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;
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"
    ]
}

Version history¶

Added in version 2.0.0.

Last update: May 1, 2022