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# 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. With the `patch` function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.

Example

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"
        ]
    }

## Diff

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

Invariant

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

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

Example

The following code shows how a JSON patch is created as a diff for two JSON values.

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

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

    int main()
    {
        // the source document
        json source = R"(
            {
                "baz": "qux",
                "foo": "bar"
            }
        )"_json;

        // the target document
        json target = R"(
            {
                "baz": "boo",
                "hello": [
                    "world"
                ]
            }
        )"_json;

        // create the patch
        json patch = json::diff(source, target);

        // roundtrip
        json patched_source = source.patch(patch);

        // output patch and roundtrip result
        std::cout << std::setw(4) << patch << "\n\n"
                  << std::setw(4) << patched_source << std::endl;
    }

Output:

    [
        {
            "op": "replace",
            "path": "/baz",
            "value": "boo"
        },
        {
            "op": "remove",
            "path": "/foo"
        },
        {
            "op": "add",
            "path": "/hello",
            "value": [
                "world"
            ]
        }
    ]

    {
        "baz": "boo",
        "hello": [
            "world"
        ]
    }

Last update: May 1, 2022
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