patch document

## Return value

patched document

## Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

## Exceptions

- Throws [`parse_error.104`](../../home/exceptions.md#jsonexceptionparse_error104) if the JSON patch does not consist of
  an array of objects.
- Throws [`parse_error.105`](../../home/exceptions.md#jsonexceptionparse_error105) if the JSON patch is malformed (e.g.,
  mandatory attributes are missing); example: `"operation 'add' must have member 'path'"`.
- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index is out of range.
- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a "path" or
  "from" member begins with '0'; example: `"array index '01' must not begin with '0'"`.
- Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a "path" or "from" member is not
  empty and does not begin with a slash (`/`); example: `"JSON pointer must be empty or begin with '/' - was: 'a'"`.
- Throws [`parse_error.108`](../../home/exceptions.md#jsonexceptionparse_error108) if a tilde (`~`) in a "path" or
  "from" member is not followed by `0` or `1`; example: `"escape character '~' must be followed with '0' or '1'"`.
- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in a "path" or
  "from" member is not a number; example: `"array index 'foo' is not a number"`.
- Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if the array index `-` is used
  where an existing element is required (the "path" of "replace", the "from" of "move" and "copy"); example:
  `"array index '-' (3) is out of range"`.
- Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if a JSON pointer inside the patch
  could not be resolved successfully in the current JSON value; example: `"key baz not found"`.
- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a reference token of a JSON
  pointer inside the patch cannot be resolved, e.g., `-` in a "remove" operation or `1a` for an array; example:
  `"unresolved reference token '-'"`.
- Throws [`out_of_range.405`](../../home/exceptions.md#jsonexceptionout_of_range405) if JSON pointer has no parent
  ("add", "remove", "move")
- Throws [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) if an "add" operation's target
  location has a parent that is neither an object nor an array.
- Throws [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) if a "remove" operation's target
  location has a parent that is neither an object nor an array.
- Throws [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) if a "move" operation's "from"
  location is a proper prefix of its "path" location.
- Throws [`other_error.501`](../../home/exceptions.md#jsonexceptionother_error501) if "test" operation was
  unsuccessful.

## Complexity

Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of
the JSON value, the complexity can usually be neglected.

## Notes

The application of a patch is atomic: Either all operations succeed and the patched document is returned or an exception
is thrown. In any case, the original value is not changed: the patch is applied to a copy of the value.

## Examples

??? 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"
    ```

??? example "Example: out_of_range.414 exception"

    The following code shows how a "move" operation whose "from" location is a proper prefix of its "path" location is
    rejected, and how the original document is left unchanged because the patch is applied to a copy.

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

    Output:

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

## See also

- [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)
- [RFC 6901 (JSON Pointer)](https://tools.ietf.org/html/rfc6901)
- [patch_inplace](patch_inplace.md) applies a JSON Patch without creating a copy of the document
- [merge_patch](merge_patch.md) applies a JSON Merge Patch

## Version history

- Added in version 2.0.0.
- Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
  target location has a non-object/non-array parent in version 3.13.0.
- Added [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) and stopped silently ignoring a "remove" operation whose target
  location has a non-object/non-array parent in version 3.13.0.
- Added [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) and rejected a "move" operation whose "from" location is a proper
  prefix of its "path" location instead of silently producing a corrupted result in version 3.13.0.