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# nlohmann::basic_json::merge_patch

    void merge_patch(const basic_json& apply_patch);

The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.

The function implements the following algorithm from Section 2 of RFC 7396 (JSON Merge Patch):

    define MergePatch(Target, Patch):
      if Patch is an Object:
        if Target is not an Object:
          Target = {} // Ignore the contents and set it to an empty Object
        for each Name/Value pair in Patch:
          if Value is null:
            if Name exists in Target:
              remove the Name/Value pair from Target
          else:
            Target[Name] = MergePatch(Target[Name], Value)
        return Target
      else:
        return Patch

Thereby, `Target` is the current object; that is, the patch is applied to the current value.

## Parameters

`apply_patch` (in)
  the patch to apply

## Complexity

Linear in the lengths of `apply_patch`.

## Examples

Example

The following code shows how a JSON Merge Patch is applied to a JSON document.

    #include <iostream>
    #include <nlohmann/json.hpp>
    #include <iomanip> // for std::setw

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

    int main()
    {
        // the original document
        json document = R"({
                    "title": "Goodbye!",
                    "author": {
                        "givenName": "John",
                        "familyName": "Doe"
                    },
                    "tags": [
                        "example",
                        "sample"
                    ],
                    "content": "This will be unchanged"
                })"_json;

        // the patch
        json patch = R"({
                    "title": "Hello!",
                    "phoneNumber": "+01-123-456-7890",
                    "author": {
                        "familyName": null
                    },
                    "tags": [
                        "example"
                    ]
                })"_json;

        // apply the patch
        document.merge_patch(patch);

        // output original and patched document
        std::cout << std::setw(4) << document << std::endl;
    }

Output:

    {
        "author": {
            "givenName": "John"
        },
        "content": "This will be unchanged",
        "phoneNumber": "+01-123-456-7890",
        "tags": [
            "example"
        ],
        "title": "Hello!"
    }

## See also

- RFC 7396 (JSON Merge Patch)
- patch apply a JSON patch

## Version history

- Added in version 3.0.0.

Last update: May 1, 2022
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