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nlohmann::basic_json::diff

static basic_json diff(const basic_json& source,
                       const basic_json& target);

Creates a JSON Patch so that value source can be changed into the value target by calling patch function.

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

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

Parameters

source (in)
JSON value to compare from
target (in)
JSON value to compare against

Return value

a JSON patch to convert the source to target

Exception safety

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

Complexity

Linear in the lengths of source and target.

Notes

Currently, only remove, add, and replace operations are generated.

Examples

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;

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"
    ]
}

See also

Version history

  • Added in version 2.0.0.

Last update: May 1, 2022
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