QJsonDocument::array() and QJsonDocument::object()

I am reading the QJsonDocument documentation and I use QJsonDocument with following line:

emit this->ueSignalNewDataArrivedPlaces(QJsonDocument::fromBinaryData(incomingData[0].toByteArray()));

and I do not understand, after this line, should I use QJsonDocument::array() or QJsonDocument::object(), i.e., in what situations does QJsonDocument create array and in what situations does create object?

A JSON array is an ordered list, it is written as:

[ <item1>, <item2>, <item3> ]

while a JSON object is a named list, written as:

{
    <name1>: <item1>,
    <name2>: <item2>
}

In Qt, a QJsonArray is equivalent to a QVariantList (QList<QVariant>) and a QJsonObject is equivalent to QVariantMap (QMap<QString, QVariant>).

Which one you have to use depends on the file you are parsing. For instance, taking Wikipedia example:

{
  "firstName": "John",
  "lastName": "Smith",
  "isAlive": true,
  "age": 25,
  "address": {
    "streetAddress": "21 2nd Street",
    "city": "New York",
    "state": "NY",
    "postalCode": "10021-3100"
  },
  "phoneNumbers": [
    {
      "type": "home",
      "number": "212 555-1234"
    },
    {
      "type": "office",
      "number": "646 555-4567"
    },
    {
      "type": "mobile",
      "number": "123 456-7890"
    }
  ],
  "children": [],
  "spouse": null
}

You would use a QJsonArray to get the list of phoneNumbers, each element of phoneNumbers is a QJsonObject whith 2 named values: type and number.

If in your code you need to manipulate a JSON element but you do not know its type you can use QJsonValue, which is one of: QJsonObject, QJsonArray, bool, double or a QString