json/docs/mkdocs/docs/api/basic_json/patch.md

nlohmann::basic_json::patch

basic_json patch(const basic_json& json_patch) const;

JSON Patch 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 if the JSON patch does not consist of an array of objects.
• Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: "operation 'add' must have member 'path'".
• Throws out_of_range.401 if an array index is out of range.
• Throws parse_error.106 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 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 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 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 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 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 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 if JSON pointer has no parent ("add", "remove", "move")
• Throws out_of_range.411 if an "add" operation's target location has a parent that is neither an object nor an array.
• Throws out_of_range.413 if a "remove" operation's target location has a parent that is neither an object nor an array.
• Throws out_of_range.414 if a "move" operation's "from" location is a proper prefix of its "path" location.
• Throws other_error.501 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)
• RFC 6901 (JSON Pointer)
• patch_inplace applies a JSON Patch without creating a copy of the document
• merge_patch applies a JSON Merge Patch

Version history

• Added in version 2.0.0.
• Added out_of_range.411 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 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 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.
• Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0, as it already did for longer ones.