nlohmannjson/doc/examples/merge_patch.cpp

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

using json = nlohmann::json;

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;
}