qt_json_so_answer.cpp

// https://stackoverflow.com/a/55297349/7670262

#include <QDebug>
#include <QJsonDocument>
#include <QJsonObject>
#include <QJsonArray>

int main()
{
    const auto data = R"({ "Name": "45", "Path": "C:\file.json" })";

    auto doc = QJsonDocument::fromJson( data );
    qDebug() << "BEFORE:\n\n"
             << qPrintable( doc.toJson( QJsonDocument::Indented ) );

    // Create an array and add objects
    const auto obj1 = QJsonObject{ { "name", "abc" }, { "default", 11 } };
    const auto obj2 = QJsonObject{ { "name", "xyz" }, { "default", 22 } };

    auto obj = doc.object();
    obj.insert( "array", QJsonArray{ obj1, obj2 } );

    doc.setObject( obj );
    qDebug() << "\nAFTER:\n"
             << qPrintable( doc.toJson( QJsonDocument::Indented ) );

    // Add more objects to array

    const auto obj3 = QJsonObject
    {
        { "name", "def" },
        { "default", 33 },
        { "new1", "1" },
        { "new2", "2" }     // Add any number of objects...
    };

    const auto obj4 = QJsonObject{ { "name", "jkl" }, { "default", 44 } };

    // Get existing array to append more elements
    auto arr = doc.object()[ "array" ].toArray();
    arr.append( obj3 );
    arr.append( obj4 );

    // Set old array to newly updated one
    obj[ "array" ] = arr;

    doc.setObject( obj );
    qDebug() << "\nAFTER THAT:\n"
             << qPrintable( doc.toJson( QJsonDocument::Indented ) );

    return 0;
}

/*

// OUTPUT:

BEFORE:

 {
    "Name": "45",
    "Path": "C:\file.json"
}


AFTER:
 {
    "Name": "45",
    "Path": "C:\file.json",
    "array": [
        {
            "default": 11,
            "name": "abc"
        },
        {
            "default": 22,
            "name": "xyz"
        }
    ]
}


AFTER THAT:
 {
    "Name": "45",
    "Path": "C:\file.json",
    "array": [
        {
            "default": 11,
            "name": "abc"
        },
        {
            "default": 22,
            "name": "xyz"
        },
        {
            "default": 33,
            "name": "def",
            "new1": "1",
            "new2": "2"
        },
        {
            "default": 44,
            "name": "jkl"
        }
    ]
}

*/