# nlohmann::basic_json::patch

```
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`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error104) if the JSON patch does not consist of an array of objects.
- Throws [`parse_error.105`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if an array index is out of range.
- Throws [`parse_error.106`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range405) if JSON pointer has no parent ("add", "remove", "move")
- Throws [`out_of_range.411`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#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`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range414) if a "move" operation's "from" location is a proper prefix of its "path" location.
- Throws [`other_error.501`](https://json.nlohmann.me/home/exceptions/#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: 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 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"
    ]
}
```

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.

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

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // the original document
    json doc = R"(
        {
          "a": { "b": 1 }
        }
    )"_json;

    // a patch that tries to move "/a" into one of its own children
    json patch = R"(
        [
          { "op": "move", "from": "/a", "path": "/a/b" }
        ]
    )"_json;

    // exception out_of_range.414
    try
    {
        json patched_doc = doc.patch(patch);
    }
    catch (const json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // the original document is unchanged
    std::cout << std::setw(4) << doc << std::endl;
}
```

Output:

```
[json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'
{
    "a": {
        "b": 1
    }
}
```

## See also

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

## Version history

- Added in version 2.0.0.
- Added [`out_of_range.411`](https://json.nlohmann.me/home/exceptions/#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 unreleased.
- Added [`out_of_range.413`](https://json.nlohmann.me/home/exceptions/#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 unreleased.
- Added [`out_of_range.414`](https://json.nlohmann.me/home/exceptions/#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 unreleased.